US20070279117A1

US20070279117A1 - Method and system for high frequency clock signal gating

Info

Publication number: US20070279117A1
Application number: US11/769,408
Authority: US
Inventors: Hayden Cranford; Stacy Garvin; Vernon Norman; Samuel Ray; Wayne Utter
Original assignee: International Business Machines Corp
Current assignee: GlobalFoundries Inc
Priority date: 2005-09-27
Filing date: 2007-06-27
Publication date: 2007-12-06
Also published as: US7279950B2; US20070069793A1

Abstract

A differential clock signal gating method and system is provided, providing a clock gating signal with a timing relationship to a clock signal and a differential pair current to a buffer differential pair load element. Switching the differential pair current from the load element to a buffer differential pair responsive to a gating signal pulse, the gating signal pulse correlated to a first clock signal pulse, the buffer differential pair buffers a second clock signal pulse occurring immediately and sequentially after the first clock signal pulse and successive clock signal pulses as a buffer clock signal output, the output comprising a plurality of pulses each having the clock signal amplitude and the clock signal pulse width.

Description

RELATED APPLICATION

This application is a continuation of application Ser. No. 11/235,758, filed Sep. 27, 2005.

FIELD OF THE INVENTION

The present invention relates to differential clock signal gating in serial link architecture, and more particularly to synchronism across multiple High Speed SerDes (HSS) cores of very high frequency clocks.

BACKGROUND OF THE INVENTION

High-speed data-communications systems typically incorporate multiple robust (gigabit-rate) serializer/deserializer (SerDes) chips that can send and receive parallel data over a serial link. Jitter is one of the most important issues in the design and operation of high-speed serial links. Although SerDes devices implement a purely digital function—serial data communications—they behave in an analog-like fashion, especially in the low-voltage differential signaling used at 10-Gbits/s speeds. And SerDes receivers and transmitters operate asynchronously: receiver and transmitter system clocks must operate within tolerances specified by the communications standard to which they conform, but they are not locked.
Synchronism across multiple High Speed SerDes (HSS) cores, requires clock signal gating finction methods and circuitry. Clock gating in many applications is a necessity, and is typically accomplished by the use of a selector circuit or other switching devices inserted in the clock path, to switch between a “static” differential clock OFF condition and the desired clock. However problems arise through inserting an additional stage in the clock path, which degrades clock path performance and results in lower total system integrity while contributing additional jitter to the clock path.
What is needed is a clock gating method and system for synchronism across multiple High Speed SerDes (HSS) cores of very high frequency clocks that do not contribute to clock path jitter.

SUMMARY OF THE INVENTION

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of a clock gating circuit according to the present invention.
FIG. 2 is a block diagram of a VCO BUFR circuit according to the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT(S)

