Method and apparatus for equalizing propagation delay

[0002] 1. Field of the Invention

[0003] The present invention relates to circuits used for equalizing propagation delays of signals in a circuit. In particular, the invention relates to the use of delay-locked loops to equalize propagation delays.

[0004] 2. Description of the Related Art

[0005] A data switch may be implemented with a number of channels for transmitting data. The channels may be grouped into a number of parts. However, variations in the fabrication process, operating voltages, and operating temperature of the various parts can undesirably increase signal skew between channels. This problem increases with the number of parts.

[0006] In other instances it is desirable to limit the number of channels implemented on each part. For example, for the DP83890 Private Data Exchange (PDX) from National Semiconductor Corp., Santa Clara, Calif. it was desired to integrate no more than 9 channels on a single part. (For example, eight channels for data and one channel for a clock signal.) A solution was needed to ensure that propagation delays through any N parts on printed circuit board would be substantially equal, to minimize skew between channels (e.g., 24 channels on three parts for the PDX). Otherwise there is no sampling eye at the receiver when a printed circuit board has both a fast part and a slow part on it.

[0007] The present invention addresses these and other problems of the prior art by providing an apparatus for and method of equalizing propagation delay.

[0008] According to one embodiment, an apparatus according to the present invention includes a circuit for generating a signal for equalizing propagation delay among parts of a transceiver. The parts each having a plurality of channels, with each channel being configured to receive the signal. The apparatus comprises a master circuit and a dummy channel circuit. The master circuit is configured to receive and lock to a reference clock signal and in accordance therewith generate a reference delay signal and an adjusted clock signal. The dummy channel circuit is configured to receive the adjusted clock signal, the reference delay signal and a dummy data signal, and in accordance therewith generate an intermediate data signal, the dummy data signal and one or more control signals. The one or more control signals correspond to a delay between the adjusted clock signal and the intermediate data signal.

[0009] According to another embodiment, a method according to the present invention equalizes propagation delay among parts of a transceiver. The parts each have a plurality of channels, with each channel being configured to receive a signal for setting a delay period. The method includes the steps of receiving and locking to a reference clock signal; and generating a reference delay signal and an adjusted clock signal, in accordance with the step of receiving and locking. The method further includes the step of receiving the adjusted clock signal, the reference delay signal and a dummy data signal. The method still further includes the step of generating an intermediate data signal, the dummy data signal and one or more control signals, in accordance with the step of receiving the adjusted clock signal, wherein the one or more control signals correspond to a delay between the adjusted clock signal and the intermediate data signal.

[0010] A better understanding of the features and advantages of the present invention will be obtained by reference to the following detailed description and accompanying drawings which set forth illustrative embodiments in which the principles of the invention are utilized.

[0011] FIG. 1 is a block diagram of a propagation delay equalizer circuit according to the present invention.

[0012] FIG. 2 is a flowchart of the process performed by the propagation delay equalizer circuit.

[0013] FIG. 3A is a block diagram of a master circuit in the propagation delay equalizer circuit.

[0014] FIG. 3B is a timing diagram of the signals generated by the master circuit of FIG. 3A.

[0015] FIG. 4 is a block diagram of a delay circuit in the master circuit.

[0016] FIG. 5 is a block diagram of a dummy channel circuit in the propagation delay equalizer circuit.

[0017] FIG. 6 is a block diagram of a dummy delay circuit in the dummy channel circuit.

[0018] FIG. 7 is a block diagram of a part with nine data channels as implemented in a preferred embodiment of the present invention.

[0019] FIG. 8 is a block diagram of separate dummy transmission and dummy reception channels, and separate transmission and reception channels, in a preferred embodiment of the present invention.

[0020] FIG. 9 is a block diagram of additional components in the master circuit.

[0021] In a preferred embodiment, this invention uses delay locked loops (DLLs) to calibrate two dummy channels (a receive and a transmit channel) such that the delay is maintained at a substantially constant value, regardless of process, voltage, and temperature variations. A variable delay stage in each channel is biased off of the DLL control current in the dummy channel. A dummy channel is used so that the part could lock to the precision delay even without any data input.

