EP1053596A1 - A precision relaxation oscillator with temperature compensation and various operating modes - Google Patents

A precision relaxation oscillator with temperature compensation and various operating modes

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Publication number
EP1053596A1
EP1053596A1 EP99963026A EP99963026A EP1053596A1 EP 1053596 A1 EP1053596 A1 EP 1053596A1 EP 99963026 A EP99963026 A EP 99963026A EP 99963026 A EP99963026 A EP 99963026A EP 1053596 A1 EP1053596 A1 EP 1053596A1
Authority
EP
European Patent Office
Prior art keywords
circuit
operating mode
accordance
clock
current
Prior art date
Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
Withdrawn
Application number
EP99963026A
Other languages
German (de)
French (fr)
Inventor
James B. Nolan
Ryan Scott Ellison
Michael S. Pyska
Current Assignee (The listed assignees may be inaccurate. Google has not performed a legal analysis and makes no representation or warranty as to the accuracy of the list.)
Microchip Technology Inc
Original Assignee
Microchip Technology Inc
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Priority claimed from US09/205,758 external-priority patent/US6052035A/en
Application filed by Microchip Technology Inc filed Critical Microchip Technology Inc
Publication of EP1053596A1 publication Critical patent/EP1053596A1/en
Withdrawn legal-status Critical Current

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Classifications

    • H—ELECTRICITY
    • H03—ELECTRONIC CIRCUITRY
    • H03K—PULSE TECHNIQUE
    • H03K3/00—Circuits for generating electric pulses; Monostable, bistable or multistable circuits
    • H03K3/01—Details
    • H03K3/011—Modifications of generator to compensate for variations in physical values, e.g. voltage, temperature

Definitions

  • This invention relates generally to integrated circuits which produce clock frequencies.
  • the present invention is a precision relaxation oscillator that produces a stable clock frequency over wide variations of ambient temperature, fabrication process and voltage.
  • the invention is implemented on a single, monolithic integrated circuit.
  • the precision relaxation oscillator is capable of several modes of operation.
  • the current state of the art describes RC relaxation oscillators which primarily depend on one of two schemes.
  • a single comparator is coupled to a pulse generator to alternately charge and discharge a capacitor to produce a clock for a "D type" flip-flop.
  • the resistor and capacitor typically have unpredictable voltage and temperature coefficients.
  • the charging current and comparator input slew are a function of the supply voltage which is also subject to drift.
  • the pulse generator output may vary with temperature and supply voltage. These factors lead to a clock frequency that varies over temperature.
  • an RC circuit provides a common input to each of two comparators . Independent reference voltages are coupled to each of the remaining inputs of the comparators . The outputs of each of the two comparators are coupled to the inputs of a "Set-Reset type" flip-flop. The output of the flip-flop serves to alternately charge and discharge the capacitor.
  • a relaxation oscillator which is capable of maintaining a stable clock frequency independent of temperature.
  • a stable clock is defined as one which maintains a stable frequency- in an environment which is subject to temperature fluctuations.
  • a precision relaxation oscillator that produces a stable clock frequency over wide variations of ambient temperature.
  • the precision relaxation oscillator is comprised of an oscillation generator, a first current generator for producing a first output current and a second current generator for producing a second output current.
  • the invention is implemented on a single, monolithic integrated circuit.
  • an external resistor may be coupled to either the first or second current generators to produce the respective output currents required for determining the clock frequency.
  • a plurality of internal resistors within the first and second current generators are provided which are used to select the clock speed of the oscillator.
  • a transition detector circuit is provided.
  • a clock inhibiter coupled to the output of the oscillation generator is provided.
  • Figure 1 is a schematic diagram of the prior art showing a simple RC Relaxation Oscillator with a pulse generator.
  • Figure 2 is a schematic diagram of the prior art showing a dual comparator RC Relaxation Oscillator.
  • FIG. 3 is a block diagram of the present invention.
  • FIG. 4 is a block diagram of the CTAT current generator found in the present invention.
  • Figure 5 is a block diagram of the PTAT current generator found in the present invention.
  • Figure 6 is a timing diagram of specific parameters of the present invention.
  • Figure 7 is a timing diagram of the clock transition between slow mode and fast mode.
  • a precision relaxation oscillator 1 that produces a stable clock frequency over wide variations of ambient temperature is shown.
  • the precision relaxation oscillator 1 produces a stable clock frequency in the range of approximately 1 KHz to 8 MHz.
  • the present invention is not limited to a specific frequency range.
  • the precision relaxation oscillator 1 is capable of 3 modes.
  • the first mode is the fast mode and is the normal operating mode.
  • the second mode is the slow mode and is selected to conserve power, yet to have some functions of the circuit to which the precision relaxation oscillator 1 serves, remain active.
  • the third mode is the sleep mode. In this mode, the precision relaxation oscillator 1 is inactive and there is no clock output, nor is there any power consumption.
  • the transition between modes may occur "on the fly," i.e. suspension of processing activity by the CPU is not required for transitioning from one mode to another. chorus• However, in the preferred embodiment, the CPU or microcontroller will have completed the current instruction cycle before switching modes.
  • the precision relaxation oscillator 1 is comprised of an oscillation generator 100, a first current generator 200 which is typically a Complementary to Absolute Temperature (CTAT) current generator, a second current generator 300 which is typically a Proportional to Absolute Temperature (PTAT) current generator, a transition detector 400 and a clock inhibiter 500.
  • CTAT Complementary to Absolute Temperature
  • PTAT Proportional to Absolute Temperature
  • the precision relaxation oscillator 1 is implemented on a single, monolithic integrated circuit.
  • CTAT 200 and PTAT 300 current generators are independently implemented and yield several important functions to the present invention.
  • the CTAT 200 and PTAT 300 current generators compensate for the effects that temperature variation has on the internal components of the device such as resistors, capacitors and comparators by providing offsetting currents CTAT current 220 and PTAT current 320, i.e. currents with opposite slopes with respect to temperature.
