Abstract:
A synchronous circuit implements a bypass mode for use in conjunction with an inductive-capacitive (“LC”) buffer. The LC buffer receives differential conventional clock signals, and generates buffered differential conventional clock signals. A synchronous circuit, such as a latch, includes at least two clock receivers. The conventional clock signal is input to the first clock receiver, such as a transistor, and an auxiliary clock is input to a second clock receiver. The conventional clock signal provides timing for the synchronous circuit under a normal mode of operation, and the auxiliary clock signal provides timing for the synchronous circuit under a test mode of operation at a frequency lower than the conventional clock signal.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention is directed toward the field of synchronous circuits, and more particularly toward testing of synchronous circuits. 
     2. Art Background 
     Data communication systems transport data at a speed defined by a predetermined data rate. The speed of transmitting data in modern broadband communication systems has rapidly increased in recent years. Today, data rates as high as 40 gigabits per second (“Gbps”) are required for the OC-768 optical networking standard. 
     These data communication systems include basic synchronous circuits, such as latches, flip-flops, and constituent logic gates. To operate at high data rates, the synchronous circuits must also switch at high speeds (i.e., the synchronous circuits require a high bandwidth of operation). Thus, the high-speed synchronous circuits require the use of a high-speed clock with low jitter. Another requirement to effectively implement high-speed synchronous circuits is to minimize power dissipation. 
     To help meet these requirements, it is becoming increasingly more common to use inductive-capacitive (“LC”) tuned amplifiers as clock buffers. The LC tuned amplifiers, used to distribute clocks in the high-speed synchronous circuits, drive large capacitive loads on the clock line. These LC tuned amplifiers operate at high impedances within a narrow frequency range (i.e., a narrow clock data rate). For example, a phase lock loop (“PLL”) with an LC tuned voltage controlled oscillator may be used to generate the clock and an LC tuned amplifier may be used to buffer the clock. 
     Synchronous circuits typically include inputs to receive test or bypass clocks. In general, the bypass clocks are input to sections of the synchronous circuits to test the circuits. When debugging timing problems, it is desirable to set the bypass clock over a wide frequency range. In one application, the frequency or clock rate of the bypass clock is relatively low. For high-speed synchronous digital circuits, the frequency or clock rate of the bypass clock may be significantly lower than the clock rate of the device under normal operation. Unfortunately, the use of LC tuned amplifiers in these environments is limited because LC tuned amplifiers have a relatively narrow tuning range. Specifically, the LC tuned amplifiers only amplify over a limited frequency band due to the bandpass response of the tuned load. Thus, traditionally, LC tuned amplifiers preclude testing circuits over a wide frequency range. 
     To overcome frequency limitations at tuned nodes, an external clock may be injected into the integrated circuit chip through a bypass mode of operation. Typically, the clock is injected through a multiplexor (“MUX”) at the output of the PLL.  FIG. 1  illustrates a prior art circuit used to implement a conventional clock bypass mode. As shown in  FIG. 1 , the conventional clock, f c , and a bypass clock, f b , are input to MUX  120 . MUX  120  is controlled by a mode selection (i.e., selecting either the conventional clock or the bypass clock). The clock, output from MUX  120 , is input to buffer  130 . The buffer  130  drives a capacitive load, illustrated by capacitor  140 , for input to a plurality of digital circuits (flip-flops  150 ,  152 ,  154  and  156 ). 
     As explained herein, it is desirable to provide a bypass clock to high-speed synchronous circuits without impeding the operation of the circuits. It is also desirable to provide a bypass clock to high-speed synchronous circuits with an LC clock buffer. 
     SUMMARY OF THE INVENTION 
     A synchronous circuit implements a bypass mode for use in conjunction with an inductive-capacitive (“LC”) buffer. The LC buffer receives at least one conventional clock signal, and generates one or more buffered conventional clock signals. A synchronous circuit, such as a latch, includes at least two clock inputs: a first clock input that receives the conventional clock signal, and a second clock input signal that receives an auxiliary clock. The conventional clock signal provides timing for the synchronous circuit under a normal mode of operation. The auxiliary clock signal provides timing for the synchronous circuit under a test mode of operation. The auxiliary clock signal has a frequency or clock rate lower than the conventional clock signal. In one embodiment, the auxiliary clock signals and the conventional clock signals are differential signals. 