[0022] A DLL can be used to ensure that the delay from transistor-transistor logic (TTL) input to low-voltage differential signaling (LVDS) output and LVDS input to TTL output is constrained within a small time period (e.g., 400 ps) across all channels across variations in process, voltage, and temperature. This uses dummy transmission and receive channels with primary inputs and outputs, and a high speed input buffer for sensing input signals and output signals on the dummy channels. On board, the dummy channel output has a dummy load which matches that of the other channels. A dedicated output for sourcing the calibration clock is also used.

[0023] In one embodiment, the LVDS driver has an onboard DLL that takes a 25 MHz reference clock and creates eight evenly spaced phases of 25 MHz. One of these phases is the input to the dummy channels, then the next phase (delayed 5 ns) is compared to the sense output for the channel. These go into a phase comparator of the second DLL, which locks when the channel propagation delay is 5 ns. The phase comparator output goes to a digital filter, which decodes a value used to turn on binary weighted capacitors in a variable delay stage inserted in the data path. This delay stage is slaved to the active channels.

[0024] The delay calibration circuit ensures that the delays through the transmit and receive sides of an LVDS transceiver are substantially the same as the delays through the transmit and receive sides of other LVDS transceiver integrated circuits on the same printed circuit board, even if they are the results of different fabrication processes, have different operating voltages, or have different operating temperatures. Of course on a printed circuit board it is expected that operating voltages and temperatures will be very similar for all integrated circuits, so fabrication process variation is the biggest factor.

[0025] FIG. 1 illustrates the three general components of a propagation delay equalizer circuit 10. Circuit 10 includes master circuit 300, dummy channel circuit 500, and data channel circuit 100. Master circuit 300 receives reference clock signal 20 and outputs adjusted clock signal 30 and reference delay signal 40. Dummy channel circuit 500 receives reference clock signal 20, adjusted clock signal 30 and reference delay signal 40, and outputs dummy data signal 50 and equalization delay signal 60. Data channel circuit 100 receives reference clock signal 20 and equalization delay signal 60, and outputs data signal 70.

[0026] As discussed in more detail below, dummy channel 500 sends dummy data signal 50 and equalizes a delay of dummy data signal 50 until the delay matches reference delay signal 40. Dummy channel 500 then identifies the delay to data channel 100 via equalization delay signal 60. Data channel 100 then adjusts a delay of data signal 70 until such delay matches that represented by equalization delay signal 60. In this manner, a plurality of data channels can send and receive data signals with uniform delays.

[0027] FIG. 2 illustrates the operations performed by propagation delay equalizer circuit 10. Circuit 10 receives reference clock signal 20 in receiving state 210. Circuit 10 then generates adjusted clock signal 30 in generating state 220, sends dummy data signal 50 in sending state 230, and receives dummy data signal 50 in receiving state 240. Also after receiving state 210, circuit 10 locks to reference clock signal 20 in locking state 250 and transmits reference delay signal 40 in transmitting state 260. After receiving state 240 and transmitting state 260, circuit 10 equalizes a delay of dummy data signal 50 with reference delay signal 40 in equalizing state 270. Then, circuit 10 sends equalization delay signal 60, based on a result of the equalization, in sending state 280. Then, circuit 10 equalizes a delay of data signal 70 based on equalization delay signal 60 in equalizing state 290.

[0028] FIG. 3A illustrates the circuits in master circuit 300. Circuit 300 includes clock adjuster circuit 310, delay circuit 400, flip flop circuit 340, and phase comparator circuit 360.

[0029] Clock adjuster circuit 310 receives reference clock signal 20 and outputs adjusted clock signal 30. As implemented, reference clock signal 20 runs at 25 MHz; however, other frequencies may be used as desired. Preferably the clock adjuster circuit 310 is a clock divider circuit (e.g., a flip-flop) and the adjusted clock signal 30 may be the reference clock signal 20 divided by eight to 3.125 MHz. A frequency different than that of reference clock signal 20 ensures that master circuit 300 does not lock to a multiple of the reference delay. Specifically, the phase comparator circuit 360 cannot detect a phase difference of above 180 degrees. In a case where the timing variation is greater than half of the period of the reference clock signal 20 (greater than, e.g., 20 ns), the detected phase difference would be inaccurate. Calculating the phase difference from the adjusted clock signal 30, with a period greater than that of the reference clock signal 20, results in a more accurate phase difference that may be detected.