  • the combining, or summing of the CTAT current 290 and PTAT current 390 occurs when introduced to the oscillation generator 100 for charging a first capacitor 110 and a second capacitor 120. Because the CTAT 290 and PTAT 390 currents are approximately linear and ⁇ -of opposite slope with respect to temperature, the result of the summation is an I ccc 190 that is nearly independent of temperature.
  • the oscillation generator 100 is comprised of a set-reset flip-flop 160, a comparator circuit 180 further comprised of two comparators 182 & 184, two capacitors 110 & 120, four transistor switches 130, 132, 134 & 136, two inverters 140 & 142 and a bandgap reference voltage circuit 150 for producing a reference voltage 152.
  • the transistor switches 130 & 134 provide charging paths for the capacitors 110 & 120, respectively.
  • the transistor switches 132 & 136 provide discharging paths for the capacitors 110 & 120, respectively.
  • the transistor switches 130, 132, 134 and 136 are MOSFET transistors, however, those skilled in the art will recognize that the invention is not limited to this technology.
  • the oscillation generator 100 operates by having one capacitor charge while the other capacitor discharges.
  • the discharge path for the capacitor 110 is connected via transistor switch 132 to an input of the comparator 182.
  • the discharge path for the capacitor 120 is connected via transistor switch 136 to an input of the comparator 184.
  • a stable reference voltage source such as a bandgap reference voltage circuit 150 is used.
  • the bandgap reference voltage circuit 150 provides a single reference voltage 152, whic is connected to second inputs of comparators 182 & 184, and is used to set the common mode voltage at each comparator 182 & 184 and at the CTAT current generator 200.
  • the P BIA ⁇ input 325 for the bandgap reference voltage circuit 150 is an output of the PTAT bias generator 310 which is described below.
  • the bandgap reference voltage circuit 150 has the advantage of stabilizing capacitor charging current and minimizing the error due to variance in comparator input slew and propagation delay.
  • the CTAT 200 current generator relies on the same reference voltage 152 as the comparators 182 & 184.
  • the CTAT current 290 (figure 4), which is equal to Vpr F /R, also increases. Without compensation elsewhere, this increased CTAT current 290 would result in a faster clock frequency 166, because a greater I ccc 190 is produced, which results in faster charging of the capacitors 110 and 120.
  • the capacitors 110 and 120 must charge to a greater level for the comparators 182 and 184 to trip with respect to the increased reference voltage 152.
  • the present invention requires a simpler, lest costly reference voltage source to achieve clock frequency stability.
  • bandgap reference voltage circuit 150 there are various embodiments of the bandgap reference voltage circuit 150, as well as other reference voltage sources such as voltage dividers, which are well known to those skilled in the art. However, the novel way in which the bandgap reference voltage circuit 150 is implemented in the present invention is not disclosed by the prior art.
  • comparator 182 is connected to the set input 162 of the flip-flop 160.
  • comparator 184 is connected to the reset input 164 of the flip-flop 160.
  • the Q output 166 of the flip-flop 160 provides a stable clock frequency INTCLK that is independent of temperature variation.
  • the Q output 166 is also routed to transistor switch 132 and via inverter 140 to transistor switch 130.
  • the Q output 166 provides the signal that controls the transistor switches 130 & 132 which in turn open and close the charging and discharging paths for capacitor 110.
  • the complementary Q output 168 of flip-flop 160 provides a second stable clock frequency that is also independent of temperature and the complement of Q output 166.
  • the complementary Q output 168 is routed to transistor switch 136 and via inverter 142 to transistor switch 134.
  • the complementary Q output 168 provides the signal that controls the transistor switches 134 & 136 which in turn open and close the charging and discharging paths for capacitor 120.
  • the transition detector 400 performs two primary functions: converting an asynchronous fast/slow signal (AS K YNCH FAST/SLOW) to a synchronous fast/slow signal (SYNCH FAST/SLOW 404) and initializing the clock inhibiter 500.
  • the purpose of the clock inhibiter 500 is to inhibit INTCLK 166 from being output as CLKOUT 502 for a programmable number of clock cycles during mode transition when the INTCLK 166 may be unstable.
  • the transition detector 400 which is comprised of simple combinational and sequential logic such as a synchronous one-shot, sends a RSTCLK 402 signal to initialize the clock inhibiter 500.
  • the clock inhibiter 500 Upon receiving the RSTCLK 402 signal from the transition detector 400, the clock inhibiter 500, which may be a simple programmable counter, will inhibit CLKOUT 502 from being sent to the connected circuitry, e.g. the CPU, for a predetermined number of clock cycles.
  • the clock inhibiter 500 serves to prevent logic anomalies as a result of an unstable clock frequency or a clock frequency in transition. After a predetermined number of clock cycles following mode transition, when presumptively the operation of the precision relaxation oscillator 1 is stable, the clock inhibiter 500 will allow the CLKOUT 502 to pass to the connected circuitry.
  • the trailing edge of the RSTCLK 402 signal triggers the SYNCH FAST/SLOW 404 signal.
  • the SYNCH FAST/SLOW 402 is used by the CTAT current generator 200 and PTAT current generator 300 to adjust the respective currents 290 and 390 for fast or slow mode operation, as described below.
  • the CTAT current generator 200 is comprised of a CTAT bias generator 210 and a current mirror 250 for producing the CTAT current 290.
  • the CTAT bias generator 210 is comprised of an amplifier circuit 220, at least one resistor 232, 233 & 234 with a small positive temperature coefficient for regulating the input current to the amplifier and a transistor 240 for providing an input current to the amplifier 220.
  • the amplifier 220 is a cascode configuration for supply and noise rejection.
  • the reference voltage 152 is coupled to an input of the amplifier 220.
  • the different resistors 232, 233 & 234, which vary in impedance, are for controlling the current sent to the current mirror 250 and thus, determine the specific stable clock frequency independent of temperature which is produced by the oscillation generator 100.