     The synchronous circuit comprises clock receiver circuits. In one embodiment, a first clock receiver circuit is coupled to the first clock input, and a second clock receiver circuit is coupled to the second clock input. In one embodiment, the first and second clock receiver circuits comprise transistors. The second clock receiver is coupled in parallel to the first clock receiver circuit, such that the second clock receiver does not affect the operation of the LC buffer and first clock receiver circuit during conventional operation of the circuit. In one embodiment, the LC buffer circuit includes a control circuit that controls the buffering of the conventional clock signal under the normal mode of operation and that disables conventional clock signal under the test mode of operation. 
     In one embodiment, the clock is buffered in an inductive-capacitive (“LC”) buffer. In addition, one or more local buffers are included in the circuit. For this embodiment, the buffered clock from the LC buffer is input to one or more local buffers. A bypass clock line is also an input to the local buffers. The bypass clock input and the clock input are parallel, such that the bypass clock operation does not affect the normal mode of operation. A single clock line output from the local buffer drives at least one clock input on a synchronous circuit. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  illustrates a prior art circuit used to implement a conventional clock bypass mode. 
         FIG. 2  illustrates a conventional clock bypass mode circuits applied to an LC tuned node. 
         FIG. 3   a  illustrates a waveform of the output of the buffer during normal high-speed clock operation. 
         FIG. 3   b  illustrates a waveform for the output of the buffer at low bypass frequencies. 
         FIG. 4  illustrates one embodiment for implementing a bypass clock mode in a high-speed synchronous circuit. 
         FIG. 5  is a block diagram illustrating one embodiment for a parallel bypass clock circuit. 
         FIG. 6  illustrates one embodiment for a current mode logic latch incorporating the parallel bypass clock techniques of the present invention. 
         FIG. 7  illustrates one embodiment for a tuned buffer circuit for use with the clock bypass techniques of the present invention. 
         FIG. 8  is a block diagram illustrating another embodiment for the parallel bypass clock techniques of the present invention. 
         FIG. 9  illustrates one embodiment of a local clock buffer with parallel bypass clock input. 
         FIG. 10  illustrates one embodiment for an emitter coupled logic (“ECL”) latch incorporating the parallel bypass clock techniques of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 2  illustrates a conventional clock bypass mode circuit applied to an LC tuned node. Circuit  200  of  FIG. 2  includes the same elements as circuit  100  of  FIG. 1 , except buffer  230  in  FIG. 2  drives an inductive/capacitive (LC) load, illustrated through the inductor  232  and capacitor  234 . The conventional clock bypass circuit, when applied to an LC load, does not operate properly. The inductive load effectively “shorts out” the buffer for frequencies of the bypass clock much lower than the conventional clock. In essence, at lower frequencies, the LC load exhibits a low effective impedance, thereby limiting the clock drive of the buffer. Specifically, the LC buffer provides a large signal of amplitude A c  at the center frequency 
         (       i   .   e   .     ,     fc   =     1     2   ⁢           ⁢   π   ⁢     LC1             )     ,         
and a smaller signal of amplitude, A b , at much lower frequencies (e.g., bypass clock frequencies).
 
       FIG. 3   a  illustrates a waveform of the output of the buffer during normal high-speed clock operation. As shown in  FIG. 3   a , the amplitude of the buffer clock, A c , is sufficient to drive multiple clock inputs.  FIG. 3   b  illustrates a waveform for the output of the buffer at low bypass frequencies (e.g., f b ). As shown in  FIG. 3   b , the amplitude of the clock output from the buffer is low, and thus is insufficient to drive clock inputs in the synchronous circuits. 