[0030] Delay circuit 400 receives adjusted clock signal 30 and master delay signal 320, outputs delayed adjusted clock signal 330 based on adjusted clock signal 30 and master delay signal 320, and outputs reference delay signal 40 based on delayed adjusted clock signal 330. Flip flop circuit 340 receives reference clock signal 20 and adjusted clock signal 30, and outputs flip flop clock signal 350. Phase comparator circuit 360 receives flip flop clock signal 350 and delayed adjusted clock signal 330, and outputs master delay signal 320 to delay circuit 400.

[0031] Reference delay signal 40 is generated as follows. Flip flop circuit 340 is clocked by reference clock signal 20, generating flip flop clock 350, which is adjusted clock signal 30 delayed by the period of reference clock signal 20. For example, when reference clock signal 20 is 25 MHz, flip flop clock signal 350 is adjusted clock signal 30 delayed by 40 ns.

[0032] Phase comparator 360 compares the phases of flip flop clock signal 350 and delayed adjusted clock signal 330. It generates master delay signal 320 to indicate whether delayed adjusted clock signal 330 matches flip flop clock signal 350.

[0033] Delay circuit 400 adjusts a delay of adjusted clock signal 30 based on master delay signal 320, to produce delayed adjusted clock signal 330. It generates reference delay signal 40 based on the delay of delayed adjusted clock signal 330. For example, when delay circuit 400 includes eight similar subdelay stages and master delay signal 320 indicates that delayed adjusted clock signal 330 is in phase with flip flop clock signal 350 (period of 40 ns), reference delay signal 40 may be set at 10 ns by tapping the output of the second subdelay stage.

[0034] FIG. 3B illustrates the signals generated in a preferred implementation of master circuit 300. Reference clock signal 20 operates at 25 MHz. Adjusted clock signal 30 operates at 3.125 MHz. Flip flop clock signal 350 is adjusted clock signal 30 delayed by reference clock signal 20. That is, the flip flop clock signal 350 follows the adjusted clock signal 30 by a time delay Δ of 40 ns. Delayed adjusted clock signal 330 is adjusted clock signal 30 delayed by delay circuit 400. Thus the delayed adjusted clock signal 330 will also follow the adjusted clock signal 30 by a time delay Δ of 40 ns. Reference delay signal 40 is adjusted clock signal 30 delayed by less than all of the subdelay circuits in delay circuit 400 (see FIG. 4.). For example, the reference delay signal 40 follows the adjusted clock signal 30 by a time delay δ of 10 ns.

[0035] FIG. 4 illustrates a preferred implementation of delay circuit 400. Delay circuit 400 includes a digital-to-analog converter circuit 410 and a plurality of subdelay circuits 430. The DAC 410 provides analog master delay signal 420 to subdelay circuits 430. Reference delay signal 40 may be tapped off subdelay circuits 430 before the last subdelay circuit to give a delay less than the time delay A of delayed adjusted clock signal 330.

[0036] Preferably, phase comparator circuit 360 (see FIG. 3A) generates an 8-bit control word as master delay signal 320. The DAC 410 converts the control word into an analog current input to subdelay circuits 430. The DAC 410 preferably uses a bandgap reference current. Subdelay circuits 430 may be implemented as single-ended current starved buffers.