  • the present invention provides for selection logic 230 which inputs SYNCH FAST/SLOW 404 and a resistor select (R SELECT 236) for selecting one of the three resistors 232, 233 or 234. If the slow mode is activated by SYNCH FAST/SLOW 404, then the internal resistor R INT/LP 233 is selected. If the fast mode is activated by SYNCH FAST/SLOW 404 then the selection logic 230 considers the input R SELECT 236 in choosing as between the internal resistor R INT 232 and the external resistor R EX ⁇ 234.
  • the fast mode ⁇ -internal resistor 232 is fabricated from polysilicon technology which provides for low impedance and thus, higher current which in turn provides for a faster clock.
  • polysilicon technology has a low temperature coefficient (ppm/deg C) , thus providing improved frequency stability over temperature.
  • the slow mode internal resistor 233 is preferably fabricated from doped silicon substrate, typically either through implantation and/or diffusion, e.g. Lightly Doped Drain (LDD) .
  • doped silicon produces a high impedance which in turn reduces the current to the current mirror 250, and thus allows for operating at low power .
  • the slow mode internal resistor 233 could be fabricated using polysilicon technology. However, the resistance per unit area of polysilicon is significantly lower than the resistance per unit area of doped silicon. Thus, a polysilicon resistor would require significantly greater semiconductor area than a doped silicon resistor for a similar resistance.
  • the power consumption of the precision relaxation oscillator 1 in typical applications ranges from 250 ua (micro amperes) in the fast mode to 20 ua or less in the slow mode. No power is consumed when in the sleep mode.
  • the current mirror 250 is comprised of a plurality of transistors 252 from one to n.
  • the output the CTAT bias generator amplifier 220 is coupled to the current mirror transistors 252. Trimming the CTAT current 290 for achieving the proper CTAT: PTAT balance is programmed digitally by selecting qr enabling one or more of the current mirror transistors 252 via the calibration switches 254, which would sum, to obtain the desired CTAT current 290.
  • the calibration switches 254 are also coupled the SYNCH FAST/SLOW 404 signal via the CTAT calibration select decode 256.
  • the calibration switches 254 are configured to trim I CTAT for a fast clock.
  • the calibration switches 254 may require a different configuration to trim I CTAT for a slow clock.
  • the calibrations switches 254 toggle between a fast mode calibration and a slow mode calibration in response to the state of the SYNCH FAST/SLOW 404 signal.
  • the current mirror 250 acts as a current divider which is well known to those skilled in the art. In other embodiments the current mirror 250 may be configured as a current multiplier.
  • the CTAT current 290 is the sum of the selected outputs from the current mirror transistors 252.
  • the PTAT current generator 300 known to those skilled in the art as a ⁇ V BE circuit, is comprised of PTAT bias generator 310 and a PTAT current mirror 350 for producing a PTAT current 390.
  • the PTAT bias generator 310 is comprised of an amplifier circuit 320, a first bias circuit 330 for producing a first bias voltage across a selectable resistor with a small linear temperature coefficient 332, 333 and 334 and a second bias circuit 340 for producing a second bias voltage.
  • the first and second bias voltages provide the inputs to the amplifier 320.
  • the output of the amplifier 320 is P B1AS 325 which is coupled to the first 330 and second 340 bias circuits, the PTAT current mirror 350 and the bandgap reference voltage generator 150 (figure 1) .
  • the different resistors 332, 333 and 334 which vary in impedance, are for controlling the current sent to the current mirror 350 and thus, determine the specific stable clock frequency independent of temperature which is produced by the oscillation generator 100.
  • the PTAT bias generator 310 provides for selection logic 330 which inputs SYNCH FAST/SLOW 404 and the resistor select (R SELEC ⁇ 236) for selecting one of the three resistors 332, 333 or 334. If the slow mode is activated by SYNCH FAST/SLOW 404, then the internal resistor R INT/LP 333 is selected. If the fast mode is activated by SYNCH FAST/SLOW 404, then the selection logic 330 considers the input R SELECT 336 in choosing as between the internal resistor R INT 332 and the external resistor R ⁇ 334.
  • the fast mode internal resistor 332 is fabricated from polysilicon technology which provides for low impedance and thus , higher current which in turn provides for a faster clock.
  • polysilicon technology has a low temperature coefficient (ppm/deg C) , thus providing improved frequency stability over temperature.
  • the slow mode internal resistor 333 is preferably fabricated from diffused technology, e.g. Lightly Doped Drain (LDD) . Diffused technology produces a high impedance which in turn reduces the current to the current mirror 350, and thus allows for operating at low power.
  • the respective resistor pairs in the CTAT and PTAT bias generators 210 and 310 are matched to each other for optimum suability, ' e.g. both resistors (R EXT 234 and 334 or R INT 232 and 332) are polysilicon in the fast mode and both resistors (R ⁇ T LP 233 and 333) are doped silicon in the slow mode.
  • the slow mode internal resistor 333 could be fabricated using polysilicon technology. However, the resistance per unit area of polysilicon is significantly lower than the resistance per unit area of diffused technology. Thus, a polysilicon resistor would require significantly greater semiconductor area than, a doped silicon resistor for a similar resistance.
  • the PTAT current mirror 350 is comprised of a plurality of transistors 352 from one to n. Trimming is performed digitally by programming the selection or enablement of one or more of the current mirror transistors 352 via the calibration switches 354 to obtain the desired PTAT current 390.
  • the calibration switches 354 are also coupled to the SYNCH FAST/SLOW 404 signal via the PTAT calibration select decode 356.
  • the calibration switches 354 are configured to trim I PTAT for a fast clock.
  • the calibration switches 354 may require a different configuration to trim I PTAT for a slow clock.
  • the calibrations switches 354 toggle between a fast mode calibration and a slow mode calibration in response to the state of the SYNCH FAST/SLOW 404 signal.
  • the current mirror 350 acts as a current divider which is well known to those skilled in the art. In other embodiments the current mirror 350 may be configured as a current multiplier.
  • the PTAT current 390 is the sum of the selected outputs from the current mirror transistors 352.
  • VI 112 reflects the charging and discharging of capacitor 110 (figure 1) .