Referring now to FIG. 1, a Clock Gating Block Diagram according to the present invention is illustrated. HSS core 101 logic develops a C2GATECLK signal 100 with correct timing relationship to a CLOCK (differential) signal 102 from a Voltage Controlled Oscillator (VCO) 104. The C2GATECLK 100 is applied to the data input D 106 of a D-type flip flop 108. The CLOCK 102 input to the D type flip flop 110 clocks the input state of the D terminal to the output (Q) when CLOCK 102 changes from positive (high) to negative (low) and therefore on a negative going transition of the CLOCK 102 clocking signal through inverter 112. When C2GATECLK 100 is made active (high) at step 120 by the HSS core 101 logic during the positive half cycle 122 of CLOCK 102, the next negative transition 124 of the CLOCK 102 signal clocks the D type flip flop ON, developing C2GATECLKQ signal 130 positive at step 132, which provides the gating finction to a VCO BUFR block 140. With C2GATECLKQ 130 active during the negative portion 124 of the CLOCK signal 102 from the VCO 104 and the VCO BUFR block 140 preset to provide a ZERO output, when C2GATECLK 100 becomes active at step 120, the next positive CLOCK pulse 150 and subsequent CLOCK pulses 152 switch the output of the VCO BUFR block 140 and provide the VCO BUFR output 160 with a full amplitude and positive half cycle clock width and subsequent outputs 162 as the CLOCK 102 signal, which is applied to the input of the next buffer stage 170 in the clock path.
The present embodiment provides a clock gating method that does not require the insertion of switching components in the clock path: rather, the function is applied to the control path of a clock buffer circuit and as a result, does not contribute to clock path jitter. The present embodiment provides a method to meet differential clock signal gating requirement of very high frequency clocks (up to 2.5 GHZ), while contributing zero additional jitter to the clock path, allowing for extremely fast turn-on of the buffer 140.
Implementation of the clock gating circuitry involves minimal complexity with only a few additional components for the differential clock buffer control path. FIG. 2 illustrates an embodiment of VCO BUFR block 140 circuitry according to the present invention. Resistors R1 270 and R2 272, paired transistors T1 201 and T2 202, and differential pair current source transistor T4 204 with bias input 264 define a prior art “differential pair structure” within the VCO BUFR block 140: the functions of these elements within the differential pair structure is well known to one skilled in the art, and it is not believed that they need to be described with particularity. VDD 262 is the most positive voltage used by the circuit, and VSS 260 is the circuit ground potential.
With the C2GATECLKQ signal 130 low (OFF), transistors T5 205 and T6 206 are both conducting relative to inputs VDD 262 and VSS 260, respectively, responsive to the inverted C2GATECLKQ 130 input 295 at T5 205 and inverter 296 signal at T6 206, “pre-setting” the differential output of the VCO BUFR block (OUTN 210 and OUTP 212) to a differential ZERO. The low (OFF) C2GATECLKQ signal 130 cuts off transistor T3 203, with the inverted C2GATECLKQ signal 130 causing transistor T7 207 to conduct the differential pair current 220, otherwise sent through transistor T3 203 to the “dummy” load resistance R3 230.
Inputs INP 240 and INN 242 are ignored since the differential transistor pair T1 201 and T2 202 have no differential pair current supplied by T4 204 and cannot affect the outputs. When C2GATECLKQ 130 becomes active during the negative half 124 of the CLOCK cycle 102, as shown in FIG. 1, T5 205 and T6 206 become cutoff and at this time T7 207 becomes cutoff, removing R3 230 as a load from T4 204 and T3 203 simultaneously becoming active (in the same negative half of the CLOCK signal 124), provides the normal differential pair current 250 to T1 201 and T2 202.
T7 207 switches current to the “dummy” load resistor R3 230, allowing the differential pair current source device T4 204 to pass the normal differential pair current with the buffer outputs in a differential OFF condition. Thus when C2GATECLKQ 130 becomes active, the normal nominal value of differential pair current 250 is available immediately to the differential pair T1 201 and T2 202, rather than the relatively slower turn on and ramp up of a differential pair current device as typically provided by prior art clock gating circuitry, which would take a few CLOCK 102 cycles if T4 204 was simply switched ON from an OFF (cutoff) condition, with no current flowing. In contrast, by accomplishing the switching of the various transistor devices T1 201 through T7 207 internal to the VCO BUFR 140 during the negative half 124 of the CLOCK signal 102 and thereby turning the VCO BUFR 140 ON, the present embodiment allows the next positive CLOCK 102 pulse 150 and subsequent CLOCK pulses 152 to be available at the differential outputs OUTP 212 and OUTN 210 of the VCO BUFR 140, without distortion of the first positive CLOCK 102 pulse 150 after gating the VCO BUFR 140 ON and with no additional jitter inserted in the clock path.
Thus, while the C2GATECLKQ signal 130 is low (OFF) current flows through a “dummy path” defined from VDD 262 through R3 230, T7 207, and T4 204 to ground VSS 260. As this current flow is already present at the time that Inputs INN 242 and INP 240 begin processing, output distortion and jitter on the outputs OUTN 210 and OUTP 212 by clock gating control circuitry is avoided. Output OUTN 210 is previously preconditioned by a direct tie into VDD 262 conducted through T5 205 responsive to the inverted C2GATECLKQ low (OFF) signal 130, and OUTP 212 is previously preconditioned by a direct tie into VSS 260 conducted through T6 206 responsive to the inverted C2GATECLKQ low (OFF) signal 130.
When C2GATECLKQ signal 130 goes high (ON), both T5 205 and T6 206 are cut off, T7 207 cuts off the dummy load current through current sink R3 230 and T3 203 conducts making the differential pair current 220 available from T4 204 to the differential pair T1 201 and T2 202, all happening during the negative half of the CLOCK signal 102. As T4 204 is conducting full amplitude current with insufficient time to reduce the current flow through the device, the next CLOCK pulse 102 is buffered through the differential pair T1 201 and T2 202 with full amplitude and full pulse width, without introducing jitter in the buffering process.
Prior art clock gating methods include multiplexer structures which introduce one or more additional stages in the signal flow, and each additional stage presents a possible source of jitter introduction in the signal flow. In contrast, the above embodiment does not introduce an additional stage(s) in series with the signal flow.
In the present embodiment the C2GATECLK signal 100 is provided within a timing relationship to the CLOCK signal 102: more particularly, the C2GATECLK signal 100 is timed to go high (ON) with the positive half of a CLOCK signal 102 pulse, and the C2GATECLKQ signal 130 to go high (ON) during the negative half of the CLOCK signal 102, enabling the next CLOCK signal 102 pulse to be buffered fully through the differential pair T1 201 and T2 202. This timing relationship is established in the HSS logic providing the control signal C2GATECLKQ 130.
While the invention has been described in combination with embodiments thereof, it is evident that many alternatives, modifications, and variations will be apparent to those skilled in the art in light of the foregoing teachings. Accordingly, the invention is intended to embrace all such alternatives, modifications and variations as fall within the spirit and scope of the appended claims.