[0037] FIG. 5 illustrates the components that make up dummy channel circuit 500. Circuit 500 includes dummy delay circuit 600, driver circuit 520, level comparator circuit 540, and phase comparator circuit 560. Dummy delay circuit 600 receives adjusted clock signal 30 and dummy delay signal 570, and outputs intermediate data signal 510 and equalization delay signal 60. Driver circuit 520 receives intermediate data signal 510 and outputs dummy data signal 50. Level comparator circuit 540 receives dummy data signal 50 and outputs calibration signal 550. (The dummy data signal 50 may be two differential signals, or it may be one signal in which case the level comparator circuit 540 compares it with a reference signal such as 531bin FIG. 8.) Phase comparator circuit 560 receives calibration signal 550 and reference delay signal 40, and outputs dummy delay signal 570.

[0038] Dummy channel 500 produces equalization delay signal 60 to be an accurate delay target for the actual data channels because dummy channel 500 is configured similarly to the actual data channels. For example, driver circuit 520 is connected to have the same termination load (50e) as the driver circuits in the actual data channels. Level comparator 540 is similar to the level comparators in the actual data channels.

[0039] FIG. 6 illustrates the components that make up dummy delay circuit 600. Circuit 600 includes digital-to-analog converter circuit 610, mirroring circuit 630, and dummy delay subcircuit 640. The DAC 610 receives dummy delay signal 570 (see FIG. 5) and generates analog dummy delay signal 620. Mirroring circuit 630 receives analog dummy delay signal 620 and generates a plurality of analog dummy delay signals as equalization delay signal 60. Dummy delay subcircuit 640 receives one of the plurality of signals 60 and adjusted clock signal 30 (see FIG. 1), and generates intermediate data signal 510.

[0040] In a preferred embodiment, phase comparator 560 (see FIG. 5) generates an 8-bit control word as dummy delay signal 570. The DAC 610 then converts the control word into a current signal. Mirroring circuit 630 mirrors this current to dummy delay subcircuit 640 and to the data channels 100. Mirroring circuit 630 generates 16 mirror currents, but more or less may be generated as desired. Thus, these mirror currents are replicas of the analog dummy delay signal 620. The DAC 610 preferably generates a bandgap reference current. Dummy delay subcircuit 640 may be implemented as a single-ended current starved buffer.

[0041] FIG. 7 shows that preferably nine data channels, plus the dummy channel 500, are implemented in each part. The data channels 100a-100ireceive the equalization delay signal 60 and delay the data signals 70a-70iby the same amount. Placing the master circuit 300 and the dummy channel 500 in the middle of the part gives the smallest distance from the furthest slave delay stage, which will help with delay variation due to time-changing voltage and channel length mismatches. Preferably, three parts are implemented in the DP83890 Private Data Exchange from National Semiconductor Corp., Santa Clara, Calif.

[0042] FIG. 8 shows that preferably the delay of dummy transmission channel 500ais generated separately from the delay of dummy reception channel 500b.This separation allows the delays to be separately matched. (The dummy transmission components have the suffix ‘a’ appended to their numerical identifiers. The dummy reception components have the suffix ‘b’.)

[0043] The driver in dummy transmission channel 500ais preferably low-voltage differential signaling (LVDS) driver 520a.The LVDS driver 520ais loaded with a termination load (50d) similar to that of LVDS driver 130.

[0044] Level comparator 540ais similar to level comparator 140. The driver in dummy reception channel 500bis preferably TTL driver 520b.Level comparator 540breceives a dummy voltage signal 531bas a reference signal in addition to dummy data signal 50b.Level comparator 540bis similar to level comparator 110. Preferably transmission channel 101 is separate from reception channel 199 as well.

[0045] Transmission level comparator 110 receives TTL input signal 104 and voltage signal 106, and outputs signal 115. Transmission slave delay 120 receives signal 115 and transmission equalization delay signal 60a,and outputs delayed signal 125. The LVDS driver 130 receives delayed signal 125 and transmits data signal 134 on the data channel.

[0046] On the reception side, reception level comparator 140 receives data signal 136 from the data channel, and outputs signal 145. Reception slave delay 150 receives signal 145 and reception equalization delay signal 60b,and outputs a delayed signal 155. The TTL driver 160 receives delayed signal 155 and outputs TTL output signal 165.

[0047] Preferably, voltage signal 106 and dummy voltage signal 531bare set the same at one-half of power supply voltage V_CC. Preferably dummy voltage signal 531bis tied to the negative input of level comparator 540b.