  • the positive slope (charging) of VI 112 is equal to I ccc 190 divided by the capacitance of capacitor 110.
  • the maximum amplitude of VI 112 is equal to the reference voltage 152.
  • CMPl reflects the output of the comparator 182 which is coupled to the set input 162 of the flip-flop 160.
  • V2122 reflects the charging and discharging of capacitor 120.
  • the positive slope of V2 122 is equal to I ccc 190 divided by the capacitance of capacitor 120.
  • CMP2 reflects the output of the comparator 184 which is coupled to the reset input 164 of the flip-flop 160.
  • CLK is the Q output 166 of the flip-flop 160.
  • the values of- capacitors 110 & 120 are identical which result in similar slopes for VI 112 and V2 122.
  • the respective comparator 182 & 184 pulses low which causes the flip-flop 160 to change state.
  • RST reset
  • FIG. 7 a timing diagram which illustrates the transition from slow mode to fast mode of the embodiment of figure 3 is shown.
  • the relaxation oscillator.1 would operate similarly when transitioning from fast mode to slow mode.
  • the relaxation oscillator 1 ( Figure 3) is first operating in the slow mode.
  • the ASYNCH FAST/SLOW signal which is generated external to the present invention, is received by the transition detector 400.
  • logic level zero indicates slow mode and logic level one indicates fast mode.
  • the signal to transition to sleep mode is a separate, active high signal.
  • the transition detector 400 Upon receiving the ASYNCH FAST/SLOW signal, that meets the required setup time in relation to the internal clock INTCLK 166, to transition to the fast mode, the transition detector 400 generates two outputs.
  • the transition detector 400 outputs a reset pulse RSTCLK 402 to the clock inhibiter 500.
  • the transition detector 400 also synchronizes the ASYNCH FAST/SLOW signal and at the ⁇ trailing edge of RSTCLK 402, the transition detector 400 outputs SYNCH FAST/SLOW 404 to the current generators 200 and 300. At this point the current generators 200 and 300 begin the switching to generate the currents required for the fast mode. As the current generators 200 and 300 begin their internal switching, several clock cycles are required to allow the bias currents to settle and for INTCLK 166 to stabilize.
  • the clock inhibiter 500 Upon receiving RSTCLK 402, the clock inhibiter 500 immediately begins inhibiting CLKOUT 502. At the trailing edge of the RSTCLK 504 pulse, the relaxation oscillator 1 begins to transition from slow to fast mode. In one embodiment, the clock inhibiter counts and inhibits eight fast clock cycles of the INTCLK 166 for stabilization, before releasing the inhibit and allowing CLKOUT 504 to proceed with the fast mode clock.
  • the present invention minimizes clock frequency drift due to fabrication process, supply voltage and temperature variances. This is accomplished by providing offsetting bias currents which when summed are independent of temperature variation, trimming via the programmable current mirrors 250 & 350 to eliminate process variations, using a stable voltage reference such as a bandgap reference voltage circuit 150 and a dual capacitor, dual comparator oscillation generator 100. Also, analog design techniques, well known to those skilled in the art, such as component matching and cascode current sources enhance the stability of the circuit.

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Abstract

A precision relaxation oscillator with temperature compensation produces a stable clock frequency over wide variations of ambient temperature. The invention has an oscillation generator (100), two independent current generators (200, 300), a transition detector (400) and a clock inhibiter (500). The outputs of the two programmable, independent current generators are combined to provide a capacitor charging current which is independent of temperature. The precision relaxation oscillator is capable of three modes of operation: fast mode, slow/low power mode and sleep mode. The precision relaxation oscillator with temperature compensation and various operating modes is implemented on a single, monolithic integrated circuit.

Description

A PRECISION RELAXATION OSCILLATOR WITH TEMPERATURE COMPENSATION
AND VARIOUS OPERATING MODES
RELATED APPLICATIONS
This application is a continuation-in-part of U.S. Patent Application Serial Number 09/044,361, filed March 19, 1998, entitled "A Precision Relaxation Oscillator With Temperature Compensation, " which is assigned to the same assignee as the present application and for which there was at least one common inventor with the present application. U.S. Patent Application Serial Number 09/044,361, filed March 19, 1998, is incorporated by reference .
BACKGROUND OF THE INVENTION
1. Field of the Invention:
This invention relates generally to integrated circuits which produce clock frequencies. Specifically, the present invention is a precision relaxation oscillator that produces a stable clock frequency over wide variations of ambient temperature, fabrication process and voltage. The invention is implemented on a single, monolithic integrated circuit. Also, the precision relaxation oscillator is capable of several modes of operation.
2. Description of the Prior Art:
The current state of the art describes RC relaxation oscillators which primarily depend on one of two schemes. In the first example as found in figure 1, a single comparator is coupled to a pulse generator to alternately charge and discharge a capacitor to produce a clock for a "D type" flip-flop. Several error sources are present in this design. The resistor and capacitor typically have unpredictable voltage and temperature coefficients. The charging current and comparator input slew are a function of the supply voltage which is also subject to drift. Also, the pulse generator output may vary with temperature and supply voltage. These factors lead to a clock frequency that varies over temperature.
In a second example as illustrated in figure 2, an RC circuit provides a common input to each of two comparators . Independent reference voltages are coupled to each of the remaining inputs of the comparators . The outputs of each of the two comparators are coupled to the inputs of a "Set-Reset type" flip-flop. The output of the flip-flop serves to alternately charge and discharge the capacitor. Although this circuit eliminates the inaccuracies of the pulse generator as discussed above in figure 1, other problems manifest themselves. A duty cycle error may occur since it is unlikely that the capacitor will charge and discharge at the same rate, especially over temperature variations. Also, error is induced by the difficulty of providing two reference voltages which track each other coincidently over temperature.
Therefore, a need existed to provide a relaxation oscillator which is capable of maintaining a stable clock frequency independent of temperature.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a relaxation oscillator which is capable of maintaining a stable clock frequency independent of temperature. A stable clock is defined as one which maintains a stable frequency- in an environment which is subject to temperature fluctuations.