Claims

1. A method, comprising:

providing a clock signal having a clock signal amplitude and a clock signal pulse width;

providing a gating signal with a timing relationship to the clock signal;

sending a differential pair current to a buffer differential pair load element;

switching the differential pair current from the load element to a buffer differential pair responsive to a gating signal pulse, the gating signal pulse correlated to a first clock signal pulse; and

in response to switching the differential pair current, the buffer differential pair buffering a second clock signal pulse occurring immediately and sequentially after the first clock signal pulse and successive clock signal pulses as a buffer clock signal output, the output comprising a plurality of pulses each having the clock signal amplitude and the clock signal pulse width.

2. The method of claim 2, wherein switching the differential pair current from the load element to the buffer differential pair is responsive to the gating signal during a first clock signal pulse negative half;

further comprising providing normal differential pair current to the buffer differential pair simultaneously with removing the differential pair current from the load element.

3. The method of claim 2, further comprising:

presetting a positive buffer differential output and a negative buffer differential output to a differential zero; and

cutting off the positive buffer differential output and negative buffer differential output from the differential zero simultaneously with the steps of switching the differential pair current from the load element to the buffer differential pair and simultaneously providing normal differential pair current to the buffer differential pair.

4. The method of claim 3 wherein the clock signal has a frequency of about 2.5 GHz.

5. A circuit structure, comprising:

a clock generator;

a data device having a clock input in communication with the clock generator, a data input and a data output;

a gating device in communication with the data device data input and configured to develop a gating signal with a timing relationship to the clock signal;

a buffer in circuit communication with the data device data output and a clock generator differential clock signal; and

the buffer comprising a differential pair and a current load device;

wherein the data device is configured to provide a gating signal data output to the buffer responsive to a first clock signal pulse; and

wherein, responsive to the first clock signal pulse, the buffer is configured to switch differential pair source current from the current load device to the buffer differential pair and buffer a second immediately subsequent clock signal pulse as a buffered clock signal output, the buffered clock signal output having the clock amplitude and the clock pulse width.

6. The circuit structure of claim 5, wherein the data device is a flip flop having a clock input in inverted circuit communication with the clock generator; and

wherein the gating device is a High Speed SerDes (HSS) core comprising logic.

7. The circuit structure of claim 6 wherein the buffer comprises a first transistor configured to conduct a highest circuit positive voltage input to a subsequent buffer stage negative output responsive to an inverted gating signal input, and a second transistor configured to conduct a circuit ground potential to a subsequent buffer stage positive output responsive to the inverted gating signal input, the buffer pre-set to provide a zero output; and

a subsequent buffer stage clock input in circuit communication with the buffer block stage outputs, a buffer block subsequent buffer stage output pre-set to a differential zero.

8. The circuit structure of claim 7, the buffer further comprising:

a third transistor configured to cut off source current to the first transistor and the second transistor responsive to the inverted gating signal input; and

a fourth transistor configured to conduct source current to a current load device responsive to the inverted clock gating signal input.

9. The circuit structure of claim 8 wherein the clock signal has a frequency of about 2.5 GHz.