[0048] In dummy transmission channel 500a,level comparator 540asees a load of phase comparator 560a,while in transmission data channel 101, level comparator 110 sees the input of slave delay 120. To ensure that the delay of data channel 101 matches the delay of dummy channel 500a,slave delay 120 has a phase comparator (not shown) similar to phase comparator 560aconnected to its input; and comparator 540ahas a delay stage (not shown) similar to slave delay 120 connected to its output. Thus, both level comparator 540aand level comparator 110 see the same capacitance, i.e., the input capacitance of a phase comparator and a delay stage.

[0049] In both the dummy transmission channel 500aand the data transmission channel 101, the LVDS driver sees the same on-chip load, i.e., either level comparator 140 or level comparator 540a.There will be a big difference in the capacitance seen at the data transmission channel LVDS driver 130 and the dummy transmission channel LVDS driver 520a.However, since these are terminated outputs, the bulk of the current will flow through a termination resistor at the frequency and edge rates contemplated for use with the present invention, so the delay will depend primarily on resistance, not on charging the cable capacitance.

[0050] In dummy receive channel 500b,level comparator 540bsees a load of phase comparator 560b,while in data receive channel 199, level comparator 140 sees the input to slave delay 150. This is fixed in the same manner as described above regarding data transmission channel 101. Slave delay 150 has a phase comparator (not shown) similar to phase comparator 560bdriving its input, and level comparator 540bdrives a delay stage (not shown) similar to slave delay 150.

[0051] The biggest mismatch for dummy receive channel 500boccurs when attempting to match a capacitor on the printed circuit board off TTL driver 520bwith the capacitance of the printed circuit board and integrated circuit input capacitance. A preferred solution is to have input/output pins for the dummy channels 500a-bon the integrated circuit to which the LVDS driver 520ainterfaces that have the sole purpose of loading the dummy channels. That way, the dummy receive channel 500bsees substantially the same capacitance (load 50c) as the data receive channel 199 on that printed circuit board. Similarly, the dummy transmission channel 500ais loaded (with load 50d) just like the transmission channel 101.

[0052] Reference delay signal 40 is taken off the same output on the subdelay circuit 430 that drives the next subdelay circuit of delay circuit 400 (see FIG. 4). To ensure a precise delay (e.g., exactly 10 ns), the other subdelay circuits 430 should preferably see a similar load; otherwise the more loaded subdelay circuit will introduce more delay, and the other subdelay circuits a little less to compensate. To overcome this concern, two phase comparator flip flops (not shown) load each subdelay circuit 430 except for the one from which is tapped the reference delay signal 40, which sees one phase comparator load in dummy transmission channel phase comparator 560a,and a second load in dummy receive channel phase comparator 560b(see FIG. 8).

[0053] The interconnect capacitance will differ, but this can be minimized by preferably laying out both dummy channels 500aand 500bclose to master circuit 300.

[0054] FIG. 9 shows additional components of master circuit 300. Between phase comparator circuit 360 and delay circuit 400 (see FIG. 3A) are two pulse generator circuits 912 and 914, two pulse attenuator circuits 932 and 934, and counter circuit 950. Preferably phase comparator 360 is a flip flop with a short symmetric setup and hold window (e.g., under 50 ps). The Q output of phase comparator 360 provides down signal 902, and the QZ (inverse of Q) output provides the up signal 904. Down signal 902 is the input to pulse generator 912 which creates a pulse synchronous to reference clock signal 20 every time down is sensed true (i.e., the Q output is asserted) at a rising edge of delayed adjusted clock signal 330 input to phase comparator 360. Up signal 904 is the input to pulse generator 914 which creates a pulse synchronous to reference clock signal 20 every time up is sensed true (i.e., the QZ output is asserted) at a rising edge of the delayed adjusted clock signal 330 input to phase comparator 360. The output of pulse generator 912 is a pulse density modulated pulse stream 922 where the pulse density is proportional to an amount of samples where the delayed adjusted clock signal 330 input to phase comparator 360 lags the flip flop clock signal 350 input. The output of pulse generator 914 is a pulse density modulated pulse stream 924 where the pulse density is proportional to an amount of samples where the delayed adjusted clock signal 330 input to phase comparator 360 leads the flip flop clock signal 350 input.