It is another object of the present invention to provide a relaxation oscillator which minimizes the temperature coefficient of the oscillator as measured in parts per million of the clock frequency divided by temperature (ppm/deg C) . For example, one part per million per degree centigrade for a clock frequency of 4 MHz equates to 4 clock cycles.
It is another object of the present invention to provide a relaxation oscillator which is immune to frequency drift due to process and supply voltage.
It is another object of the present invention to provide three modes of operation which include a fast mode, a slow/low power mode and a sleep mode.
It is another object of the present invention to reduce the power consumption when operating in the slov; mode and the sleep mode.
In accordance with one embodiment of the present invention, a precision relaxation oscillator that produces a stable clock frequency over wide variations of ambient temperature is disclosed. The precision relaxation oscillator is comprised of an oscillation generator, a first current generator for producing a first output current and a second current generator for producing a second output current. The invention is implemented on a single, monolithic integrated circuit.
In accordance with another embodiment of the,-present invention an external resistor may be coupled to either the first or second current generators to produce the respective output currents required for determining the clock frequency.
In accordance with another embodiment of the invention a plurality of internal resistors within the first and second current generators are provided which are used to select the clock speed of the oscillator.
In accordance with another embodiment of the invention a transition detector circuit is provided.
In accordance with another embodiment of the present invention a clock inhibiter coupled to the output of the oscillation generator is provided.
The foregoing and other objects, features, and advantages of the invention will be apparent from the following, more particular, description of the preferred embodiments of the invention, as illustrated in the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS
Figure 1 is a schematic diagram of the prior art showing a simple RC Relaxation Oscillator with a pulse generator.
Figure 2 is a schematic diagram of the prior art showing a dual comparator RC Relaxation Oscillator.
Figure 3 is a block diagram of the present invention.
Figure 4 is a block diagram of the CTAT current generator found in the present invention.
Figure 5 is a block diagram of the PTAT current generator found in the present invention.
Figure 6 is a timing diagram of specific parameters of the present invention.
Figure 7 is a timing diagram of the clock transition between slow mode and fast mode.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
Referring to Figure 3, a precision relaxation oscillator 1 that produces a stable clock frequency over wide variations of ambient temperature is shown. Preferably, the precision relaxation oscillator 1 produces a stable clock frequency in the range of approximately 1 KHz to 8 MHz. However, those skilled in the art will recognize that the present invention is not limited to a specific frequency range.
The precision relaxation oscillator 1 is capable of 3 modes. The first mode is the fast mode and is the normal operating mode. The second mode is the slow mode and is selected to conserve power, yet to have some functions of the circuit to which the precision relaxation oscillator 1 serves, remain active. The third mode is the sleep mode. In this mode, the precision relaxation oscillator 1 is inactive and there is no clock output, nor is there any power consumption. The transition between modes may occur "on the fly," i.e. suspension of processing activity by the CPU is not required for transitioning from one mode to another. „• However, in the preferred embodiment, the CPU or microcontroller will have completed the current instruction cycle before switching modes.
The precision relaxation oscillator 1 is comprised of an oscillation generator 100, a first current generator 200 which is typically a Complementary to Absolute Temperature (CTAT) current generator, a second current generator 300 which is typically a Proportional to Absolute Temperature (PTAT) current generator, a transition detector 400 and a clock inhibiter 500. In the preferred embodiment of the present invention, the precision relaxation oscillator 1 is implemented on a single, monolithic integrated circuit.
The CTAT 200 and PTAT 300 current generators are independently implemented and yield several important functions to the present invention. The CTAT 200 and PTAT 300 current generators compensate for the effects that temperature variation has on the internal components of the device such as resistors, capacitors and comparators by providing offsetting currents CTAT current 220 and PTAT current 320, i.e. currents with opposite slopes with respect to temperature. CTAT current 290 and PTAT current 390 (figures 4 & 5) are combined to form a capacitor charging current Iccc 190 (Iccc 190 = CTAT current 290 + PTAT current 390) . The combining, or summing of the CTAT current 290 and PTAT current 390 occurs when introduced to the oscillation generator 100 for charging a first capacitor 110 and a second capacitor 120. Because the CTAT 290 and PTAT 390 currents are approximately linear and^-of opposite slope with respect to temperature, the result of the summation is an Iccc 190 that is nearly independent of temperature.
In the preferred embodiment, the oscillation generator 100 is comprised of a set-reset flip-flop 160, a comparator circuit 180 further comprised of two comparators 182 & 184, two capacitors 110 & 120, four transistor switches 130, 132, 134 & 136, two inverters 140 & 142 and a bandgap reference voltage circuit 150 for producing a reference voltage 152.
The transistor switches 130 & 134 provide charging paths for the capacitors 110 & 120, respectively. The transistor switches 132 & 136 provide discharging paths for the capacitors 110 & 120, respectively. In the preferred embodiment, the transistor switches 130, 132, 134 and 136 are MOSFET transistors, however, those skilled in the art will recognize that the invention is not limited to this technology.
The oscillation generator 100 operates by having one capacitor charge while the other capacitor discharges. The discharge path for the capacitor 110 is connected via transistor switch 132 to an input of the comparator 182. The discharge path for the capacitor 120 is connected via transistor switch 136 to an input of the comparator 184.
In the preferred embodiment and for best performance, a stable reference voltage source such as a bandgap reference voltage circuit 150 is used. The bandgap reference voltage circuit 150 provides a single reference voltage 152, whic is connected to second inputs of comparators 182 & 184, and is used to set the common mode voltage at each comparator 182 & 184 and at the CTAT current generator 200. The PBIAΞ input 325 for the bandgap reference voltage circuit 150 is an output of the PTAT bias generator 310 which is described below. The bandgap reference voltage circuit 150 has the advantage of stabilizing capacitor charging current and minimizing the error due to variance in comparator input slew and propagation delay.