     The technique of the present invention does not inject the bypass clock into an LC tuned buffer. Instead, the bypass clock is injected directly into synchronous circuits. To effectively implement the bypass clock mode, the performance of the synchronous circuits and tuned clock buffer circuits during normal mode of operation must not be impeded. One technique to inject a bypass clock is to add a switch in series with the conventional clock or differential clocks. However, the introduction of the series switch reduces the impedance at the tuned node, and therefore lowers the voltage swing at the output of the tuned buffer. As a result, in one embodiment, the bypass clock is injected into the circuit “parallel” to the conventional clock or clocks. 
       FIG. 4  illustrates one embodiment for implementing a bypass clock mode in a high-speed synchronous circuit. A clock, f c , is generated from PLL circuit  410  for normal or conventional operation of the circuit. The clock, f c , is input to buffer  420 , which in turn, drives an inductive-capacitive (“LC”) load, illustrated by inductor  430  and capacitor  440  in  FIG. 4 . The buffered clock, f c , is input to a plurality of synchronous circuits (e.g., flip-flops  460 ,  462 ,  464 , and  466 ). For this embodiment, the bypass clock, f b , is directly input to the synchronous circuits (e.g., flip-flops  460 ,  462 ,  464  and  466 ), parallel to the conventional clock, f c . Although  FIG. 4  illustrates parallel inputs to D type flip-flops, the technique may be applied to any synchronous circuit without deviating from the spirit or scope of the invention. For the embodiment of  FIG. 4 , a capacitor, C b  ( 450 ), represents the capacitance of the bypass clock input. 
       FIG. 5  is a block diagram illustrating one embodiment for a parallel bypass clock circuit. The parallel bypass clock circuit  500  is implemented with synchronous circuit  510 . The synchronous circuit  510  employs a differential configuration, and thus receives differential clocks. The synchronous circuit  510  may be any type of digital logic circuit implemented with any type of logic family. A current-mode logic (“CML”) latch is described below in conjunction with a discussion of  FIG. 6 . As shown in  FIG. 5 , conventional clocks, fc(p) and fc(n), are input to clock receiving circuits  520  and  530 , respectively. Clock receiving circuits  515  and  525  are configured in parallel to clock receiving circuits  520  and  530 . Clock receiving circuits  515  and  525  receive differential clocks fb(p) and fb(n). In one embodiment, clock receiving circuits comprise transistors, such that the gate or base of the transistors receives the clock input. As shown in  FIG. 5 , bypass clock circuit  500  includes a bias circuit  540  to bias the clock receiving circuits  515 ,  520 ,  525  and  530 . 
       FIG. 6  illustrates one embodiment for a current mode logic latch incorporating the parallel bypass clock techniques of the present invention. As shown in  FIG. 6 , the latch  600  includes a cross-coupled transistor pair M 5  and M 6 . The sources of the cross-coupled transistor pair (M 5  and M 6 ) are coupled to the drains of MOS transistors M 2  and M 8 . One of the conventional differential clocks, clk n , is input to the gate of transistor M 2 , and one of the bypass differential clocks, auxclk n , is input to the gate of transistor M 8 . Differential inputs, d p  and d n , are input to the gates of MOS transistors M 3  and M 4 , respectively. The drains of transistors M 3  and M 4  are coupled to the differential outputs of the latch, q n  and q p . The source of transistors M 3  and M 4  are coupled to the drains of transistors M 7  and M 1 , respectively. Transistor M 1  receives, at its gate, one of the conventional differential clock signals, clk p , and transistor M 7  receives, at its gate, one of the bypass differential clock signals, auxclk p . The source of transistor M 1  is coupled to the source of transistors M 2 , M 7  and M 8 , and to the drain of transistor M 0 . Transistor M 0  is biased as a constant current sourcewith appropriate voltage “bias” set at its gate, as shown in  FIG. 6 . 
     When differential clock clk p  is in a high logic level, and clk n  clock signal is in a low logic level, the cross-coupled transistor pair, M 5  and M 6 , do not latch the input data (d p  and d n ), and thus the differential inputs d p  and d n  are propagated directly to the outputs of the latch, q p  and q n . Alternatively, when clock signal clkn attains a high logic level and clock signal clk p  attains a low logic, the latch (cross coupled transistors M 5  and M 6 ) holds the previous values presented on the differential output, q p  and q n . 
