TWI439053B - Spread spectrum clock system and spread spectrum clock generator - Google Patents

Description

Spread spectrum clock system and its spread spectrum clock generator

The present invention relates to a spread spectrum clock generator, and more particularly to an adjustable frequency spread spectrum clock generator.

In high-speed electronic systems, such as wireless telephone systems, fiber-optic network systems, microcomputer systems, and high-end system-on-chip (SOC), operating speeds have reached the level of one billion hertz (GHZ). Therefore, electronic devices that work with these systems also need to increase their operating frequency to keep up with these high-speed systems. In addition, since many circuits are integrated on the same wafer, the clock signal is spread throughout the wafer, causing clock skew to occur in the clock signal.

For example, when the input clock signal drives a chip, there is a variation and a fixed delay time between the input clock signal and the internal clock signal. This unfixed delay time may cause the wafer to work abnormally. In order to synchronize the clock signals inside the system, many high-speed circuit systems have adopted a phase locked loop (PLL) and a delay-locked loop to eliminate clock signal deviation.

As the slew rate increases, the electromagnetic interference that affects or destroys the performance of peripheral electronic components is also exacerbated, causing these peripheral electronic components to be unusable. Since the clock signal is the main cause of the electromagnetic interference effect, how to reduce the electromagnetic interference effect caused by the clock signal is a top priority.

Recently, the Spread Spectrum Clock Generator has been widely used to reduce electromagnetic interference effects. As the name suggests, "spreading frequency" means that the center frequency of the clock signal is slightly spread, and the energy is dispersed to other frequencies to reduce the energy at the main frequency. Since the energy at the main frequency is reduced, the electromagnetic interference effect can be reduced.

In other words, "spreading frequency" is to make a slight change in the frequency of the clock signal, thereby reducing the electromagnetic interference effect. Even though the spread spectrum method can reduce the electromagnetic interference effect, the jitter of the clock signal (Jitter) will deteriorate, and the user must make trade-offs and compromises between reducing the electromagnetic interference effect and reducing the clock jitter phenomenon.

Unfortunately, the traditional spread-spectrum clock generator only expands the frequency of the clock signal in a multiplication manner, and cannot continuously and elastically adjust the spread frequency of the clock signal, thereby failing to reduce the electromagnetic interference effect and reduce the clock. Making trade-offs between jitters leads to a decline in overall performance.

Therefore, a new spread-spectrum clock generator is needed to more flexibly adjust the average frequency of the clock signal.

Accordingly, one aspect of the present invention is to provide a spread spectrum clock generator that continuously and elastically adjusts the frequency of a clock signal.

According to an embodiment of the invention, the spread spectrum clock generator includes a triangular wave generator, a digital waveform modulator, a triangular modulator, and a selector. The triangular wave generator converts one of the plurality of input clock signals into an original triangular wave signal, wherein the input clock signals have the same frequency and different phases from each other. The digital waveform modulator adjusts the waveform of the original triangular wave signal according to an input control signal to generate a modulated triangular wave signal and a first square wave signal corresponding to one of the modulated triangular wave signals. The triangular modulator is electrically connected to the digital waveform modulator, and the triangular wave modulator is used to accumulate the value of the modulated triangular wave signal to generate a second square wave signal. The selector selects one of the input clock signals as an output clock signal according to the voltage level of the second square wave signal and the first square wave signal, wherein the average frequency of one of the output clock signals and the input clock signal The frequency is different.

Accordingly, one aspect of the present invention is to provide a spread spectrum clock system that continuously and elastically adjusts the frequency of a clock signal.