Furthermore, in order to cancel, or at the very least minimize, the effects of reference voltage drift, the CTAT 200 current generator relies on the same reference voltage 152 as the comparators 182 & 184. For example, if the reference voltage 152 increases, the CTAT current 290 (figure 4), which is equal to VprF/R, also increases. Without compensation elsewhere, this increased CTAT current 290 would result in a faster clock frequency 166, because a greater Iccc 190 is produced, which results in faster charging of the capacitors 110 and 120. However, the capacitors 110 and 120 must charge to a greater level for the comparators 182 and 184 to trip with respect to the increased reference voltage 152. Thus, the present invention requires a simpler, lest costly reference voltage source to achieve clock frequency stability. There are various embodiments of the bandgap reference voltage circuit 150, as well as other reference voltage sources such as voltage dividers, which are well known to those skilled in the art. However, the novel way in which the bandgap reference voltage circuit 150 is implemented in the present invention is not disclosed by the prior art.
The output of comparator 182 is connected to the set input 162 of the flip-flop 160. The output of comparator 184 is connected to the reset input 164 of the flip-flop 160. Thus, as the capacitors 110 and 120 alternatively charge and discharge, the outputs of the comparators 182 & 184 alternatively set and reset the flip-flop 160 thus producing a clock output.
The Q output 166 of the flip-flop 160 provides a stable clock frequency INTCLK that is independent of temperature variation. In the preferred embodiment, the Q output 166 is also routed to transistor switch 132 and via inverter 140 to transistor switch 130. Thus the Q output 166 provides the signal that controls the transistor switches 130 & 132 which in turn open and close the charging and discharging paths for capacitor 110.
The complementary Q output 168 of flip-flop 160 provides a second stable clock frequency that is also independent of temperature and the complement of Q output 166. The complementary Q output 168 is routed to transistor switch 136 and via inverter 142 to transistor switch 134. Thus the complementary Q output 168 provides the signal that controls the transistor switches 134 & 136 which in turn open and close the charging and discharging paths for capacitor 120.
The transition detector 400 performs two primary functions: converting an asynchronous fast/slow signal (ASKYNCH FAST/SLOW) to a synchronous fast/slow signal (SYNCH FAST/SLOW 404) and initializing the clock inhibiter 500. The purpose of the clock inhibiter 500 is to inhibit INTCLK 166 from being output as CLKOUT 502 for a programmable number of clock cycles during mode transition when the INTCLK 166 may be unstable. When the circuit 1 transitions from one operating mode to another, e.g. from slow mode to fast mode, the transition detector 400, which is comprised of simple combinational and sequential logic such as a synchronous one-shot, sends a RSTCLK 402 signal to initialize the clock inhibiter 500.
Upon receiving the RSTCLK 402 signal from the transition detector 400, the clock inhibiter 500, which may be a simple programmable counter, will inhibit CLKOUT 502 from being sent to the connected circuitry, e.g. the CPU, for a predetermined number of clock cycles. Thus, the clock inhibiter 500 serves to prevent logic anomalies as a result of an unstable clock frequency or a clock frequency in transition. After a predetermined number of clock cycles following mode transition, when presumptively the operation of the precision relaxation oscillator 1 is stable, the clock inhibiter 500 will allow the CLKOUT 502 to pass to the connected circuitry.
Furthermore, the trailing edge of the RSTCLK 402 signal triggers the SYNCH FAST/SLOW 404 signal. The SYNCH FAST/SLOW 402 is used by the CTAT current generator 200 and PTAT current generator 300 to adjust the respective currents 290 and 390 for fast or slow mode operation, as described below.
Referring to figure 4, wherein like numerals reflect like elements, the CTAT current generator 200 is comprised of a CTAT bias generator 210 and a current mirror 250 for producing the CTAT current 290. The CTAT bias generator 210 is comprised of an amplifier circuit 220, at least one resistor 232, 233 & 234 with a small positive temperature coefficient for regulating the input current to the amplifier and a transistor 240 for providing an input current to the amplifier 220. The amplifier 220 is a cascode configuration for supply and noise rejection. The reference voltage 152 is coupled to an input of the amplifier 220.
The different resistors 232, 233 & 234, which vary in impedance, are for controlling the current sent to the current mirror 250 and thus, determine the specific stable clock frequency independent of temperature which is produced by the oscillation generator 100. The present invention provides for selection logic 230 which inputs SYNCH FAST/SLOW 404 and a resistor select (RSELECT 236) for selecting one of the three resistors 232, 233 or 234. If the slow mode is activated by SYNCH FAST/SLOW 404, then the internal resistor RINT/LP 233 is selected. If the fast mode is activated by SYNCH FAST/SLOW 404 then the selection logic 230 considers the input RSELECT 236 in choosing as between the internal resistor RINT 232 and the external resistor REXτ 234.
In the preferred embodiment, the fast modeκ-internal resistor 232 is fabricated from polysilicon technology which provides for low impedance and thus, higher current which in turn provides for a faster clock. In addition, polysilicon technology has a low temperature coefficient (ppm/deg C) , thus providing improved frequency stability over temperature.
By contrast, the slow mode internal resistor 233 is preferably fabricated from doped silicon substrate, typically either through implantation and/or diffusion, e.g. Lightly Doped Drain (LDD) . Doped silicon produces a high impedance which in turn reduces the current to the current mirror 250, and thus allows for operating at low power .
The slow mode internal resistor 233 could be fabricated using polysilicon technology. However, the resistance per unit area of polysilicon is significantly lower than the resistance per unit area of doped silicon. Thus, a polysilicon resistor would require significantly greater semiconductor area than a doped silicon resistor for a similar resistance. The power consumption of the precision relaxation oscillator 1 in typical applications ranges from 250 ua (micro amperes) in the fast mode to 20 ua or less in the slow mode. No power is consumed when in the sleep mode.
The current mirror 250 is comprised of a plurality of transistors 252 from one to n. The output the CTAT bias generator amplifier 220 is coupled to the current mirror transistors 252. Trimming the CTAT current 290 for achieving the proper CTAT: PTAT balance is programmed digitally by selecting qr enabling one or more of the current mirror transistors 252 via the calibration switches 254, which would sum, to obtain the desired CTAT current 290.
