Generating a clock signal

[0002] This invention relates to generating a clock signal.

[0003] Clocking circuitry for electronic devices, such as a slave RAC (RAMBUS Application-Specific Integrated Circuit Cell), generates clock signals based on a master clock. Typically, the master clock is a strobe clock, such as a CTM or Clock To Master, for sampling data received from a transmission line. The strobe clock runs about 90 degrees out of phase with the leading edge of the data so that the center of each bit is sampled. By contrast, the data is clocked to/from an electronic device at roughly one “tco” earlier than the leading edge of a bit, where “tco” corresponds to the delay between a data clock and valid output data, which is usually an output buffer delay. The data clock is therefore 90 degrees out of phase with the master clock. Clocking circuitry in the device is used to preserve this phase relationship between the master clock and the data clock.

[0004] In general, in one aspect, the invention relates to generating a clock signal. This aspect of the invention features generating a first clock signal based on a first set of reference clocks, selecting a second set of reference clocks based on the first clock signal and a predetermined delay, and generating a second clock signal, based on the second set of reference clocks, which substantially compensates for the predetermined delay.

[0005] Among the advantages of this aspect of the invention may be one or more of the following. Generating the second clock signal based on the first clock signal makes it possible to obtain a proper phase relationship between the second clock signal and a master clock signal. Moreover, taking into account a predetermined delay in the second clock signal reduces phase discrepancies brought about, e.g., by hardware such as input/output (I/O) buffers and package trace delays. A digital control circuit may be used to introduce the predetermined delay. Use of a digital control circuit reduces phase errors that may be introduced from less reliable circuitry.

[0006] Other features and advantages of the invention will become apparent from the following description, drawings, and claims.

[0007] FIG. 1 is a view of clocking circuitry for a slave RAC.

[0008] FIG. 2 is a timing diagram which depicts sampling data in accordance with a master clock.

[0009] FIG. 3 is a timing diagram which depicts outputting data in accordance with an output clock.

[0010] FIG. 4 is a block diagram of the clocking circuitry.

[0011] FIG. 5 is a flowchart which shows the operation of the clocking circuitry.

[0012] FIG. 6 is a phase diagram showing reference clocks and output data clocks.

[0013] FIG. 1 shows a slave RAC 10 which includes clocking circuitry 11. Clocking circuitry 11 receives a master clock 19 (CTM) from line 12 and outputs data 17 from slave RAC 10 onto data bus 14. A clocking signal for outputting data 17 is determined in accordance with master clock 19.

[0014] In this embodiment, master clock 19 is a strobe clock that is used to sample data from a transmission line or the like. Data is sampled at the center of each bit, making the master clock roughly 90 degrees out of phase with the leading edge of each bit. For example, as shown in FIG. 2, data 15 is sampled from a transmission line at bits 15a, 15band 15c. As shown, the rising edges 16a, 16band 16cof master clock 19 coincide with the centers of bits 15a, 15band 15cin order to reduce data sampling errors. Alternatively, the bus scheme could be a “double pump” bus scheme, where data is sampled on both the rising and falling edges of master clock 19.

[0015] Referring back to FIG. 1, data 17 is transmitted between slave RAC 10 and data bus 14 in accordance with master clock 19. Master clock 19 is provided to clocking circuitry 11 which determines an output clocking signal 20, based on master clock 19, for transmitting data 17 to data bus 14. The phase of clocking signal 20 is shifted relative to master clock 19 such that the triggering edges of clocking signal 20 are ahead of corresponding leading edges of data 17 by the sum of delays of package delays 21 and 22 and I/O buffer 24 (see FIG. 3).

[0016] Package delay lines 21 and 22 are interposed between clocking circuitry 11, line 12, and bus 14 to introduce delays in the master clock for accessing data stored in slave RAC 10 and adjusting the phase of the output data. An I/O buffer 24 is also provided for buffering the output data. Clocking circuitry 11 determines the phase of the output clocking signal based, in addition, on the delays associated with I/O buffer 24 and package delay lines 21 and 22.

[0017] FIG. 4 shows a block diagram of circuitry 11 for determining output clocking signal 20. This circuitry may be implemented using hardware components, such as multipliers, adders, and various logic gates. Circuitry 11 may also be implemented using programmable logic, such as a field programmable gate array (FPGA) or other device.

[0018] Circuitry 11 includes phase mixers 25 and 26, digital control circuit 27, mixer controller 29, small-swing CMOS (Complementary Metal Oxide Semiconductor) converters 30 and 31, duty cycle correction circuits 32 and 34, quadrature phase detection (QPD) circuit 35, and feedback buffer 36. Phase mixer 26, small-swing CMOS converter 31, duty cycle correction circuit 34, QPD circuit 35, buffer 36, and mixer controller 29 make up a delay-locked loop (DLL) 37. The operation of circuitry 11 is described in the flowchart of FIG. 5.

[0019] In FIG. 5, a first clock signal 39 is generated (501) based on reference clocks 40. Clock signal 39 passes through buffer 36, where a delay is added to produce clock signal 42. Reference clocks 40 are provided by an electronic device, such as an oscillator (not shown). In this embodiment, eight reference clocks are provided which differ in phase from one another by 45 degrees (or ⅛ of a 360 degree cycle) (see FIG. 6). However, any number of reference clocks may be provided, and the phase angle between those reference clocks may differ by an amount other than 45 degrees.

[0020] Delayed master clock 99 (FIG. 1) is input to QPD circuit 35, which outputs a signal 41 indicating the phase relationship of delayed master clock 99 to clock signal 42. Signal 41 indicates whether delayed master clock 99 is leading or lagging clock signal 42. Based on this indication, mixer controller 29 instructs phase mixer 26 to select two reference clocks. Phase mixer 26 then generates a new clock signal 39 based on the selected reference clocks.