According to another embodiment of the present invention, the spread spectrum clock system includes an oscillator, a phase locked loop, a triangular wave generator, a digital waveform modulator, a triangular modulator, and a selector. The oscillator generates a periodic signal, and the phase locked loop modulates the frequency of the periodic signal to generate a plurality of input clock signals, wherein the input clock signals have the same frequency and different phases from each other. The triangular wave generator converts one of the plurality of input clock signals into an original triangular wave signal. The digital waveform modulator adjusts the waveform of the original triangular wave signal according to an input control signal to generate a modulated triangular wave signal and a first square wave signal corresponding to one of the modulated triangular wave signals. The triangular modulator is electrically connected to the digital waveform modulator, and the triangular wave modulator is configured to accumulate the value of the modulated triangular wave signal to generate a second square wave signal. The selector selects one of the input clock signals as an output clock signal according to the voltage level of the second square wave signal and the first square wave signal, wherein the average frequency of one of the output clock signals and the input clock signal The frequency is different.

The spread spectrum clock generator and the spread spectrum clock system of the above embodiments can flexibly adjust the frequency of the clock signal to properly reduce the electromagnetic interference effect and the clock jitter phenomenon.

Referring to FIG. 1 and FIG. 2A to FIG. 2D, FIG. 2 is a block diagram and a signal waveform diagram of a spread spectrum clock system according to an embodiment of the present invention, wherein FIG. 2A shows an input clock signal 201d, 201e, 201f, and 2D are diagrams showing the time-frequency relationship of the output clock signals 207a, 207b, and 207c.

The spread spectrum clock system 115 includes an oscillator 113, a phase locked loop 111 (Phase locked loop), and a spread spectrum clock generator 101. The oscillator 113 generates a periodic signal S1. The phase-locked loop 111 modulates the frequency/phase of the periodic signal S1 to generate a plurality of input clock signals S5 (that is, the input clock signals 201d, 201e, and 201f shown in FIG. 2A), wherein the input clock signals S5 Have the same frequency and different phases from each other.

The spread spectrum clock generator 101 performs a selected action to select one of the input clock signals S5 to generate an output clock signal S7 (that is, an output clock signal 207a, 207b or 207c in the 2D map), these inputs. The clock signal S5 has the same frequency and a phase different from each other, and the instantaneous frequency of the output clock signal S7 changes with time. For example, at different points in time, the instantaneous frequency of the output clock signal S7 can be 210 MHz, 211 MHz, and 209 MHz, respectively, as depicted by 207b in Figure 2D.

The spread spectrum clock generator 101 includes a triangular wave generator 103, a digital waveform modulator 105, a Sigma delta modulator 107, and a selector 109. 2B is a waveform diagram of the original triangular wave signal 203a and the two modulated triangular wave signals 203b, 203c, and FIG. 2C is a schematic diagram showing square wave signals 205a, 205b and 205c having various duty cycles (Duty cycle). Corresponding to the original triangular wave signal 203a and the two modulated triangular wave signals 203b, 203c, respectively. The triangular wave generator 103 converts one of the plurality of input clock signals S5 into the original triangular wave signal S3 (that is, 203a of FIG. 2B).

The digital waveform modulator 105 adjusts the waveform of the original triangular wave signal S3 (that is, 203a of FIG. 2B) according to the input control signal CS adjusted by the user to generate the modulated triangular wave signal S4, corresponding to the first of the modulated triangular wave signals. Square wave signal S2 (that is, 205b or 205c of Figure 2C). If the currently selected input clock signal needs to be changed, the selector 109 can select the specific phase (ie, the leading phase or the lagging phase) by using the first square wave signal S2 as an index. Enter the clock signal S5. For example, the user may first adjust the input control signal CS according to the average frequency and the spread frequency of the current output clock signal, and then adjust the original triangular wave signal according to the adjusted input control signal CS by the digital waveform modulator 105. The frequency and/or the amplitude of S3 (that is, 203a of FIG. 2B) generates a modulated triangular wave signal S4 (203c in FIG. 2B) and a first square wave signal S2 (205c in FIG. 2C).

The triangular modulator 107 is electrically connected to the digital waveform modulator 105. The triangular modulator 107 accumulates the value of the modulated triangular wave signal S4 to generate a second square wave signal S6 having a value of logic 1 or logic 0. The selector 109 can use the second square wave signal S6 as an index to determine whether the current input clock signal needs to be modulated. In this embodiment, the feedback output clock signal S7 triggers the triangular modulator 107 to generate the second square wave signal S6.