[0055] Pulse stream 922 from pulse generator 912 is attenuated by pulse stream attenuator 932, while pulse stream 924 from pulse generator 934 is attenuated by pulse stream attenuator 934. A pulse stream attenuator is implemented as a programmable divider with a counter that counts input pulses and creates an output pulse upon counting a programmable number of input pulses. This allows programming the amount of proportional control for the loop. The outputs 942 and 944 of pulse stream attenuators 932 and 934 are input to counter 950. Counter 950 starts at a full count (i.e., minimum delay) and counts up or down in response to the pulse outputs 942 and 944 from pulse attenuators 932 and 934. A pulse input from pulse attenuator 932 will cause the count of counter 950 to increase by one, while a pulse input from pulse attenuator 934 will cause the count to decrease by one. The counter output is master delay signal 320, used to adjust the output of delay circuit 400 (see FIG. 3A). The components shown in FIG. 9 run off of the reference clock 20, and lead and lag errors are synchronized to such clock by pulse generators 912 and 914.

[0056] Referring again to FIG. 8, the delay locked loop operating point for one example embodiment should preferably be chosen such that the following requirements are met at any operating point at which a locked loop might result. First, slave delays 120 and 150 should introduce no data dependent jitter (DDJ) (i.e., the same delay should exist from zero to 50 MHz). Second, slave delays 120 and 150 should introduce no digital clock delay (DCD) (with a data signal having a frequency up to 50 MHz). Dither related to the least significant bit should be kept under 100 ps. Third, saturation voltages V_satfor all current mirrors and delay stages should be kept below at least 400 mV, and the combined effect of worst case channel length mismatch and threshold voltage V_tmismatch should result in a delay difference of under 100 ps. Fourth, direct current power supply rejection ratio should result in no more than an N% change in delay for an N% change in the power supply voltage V_DD.

[0057] The desire for small DDJ and DCD comes from the fact that we are using this to delay a data signal, so any distortion will deplete the total jitter budget.

[0058] The extremes of operating points contemplated for the circuit of the present invention is an operating temperature and voltage of −30° C. and 2.7 V for fast processes, and an operating temperature and voltage of −150° C. and 3.8 V for slow processes. The fastest delay is at the lowest voltage because it is a constant current delay stage, and a higher power supply voltage V_CCmeans it needs to swing further with a constant rise time.

[0059] It is preferred that the delay through the comparator in the following two cases is as close as possible. The first case is when a LVDS signal (e.g., a few hundred millivolts at approximately 1.3 V) is applied to the positive and negative inputs zero crossing to the output crossing V_DD/2. The second case is when a signal with a high state equal to V_DDand a low state equal to V_SSis applied to the positive input, with the negative input tied to V_DD/2, with the output measured at the V_DD/2 crossing. Satisfaction of these two cases helps ensure that the delay through the comparator is equal for both large and small voltage swings.

[0060] This implies good common mode rejection ratio in that the output current from the differential stage should not depend on common mode voltage, and also implies high gain, in that the magnitude of the differential voltage should not affect the amount of current through the branch, i.e., operated well out of linear amplifier region. Specifically, high gain means that both small and large differential input signals will provide a relatively large amount of current between the sides of the differential input stage. That is, both these cases operate in the nonlinear region.

[0061] It is not critical that the delays be perfectly matched, provided that the variation in the delay mismatch is small over process variations (because temperature and V_CCtrack on a given printed circuit board). This is because in this application, an exact 8 ns±200 ps channel delay is not required, provided that mismatch is no more than approximately 200 ps between all channels on a given printed circuit board.

[0062] It should be understood that various alternatives to the embodiments of the invention described herein may be employed in practicing the invention. It is intended that the following claims define the scope of the invention and that structures within the scope of these claims and their equivalents are covered thereby.