The calibration switches 254 are also coupled the SYNCH FAST/SLOW 404 signal via the CTAT calibration select decode 256. In the fast mode, the calibration switches 254 are configured to trim ICTAT for a fast clock. In the slow mode, the calibration switches 254 may require a different configuration to trim ICTAT for a slow clock. Thus, the calibrations switches 254 toggle between a fast mode calibration and a slow mode calibration in response to the state of the SYNCH FAST/SLOW 404 signal.
In the preferred embodiment, the current mirror 250 acts as a current divider which is well known to those skilled in the art. In other embodiments the current mirror 250 may be configured as a current multiplier. The CTAT current 290 is the sum of the selected outputs from the current mirror transistors 252. Referring to figure 5, wherein like numerals reflect like elements, the PTAT current generator 300, known to those skilled in the art as a ΔVBE circuit, is comprised of PTAT bias generator 310 and a PTAT current mirror 350 for producing a PTAT current 390. The PTAT bias generator 310 is comprised of an amplifier circuit 320, a first bias circuit 330 for producing a first bias voltage across a selectable resistor with a small linear temperature coefficient 332, 333 and 334 and a second bias circuit 340 for producing a second bias voltage. The first and second bias voltages provide the inputs to the amplifier 320. The output of the amplifier 320 is PB1AS 325 which is coupled to the first 330 and second 340 bias circuits, the PTAT current mirror 350 and the bandgap reference voltage generator 150 (figure 1) .
The different resistors 332, 333 and 334, which vary in impedance, are for controlling the current sent to the current mirror 350 and thus, determine the specific stable clock frequency independent of temperature which is produced by the oscillation generator 100.
Similar to the CTAT bias generator 210, the PTAT bias generator 310 provides for selection logic 330 which inputs SYNCH FAST/SLOW 404 and the resistor select (RSELECτ 236) for selecting one of the three resistors 332, 333 or 334. If the slow mode is activated by SYNCH FAST/SLOW 404, then the internal resistor RINT/LP 333 is selected. If the fast mode is activated by SYNCH FAST/SLOW 404, then the selection logic 330 considers the input RSELECT 336 in choosing as between the internal resistor RINT 332 and the external resistor R^ 334.
In the preferred embodiment, the fast mode internal resistor 332 is fabricated from polysilicon technology which provides for low impedance and thus , higher current which in turn provides for a faster clock. In addition, polysilicon technology has a low temperature coefficient (ppm/deg C) , thus providing improved frequency stability over temperature.
By contrast, the slow mode internal resistor 333 is preferably fabricated from diffused technology, e.g. Lightly Doped Drain (LDD) . Diffused technology produces a high impedance which in turn reduces the current to the current mirror 350, and thus allows for operating at low power. The respective resistor pairs in the CTAT and PTAT bias generators 210 and 310 are matched to each other for optimum suability,' e.g. both resistors (REXT 234 and 334 or RINT 232 and 332) are polysilicon in the fast mode and both resistors (R^T LP 233 and 333) are doped silicon in the slow mode.
The slow mode internal resistor 333 could be fabricated using polysilicon technology. However, the resistance per unit area of polysilicon is significantly lower than the resistance per unit area of diffused technology. Thus, a polysilicon resistor would require significantly greater semiconductor area than, a doped silicon resistor for a similar resistance.
The PTAT current mirror 350 is comprised of a plurality of transistors 352 from one to n. Trimming is performed digitally by programming the selection or enablement of one or more of the current mirror transistors 352 via the calibration switches 354 to obtain the desired PTAT current 390.
The calibration switches 354 are also coupled to the SYNCH FAST/SLOW 404 signal via the PTAT calibration select decode 356. In the fast mode, the calibration switches 354 are configured to trim IPTAT for a fast clock. In the slow mode, the calibration switches 354 may require a different configuration to trim IPTAT for a slow clock. Thus, the calibrations switches 354 toggle between a fast mode calibration and a slow mode calibration in response to the state of the SYNCH FAST/SLOW 404 signal.
In the preferred embodiment, the current mirror 350 acts as a current divider which is well known to those skilled in the art. In other embodiments the current mirror 350 may be configured as a current multiplier. The PTAT current 390 is the sum of the selected outputs from the current mirror transistors 352.
Referring to figure 6, wherein like numerals reflect like elements, a general timing diagram (i.e. not mode dependent) for the relaxation oscillator 1 is shown. VI 112 reflects the charging and discharging of capacitor 110 (figure 1) . Note that the positive slope (charging) of VI 112 is equal to Iccc 190 divided by the capacitance of capacitor 110. The maximum amplitude of VI 112 is equal to the reference voltage 152. CMPl reflects the output of the comparator 182 which is coupled to the set input 162 of the flip-flop 160.
V2122 reflects the charging and discharging of capacitor 120. In this case the positive slope of V2 122 is equal to Iccc 190 divided by the capacitance of capacitor 120. CMP2 reflects the output of the comparator 184 which is coupled to the reset input 164 of the flip-flop 160. CLK is the Q output 166 of the flip-flop 160.
For a 50 percent duty cycle, the values of- capacitors 110 & 120 are identical which result in similar slopes for VI 112 and V2 122. As the capacitor voltage exceeds the reference voltage 152, the respective comparator 182 & 184 pulses low which causes the flip-flop 160 to change state. RST (reset) is used to initialize the comparators 182 & 184 and the flip-flop 160 to a known state.
Referring to' Figure 7, wherein like numerals reflect like elements, a timing diagram which illustrates the transition from slow mode to fast mode of the embodiment of figure 3 is shown. The relaxation oscillator.1 would operate similarly when transitioning from fast mode to slow mode. In the timing diagram, the relaxation oscillator 1 (Figure 3) is first operating in the slow mode. The ASYNCH FAST/SLOW signal, which is generated external to the present invention, is received by the transition detector 400. In the preferred embodiment, logic level zero indicates slow mode and logic level one indicates fast mode. The signal to transition to sleep mode is a separate, active high signal.