The selector 109 selects one of the input clock signals S5 as the output clock signal S7 according to the received second square wave signal S6 and the voltage level of the first square wave signal S2.

Table 1 below shows the operation of the selector 109, and displays the first square wave signal S2, the second square wave signal S6 (output by the triangular modulator 107), and the selected input clock signal S5 between the three signals. Relationship. According to Table 1, the selector 109 selects one of the input clock signals S5 according to the first square wave signal S2 and the second square wave signal S6. In more detail, whether the currently selected input clock signal S5 needs to be modulated may be determined according to the second square wave signal S6. If the second square wave signal S6 indicates that the currently selected input clock signal S5 needs to be modulated (that is, state 2 and state 4 of Table 1), then the first square wave signal S2 can be used to decide to select the previous one. The clock signal 201d (backward phase) or the subsequent input clock signal 201f (leading phase) is input as the subsequent output clock signal S7.

For example, if the currently selected input clock signal is 201e and the second square wave signal S6 is logic 1, the selector 109 reselects the input clock signal according to the logic level of the first square wave signal S2. One of S5, in place of the currently selected input clock signal 201e. In this case, if the first square wave signal S2 is logic 0, the input clock signal 201d is selected, and if the first square wave signal S2 is logic 1, the input clock signal 201f is selected. In addition, when the second square wave signal S6 is logic 0 (that is, states 1 and 3 in Table 1), the selector 109 maintains the currently selected input clock signal 201e as the output clock signal S7.

If the selected input clock signal changes, the period of the output clock signal S7 (Period/cycle) is changed (that is, the frequency of the output clock signal S7 changes). The average frequency of the output clock signal S7 may be the same or different from the frequency of the input clock signal S5, the frequency is the same or the difference is determined by the input control signal CS. For example, if the frequency of the input clock signal S5 is 200 MHz, the input control signal CS can be used to adjust the average frequency of the output clock signal to 210 MHz (that is, the original triangular wave signal 203a in FIG. 2B is adjusted to Modulated triangular wave signal 203b). In this example, the frequency of the output clock signal S7 is not a multiple of the frequency of the input clock signal S5. In other examples, if the input control signal CS keeps the original triangular wave signal 203a unchanged, the average frequency of the output clock signal S7 will be equal to the frequency of the input clock signal S5; in addition, if the triangular modulator 107 has not been fed back The subsequent output clock signal S7 is triggered, and the average frequency of the output clock signal S7 is equal to the frequency of the input clock signal S5. Therefore, the digital waveform modulator 105 can elastically adjust the spread amount of the output clock signal S7.

As a result, as shown in Fig. 2D, the average frequency of the frequency/time curve 207b of the output clock signal is 210 MHz, and the average frequency of the frequency/time curve 207c (S7) is 190 MHz, both of which are input clocks. There is a slight difference in the frequency of the signal at 200MHz.

According to the above embodiment, the average frequency of the output clock signal can be the same as the frequency of the input clock signal, or can be slightly different from the frequency of the input clock signal instead of the multiple of the input clock signal frequency, so that the output can be expanded more flexibly. The frequency of the clock signal.

The present invention has been disclosed in the above embodiments, and is not intended to limit the present invention. Any one of ordinary skill in the art to which the present invention pertains may make various changes without departing from the spirit and scope of the invention. And the scope of the present invention is defined by the scope of the appended claims.

101. . . Spread spectrum clock generator

103. . . Triangle wave generator

105. . . Digital waveform modulator

107. . . Triangular modulator

109. . . Selector

111. . . Phase-locked loop

113. . . Oscillator

115. . . Spread spectrum clock system

201d. . . Input clock signal

201e. . . Input clock signal

201f. . . Input clock signal

203a. . . Original triangular wave signal

203b. . . Modulated triangular wave signal

203c. . . Modulated triangular wave signal

205a. . . Square wave signal

205b. . . Square wave signal

205c. . . Square wave signal

207a. . . Frequency/time curve

207b. . . Frequency/time curve

207c. . . Frequency/time curve

S1. . . Periodic signal

S2. . . First square wave signal

S3. . . Original triangular wave signal

S4. . . Modulated triangular wave signal

S5. . . Input clock signal

S6. . . Second square wave signal

S7. . . Output clock signal

The above and other objects, features, advantages and embodiments of the present invention will become more apparent and understood.