     In general, to add a parallel bypass clock to the CML latch, the auxiliary differential transistors pair, M 7  and M 8 , is added to the basic CML latch. For this embodiment, transistor M 7  is coupled in parallel with transistor M 1 , and transistor M 8  is coupled in parallel with transistor M 2 . Transistor M 7  receives, at its gate, a bypass or auxiliary clock p  (auxclk p ), and transistor M 8  receives, at its gate, an auxiliary or bypass clock n  (auxclk n ). Similar to the transistors M 1  and M 2 , the sources of transistors M 7  and M 8  are coupled to a bias transistor, M 0 . 
     During normal operation, the external bypass clock signals (auxclk p  and auxclk n ) are pulled to ground. A low logic level from signals auxclk p  and auxclk n  turn off transistors M 7  and M 8 . With transistors M 7  and M 8  off, the CML latch operates in normal mode (i.e., the transistor pair M 7  and M 8  do not affect the operation of the latch). However, when an external bypass clock is injected, clock signals auxclk p  and auxclk n  bias transistors M 7  and M 8  at the appropriate common mode level to turn on transistors M 7  and M 8 . Also, clock signals clkp and clkn are pulled to ground, thereby turning off transistors M 1  and M 2 . Thus, for this embodiment, an auxiliary or bypass clock is added to the CML latch without affecting the operation of the LC tuned buffer (i.e., the buffer that drives the conventional clock clk p  and clk n ). 
       FIG. 7  illustrates one embodiment for a tuned buffer circuit for use with the clock bypass techniques of the present invention. For this embodiment, the tuned buffer circuit  700  includes transistors M 9  M 10 , M 11 , M 12 , M 13  and M 14 . Also, the tuned buffer  700  includes inductors  710  and  720 . A bypass enable signal (bypassen) is input to the gate of transistor M 9 , and input to the gates of transistors M 13  and M 14 . Differential clock signals clkip and clkin (e.g., output from a PLL circuit) are input to transistor pair M 10  and M 11 . The drains of transistors M 14  and M 11  drive the output clock signal clk p , and the drains of transistors M 10  and M 13  drive the output clock signal clk n . 
     In the normal mode of operation, the control signal, bypassen, is a low logic level. This enables buffer circuit  700  by turning on transistor M 9  and turning off transistors M 13  and M 14 . To operate in the bypass mode, the control signal, bypassen, is set to a high logic level. In turn, the high logic level of bypassen signal turns off transistor M 9  to disable the buffer. In addition, the control signal bypassen turns on transistors M 13  and M 14  to pull clock signals clk p  and clk n  to ground. 
       FIG. 8  is a block diagram illustrating another embodiment for the parallel bypass clock techniques of the present invention. Circuit  800  includes a tuned buffer  830  for buffering the clock, f c , during normal mode of operation. The clock, f c , is generated from the phase locked loop (“PLL”) circuit  810 . The buffer  830  drives an inductive-capacitive (“LC”) load, illustrated by inductor  832  and capacitor  834  in  FIG. 8 . In some implementations of synchronous circuits, additional clock buffering, without LC tuning, is required for proper operation of the circuits. The buffered clock output from tuned buffer  830  is input to one or more local buffers (e.g., buffers  820  and  825  in  FIG. 8 ). The local buffers ( 820  and  825 ) are not tuned buffers. For this embodiment, a bypass clock, along with the clock, f c , are input to the local buffers. The local buffers ( 820  and  825 ) drive the clock input to one or more synchronous circuits (e.g., circuits  840 ,  842 ,  844  and  846 ). A local buffer may be configured to drive the clock inputs of one or more circuits, as appropriate. 
       FIG. 9  illustrates one embodiment of a local clock buffer with parallel bypass clock input. For this embodiment, local clock buffer  900  is implemented using MOS transistor (e.g., current-mode logic). The local buffer  900  includes transistors M 15  and M 16 , for normal clock mode operation, transistors M 17  and M 18 , for bypass clock mode operation, and transistor M 19  for biasing. Resistors R 2  and R 3 , coupled to a power supply voltage, V dd , provide gain in the differential buffer. 