Upon receiving the ASYNCH FAST/SLOW signal, that meets the required setup time in relation to the internal clock INTCLK 166, to transition to the fast mode, the transition detector 400 generates two outputs. The transition detector 400 outputs a reset pulse RSTCLK 402 to the clock inhibiter 500. The transition detector 400 also synchronizes the ASYNCH FAST/SLOW signal and at the ■ trailing edge of RSTCLK 402, the transition detector 400 outputs SYNCH FAST/SLOW 404 to the current generators 200 and 300. At this point the current generators 200 and 300 begin the switching to generate the currents required for the fast mode. As the current generators 200 and 300 begin their internal switching, several clock cycles are required to allow the bias currents to settle and for INTCLK 166 to stabilize.
Upon receiving RSTCLK 402, the clock inhibiter 500 immediately begins inhibiting CLKOUT 502. At the trailing edge of the RSTCLK 504 pulse, the relaxation oscillator 1 begins to transition from slow to fast mode. In one embodiment, the clock inhibiter counts and inhibits eight fast clock cycles of the INTCLK 166 for stabilization, before releasing the inhibit and allowing CLKOUT 504 to proceed with the fast mode clock.
The present invention minimizes clock frequency drift due to fabrication process, supply voltage and temperature variances. This is accomplished by providing offsetting bias currents which when summed are independent of temperature variation, trimming via the programmable current mirrors 250 & 350 to eliminate process variations, using a stable voltage reference such as a bandgap reference voltage circuit 150 and a dual capacitor, dual comparator oscillation generator 100. Also, analog design techniques, well known to those skilled in the art, such as component matching and cascode current sources enhance the stability of the circuit.
Although the invention has been ■ particularly shown and described with reference to a preferred embodiment thereof, it will be understood by those skilled in the art that changes in form and detail may be made therein without departing from the spirit and scope of the invention.

Claims

WHAT IS CLAIMED IS:
" -." A precision relaxation oscillator with temperature compensation circuit comprising: an oscillation generator; a first current generator coupled to the oscillation generator; a second current generator coupled to the oscillation generator; a clock inhibiter coupled to the oscillation generator; and a transition detector coupled to the clock inhibiter; wherein the circuit is implemented on a single, integrated circuit .
^^J. The circuit in accordance with claim 1 wherein the circuit may produce a clock output and has a plurality of operating modes comprising: a first operating mode; and a second operating mode wherein the clock output of the second operating mode is of a slower frequency then the clock output of the first operating mode.
3. The circuit in accordance with Claim 2 wherein: the first operating mode produces a capacitor charging current ; and the second operating mode produces a second capacitor charging current such that the second capacitor charging current is less than the first capacitor charging current.
4. The circuit in accordance with Claim 2 wherein; the first operating mode has a nominal power consumption; and the second operating mode has a second nominal power consumption such that the second nominal power consumption is less than the first nominal power consumption.
5. The circuit in accordance with Claim 2 wherein the circuit has a third operating mode wherein the circuit does not produce a clock output .
6. The circuit in accordance with Claim 5 wherein the third operating mode does not produce a capacitor charging current.
7. The circuit in accordance with Claim 5 wherein the third operating mode that has a nominal power consumption of approximately zero .
8. The circuit in accordance with Claim 2 wherein the first operating mode is determined by the selection of a resistor which is internal to the first current generator and the selection of a resistor which is internal to the second current generator.
9. The circuit in accordance with Claim 8 wherein the first operating mode is further determined by programming a plurality of calibration switches which are internal to the first current generator and by programming a plurality of calibration switches which are internal to the second current generator.
10. The circuit in accordance with Claim 2 wherein the second operating mode is determined by the selection of a second resistor which is internal to the first current generator and the selection of a second resistor which is internal to the second current generator .
11. The circuit in accordance with Claim 10 wherein the second operating mode is further determined by programming the plurality of calibration switches which are internal to the first current generator and by programming the plurality of calibration switches which are internal to the second current generator.
lή . The circuit in accordance with Claim 1 wherein the clock
/ inhibiter is comprised of a counter. 1-3. The circuit in accordance with Claim 12 wherein the clock inhibiter receives a transition signal from the transition detector and upon receiving the transition signal blocks a clock output of the oscillation generator for a predetermined number of clock cycles.
14. The circuit m accordance with Claim 13 wherein the clock inhibiter permits transmission of the clock .output after the predetermined number of clock cycles has. expired.
15. The circuit in accordance with Claim 13 wherein the predetermined number of clock cycles is programmable.
1.6. A precision relaxation oscillator with temperature compensation circuit wherein the circuit produces a clock output and has a plurality of operating modes comprising: a first operating mode; and a second operating mode wherein the clock output of the second operating mode is of a slower frequency then the clock output of the first operating mode.
17. The circuit in accordance with Claim 16 wherein: the first operating mode produces a capacitor charging current; and the second operating mode produces a second capacitor charging current such that the second capacitor charging current is less than the first capacitor charging current.
18. The circuit in accordance with Claim 16 wherein; the first operating mode has a nominal power consumption; and the second operating mode has a second nominal power consumption such that the second nominal power consumption is less than the first nominal power consumption.
19. The circuit in accordance with Claim 16 wherein the circuit has a third operating "mode wherein the circuit does not produce a clock output .
20. The circuit in accordance with Claim 19 wherein the third operating mode does not produce a capacitor charging current.
21. The circuit in accordance with Claim 19 wherein the third operating mode that has a nominal power consumption of approximately zero .
EP99963026A 1998-12-04 1999-12-06 A precision relaxation oscillator with temperature compensation and various operating modes Withdrawn EP1053596A1 (en)

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US205578 1980-11-10
US09/205,758 US6052035A (en) 1998-03-19 1998-12-04 Oscillator with clock output inhibition control
PCT/US1999/028910 WO2000036745A1 (en) 1998-03-19 1999-12-06 A precision relaxation oscillator with temperature compensation and various operating modes

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