1 is a block diagram showing a spread spectrum clock system according to an embodiment of the present invention.

2A to 2D are diagrams showing signal waveforms of a spread spectrum clock system according to an embodiment of the present invention.

101. . . Spread spectrum clock generator

103. . . Triangle wave generator

105. . . Digital waveform modulator

107. . . Triangular modulator

109. . . Selector

111. . . Phase-locked loop

113. . . Oscillator

115. . . Spread spectrum clock system

S1. . . Periodic signal

S2. . . First square wave signal

S3. . . Original triangular wave signal

S4. . . Modulated triangular wave signal

S5. . . Input clock signal

S6. . . Second square wave signal

S7. . . Output clock signal

Claims (10)

A spread spectrum clock generator includes: a triangular wave generator for converting one of a plurality of input clock signals into an original triangular wave signal, wherein the input clock signals have the same frequency and are different from each other a digital waveform modulator for adjusting a waveform of the original triangular wave signal according to an input control signal to generate a modulated triangular wave signal and a first square wave signal corresponding to the modulated triangular wave signal; Electrically connected to the digital waveform modulator for accumulating the value of the modulated triangular wave signal to generate a second square wave signal; and a selector for relying on the second square wave Selecting one of the input clock signals as an output clock signal, wherein the signal and the voltage level of the first square wave signal are an output clock signal, wherein an average frequency of the output clock signal is proportional to a frequency of the input clock signal different. The spread spectrum clock generator of claim 1, wherein the digital waveform modulator further adjusts a frequency of the original triangular wave signal to generate the modulated triangular wave signal. The spread spectrum clock generator of claim 1, wherein the digital waveform modulator further adjusts an amplitude of the original triangular wave signal to generate the modulated triangular wave signal. The spread spectrum clock generator of claim 1, wherein the output clock signal is fed back to the triangular modulator to trigger the triangular modulator to generate the second square wave signal. The spread spectrum clock generator of claim 4, wherein an average frequency of the output clock signal is equal to a frequency of the input clock signal before the triangular modulator has been triggered. A spread spectrum clock system comprising: an oscillator to generate a periodic signal; a phase locked loop to modulate a frequency of the periodic signal to generate a plurality of input clock signals, wherein the input clock signals have a same frequency and a different phase; a triangular wave generator for converting one of the input clock signals into an original triangular wave signal; a digital waveform modulator for adjusting the original triangular wave according to an input control signal a waveform of the signal to generate a modulated triangular wave signal and a first square wave signal corresponding to one of the modulated triangular wave signals; a triangular modulator electrically connected to the digital waveform modulator, the triangular wave modulator system Generating a second square wave signal by accumulating the value of the modulated triangular wave signal; and selecting a selector to select the input clock according to the voltage level of the second square wave signal and the first square wave signal One of the signals acts as an output clock signal, wherein an average frequency of one of the output clock signals is different from the frequency of the input clock signal. The spread spectrum clock system of claim 6, wherein the digital waveform modulator further adjusts a frequency of the original triangular wave signal to generate the modulated triangular wave signal. The spread spectrum clock system of claim 6, wherein the digital waveform modulator further adjusts an amplitude of the original triangular wave signal to generate the modulated triangular wave signal. The spread spectrum clock system of claim 6, wherein the output clock signal is fed back to the triangular modulator to trigger the triangular modulator to generate the second square wave signal. The spread spectrum clock system of claim 9, wherein the average frequency of the output clock signal is equal to the frequency of the input clock signal before the triangular modulator has been triggered.