[0021] More specifically, phase mixer 26 selects (501a) two adjacent reference clock signals that are 45 degrees apart in phase (see FIG. 6). In this embodiment, the 45 degree phase difference corresponds to a 312 picosecond delay. These reference clock signals are selected so that delayed master clock 99 and clocking signal 42 differ in phase by 90 degrees. Phase mixer 26 generates (501b) clocking signal 39 by interpolating between clocks 46aand 46b(FIG. 6) based on information provided from mixer controller 29. Clocking signal 39 could be at any point between clocks 46aand 46b. This is done by adjusting the phase of one of the selected reference clocks towards the phase of the other selected reference clock. Phase mixer 26 may thus generate a clock signal 39 that is roughly half-way between the two reference clocks 46aand 46bas shown, for example, in FIG. 6. The resulting clocking signal 39 passes through small-swing CMOS converter 31. Small-swing CMOS converter 31 processes clocking signal 39 so that it ranges between a predetermined voltage value (e.g., 5 volts) and ground. Small-swing CMOS converter 31 is also capable of adjusting the duty cycle of its input signals so that its output signal has a different duty cycle as compared to its inputs.

[0022] Clocking signal 39 passes through inner loop 49, which includes duty cycle correction circuit 34. Duty cycle correction circuit 34 detects duty cycle errors in clocking signal 39 and provides an error message to small-swing CMOS converter 31 instructing converter 31 to correct the duty cycle of clocking signal 39. Clocking signal 39 also passes through buffer 36. Buffer 36 adds (501c) a delay to clocking signal 39 and provides the resulting clocking signal 42 to QPD circuit 35. The delay added by buffer 36 corresponds to the delay added by I/O buffer 24 (FIG. 1).

[0023] QPD circuit 35 then determines (501d) the difference (if any) in phase between delayed master clock 19 and clocking signal 42. An indication of the difference is output to mixer controller 29. If the phase difference between delayed master clock 99 and clocking signal 42 is 90 degrees, the process proceeds from 501eto 502; otherwise, the process returns to 501awhere phase mixer 26 selects new reference clocks and 501ato 501eare repeated. Instead of phase mixer 26 selecting new reference clocks in 501a, mixer controller 29 may simply instruct phase mixer 26 to generate a new clocking signal based on the previously-selected reference clocks.

[0024] In 502, mixer controller 29 passes its reference clock information to digital controller 27. Digital controller 27 uses this information to select a second set of reference clocks and to interpolate a clock signal 51 such that it is offset by a predetermined amount from clock 39. In this embodiment, digital controller 27 instructs phase mixer 25 to select a second set of reference clocks 46aand 46cthat leads the first set of reference clocks 46aand 46bby 45 degrees (see FIG. 6). As shown, a single clock 46amay be common to both sets of reference clocks. Leading reference clocks are selected to compensate for delays introduced by package delay circuits 21 and 22 (FIG. 1). More specifically, to compensate for the package delay of package delay lines 21 and 22, the second set of reference clocks are used by phase mixer 25 to generate clock 51 which is earlier in phase than clocking signal 39 by the sum of the delays of package delay lines 21 and 22.

[0025] Digital controller 27 may be constructed of programmable registers, adders, and other circuitry on die or from discrete hardware components. In either case, control signals 50 are provided to digital controller 27 (from, e.g., an external controller—not shown). These control signals combine with inputs 29a(FIG. 4) from mixer controller 29 to implement a predetermined delay through mixer controller 25. Thus, different delays may be added via digital controller 27, depending upon the content of control signals 50.

[0026] Phase mixer 25 selects (502) the second set of reference clocks 46aand 46cbased on instructions received from digital controller 27. Phase mixer 25 selects reference clocks that can be used to generate an output clock signal 51 having a phase that leads master clock 19 by a sum corresponding to 90 degrees plus the combined delays of package delay lines 21 and 22 and the delay associated with I/O buffer 24. For example, in FIG. 6, reference clocks 46aand 46care selected, which differ in phase by 45 degrees and which also lead reference clocks 46aand 46bby 45 degrees.

[0027] Phase mixer 25 generates (503) the output clock signal (as specified above) based on the selected reference clocks. The output clock signal then passes through small-swing CMOS converter 30 where it is processed so that it ranges between a predetermined voltage (e.g., 5 volts) and ground. Duty cycle correction circuit 32 corrects the duty cycle of output clock signal 51 and passes output clock signal 51 through small-swing CMOS converter 30. Clock signal 51 is then output (504) to clock data from I/O buffer 24 to data bus 14.

[0028] The invention is not limited to the specific configuration or environment set forth above. For example, digital controller 27 may not be required if the combined delay of the package delay circuits 20 and 21 is roughly ⅛ (45 degrees lagging) of a period of master clock 12. Circuitry 11 may be used to provide clocking for devices other than a slave RAC, such as an RDRAM (RAMBUS Dynamic Random Access Memory). In fact, the invention can be used on any source bus synchronous channel.

[0029] The invention can be used to compensate for delays which do not include the delays of package delays 21 and 22. For example, the invention could be used in connection with a “master” device (vs. a slave RAC), where compensation is not required for package delays. For example, the invention could be used to provide programmable delays between clocks 42 and 51/20 (FIG. 4) and/or to compensate for delay skews between I/O buffer 24 and buffer 36.

[0030] Other embodiments not described herein are also within the scope of the following claims.