     Under normal clock operation, differential inputs, clkp and clkn, input to the gates of transistors M 15  and M 16 , respectively, are driven by a clock signal output from a tuned buffer (e.g., tuned buffer  830  of  FIG. 8 ). The bypass clock inputs, auxclkp and auxclkn, are input to the gates of transistors M 17  and M 18 . During normal clock operation, bypass clock inputs, auxclkp and auxclkn, are held to ground. A low logic level on auxclkp and auxclkn lines turn off transistors M 17  and M 18 . Under bypass clock mode operation, the tuned buffer pulls the clock lines, clkp and clkn, to ground. One embodiment to pull clock lines clkp and clkn to ground in a tuned buffer is shown in  FIG. 7 . Also, under bypass clock mode operation, the differential bypass clocks, auxclkp and auxclkn, are driven for input to the gates of transistor M 17  and M 18 . During both modes of operation, a differential clock output is developed across resistor R 2  and R 3  on output nodes outn and outp. 
       FIG. 10  illustrates one embodiment for an emitter coupled logic (“ECL”) latch incorporating the parallel bypass clock techniques of the present invention. As shown in  FIG. 10 , latch  1000  includes cross-coupled bipolar transistor pair  1045  and  1050 . The emitter of the cross-coupled transistor pair ( 1045  and  1050 ) is coupled to the collectors of npn transistors  1025  and  1030 . One of the conventional differential clocks, clk n , is input to the base of transistor  1025 , and one of the bypass differential clocks, auxclk n , is input to the base of transistor  1030 . Differential inputs, d p  and d n , are input to the bases of npn transistors  1035  and  1040 , respectively. The collectors of transistors  1035  and  1040  are coupled to the differential outputs of the latch, q n  and q p . The emitters of transistors  1035  and  1040  are coupled to the collectors of transistors  1020  and  1015 , respectively. Transistor  1015  receives, at its base, one of the conventional differential clock signals, clk p , and transistor  1020  receives, at its base, one of the bypass differential clock signals, auxclk p . Transistor  1010  is biased with a constant current source, “bias”, as shown in  FIG. 10 . 
     When clock signal clk p  is in a high logic level, and clock signal clk n  is in a low logic level, the cross coupled transistor pair,  1045  and  1050 , do not latch the input data (d p  and d n ), and thus the differential inputs d p  and d n  are propagated directly to the outputs of the latch, q p  and q n . Alternatively, when clock signal clk n  attains a high logic level and clock signal clk p  attains a low logic, the latch (cross coupled transistors  1045  and  1050 ) holds the previous values presented on the differential output, q p  and q n . 
     To add a parallel bypass clock to the ECL latch, the auxiliary differential transistor pair,  1020  and  1030 , is added to the basic ECL latch as shown in  FIG. 10 . Specifically, transistor  1020  receives, at its base, auxiliary clock p  (auxclk p ), and transistor  1030  receives, at its base, auxiliary clock n  (auxclk n ). During normal operation mode, the external bypass clock signals (auxclk p  and auxclk n ) are pulled to ground. A low logic level from signals auxclk p  and auxclk n  turn off transistors  1020  and  1030 . With transistors  1020  and  1030  off, the ECL latch operates in normal mode. However, when an external bypass clock is injected, clock signals auxclk p  and auxclk n  bias transistors  1025  and  1030  at the appropriate level to turn on transistors  1020  and  1030 . Also, clock signals clkp and clkn are pulled to ground, thereby turning off transistors  1015  and  1025 . Thus, for this embodiment, an auxiliary or bypass clock is added to the ECL latch without affecting the operation of the LC tuned buffer (i.e., the buffer that drives the conventional clock clk p  and clk n ). 
     Although the present invention has been described in terms of specific exemplary embodiments, it will be appreciated that various modifications and alterations might be made by those skilled in the art without departing from the spirit and scope of the invention.