Abstract:
A method of testing connectivity through a plurality of dual purpose current mode logic (“CML”) latch circuits connected in a series is provided. Each of the CML latch circuits are operable to latch at least one output signal at a timing in accordance with at least one clock signal and having a mode control device for operating the CML latch circuit as a buffer amplifier when the at least one clock signal is inactive. The method comprises the steps of activating the mode control devices of each of the CML latches to operate each of the CML latches as a buffer; inputting a first signal to a first CML latch of the series; latching an output signal of a second CML latch of the series, the second CML latch being connected at a point in the series downstream from the first CML latch; and determining whether the output signal changes in accordance with a change in the first signal.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
   This application is a divisional of U.S. patent application Ser. No. 11/307,923 filed Feb. 28, 2006, now U.S. Pat. No. 7,358,787, the disclosure of which is hereby incorporated herein by reference. 

   BACKGROUND OF THE INVENTION 
   The invention relates to latch circuits, and more particularly to a method of testing connectivity using latch signals which are transmitted as differential pairs of signals. 
   Latches take a variety of forms and are used in a variety of applications. Latches are basic building blocks of many types of sequential digital circuits including flip-flops, registers, adders, multipliers, etc., and are used at interfaces between digital circuits and analog circuits. In its simplest form, a binary digital latch is implemented by a circuit which generates an output signal having one of two binary states determined in accordance with a state of at least one input signal. A clock signal times the operation of the binary latch such that the output signal transitions between states at times determined in accordance with the clock signal. 
   A current mode logic (“CML”) latch is a particular type of latch which is usable when signals are transmitted as differential pairs of signals. Signals transmitted at relatively high frequencies require noise rejection to a greater degree than signals transmitted at lower frequencies. One way to achieve greater noise rejection is to transmit one signal differentially as a pair of signals which have opposite states. In each such pair, the differential signals either remain together at their respective opposite states or swing between the opposite states simultaneously. Data-carrying signals are input to a CML latch as a pair of differential data signals. Clock signals are input to the CML latch as a pair of differential clock signals. A CML latch rejects noise that affects (e.g., slows, advances, raises or lowers) both of the differential signals in the same way so as to latch the output signal reliably at a correct state despite noise affecting the differentially transmitted pair of signals. With differential signal transmission, even in the presence of noise, the differential clock signals accurately time the operation of the CML latch and the CML correctly latches the states of the differential data signals. 
     FIG. 1  is a schematic diagram illustrating a CML latch  100  in accordance with the prior art. As illustrated in  FIG. 1 , the CML latch  100  includes a first input device  102  and a second input device  104 , the first and second input devices being operable to receive first and second differentially transmitted input signals AP and AN, respectively. A first tail device  110  controls the flow of current between the first and second input devices and a current source  114  which is connected to ground. The first and second input devices  102 ,  104  become active when one clock signal CP input to the first tail device  110  is active. Such clock signal CP is one of a pair of differential clock signals CP and CN having phases 180 degrees apart, the clock signals swinging simultaneously between opposite levels. The differential clock signals operate at a relatively high frequency, such as a frequency of a few hundred megahertz (MHz) to several gigahertz (GHz) or tens of gigahertz. 
   When clock signal CP is active, one of the first and second input devices conducts a current I 1  or I 2 , respectively, in accordance with the states of the first and second input signals AP and AN, respectively. The states of output signals ZP and ZN change according to the currents I 1  and I 2  across loads L 1  and L 2 , respectively. In such way, when input signal AP is active, current I 1  across load L 1  pulls down the voltage at node ZN such that the output signal ZN becomes low. The input signal AN at such time is inactive, causing input device  104  to be turned off. In that case, current I 2  does not flow and the output signal at node ZP remains high. On the other hand, when input signal AN is active, current I 2  across load L 2  pulls down the voltage at node ZP such that the output signal ZP becomes low. At such time, the input signal AP is inactive, causing input device  102  to be turned off such that current I 1  does not flow and the output signal at node ZN remains high. 
   A pair of cross-coupled devices  106  and  108  are operable to latch the output signals ZP and ZN when the differential clock signal CN is active. When clock signal CP is active, the clock signal CN is inactive, such that output signals ZP and ZN change when the input signals AN and AP change. On the other hand, when clock signal CP is inactive and the clock signal CN is active, the cross-coupled devices  106 ,  108  latch the current states of the output signals ZP and ZN and hold them until clock signal CP becomes active again. 
   One problem of the CML latch  100  is that it is only usable when the differential clock signals CP and CN are active. The high switching frequency of these clock signals precludes them from being supplied to the CML latch by any means other than internal generation on an integrated circuit (“IC”) or chip which incorporates the CML latch or on a card to which the chip is mounted. Signals cannot be propagated through the CML latch unless the differential clock signals are present. 
   However, it is desirable to test chips which include CML latches at times when it is not possible to supply the differential clock signals CP and CN to the latches. 
   SUMMARY OF THE INVENTION 
   In accordance with an embodiment of the invention, a method of testing connectivity through a plurality of dual purpose current mode logic (“CML”) latch circuits connected in a series is provided. Each of the CML latch circuits are operable to latch at least one output signal at a timing in accordance with at least one clock signal and having a mode control device for operating the CML latch circuit as a buffer amplifier when the at least one clock signal is inactive. The method comprises the steps of activating the mode control devices of each of the CML latches to operate each of the CML latches as a buffer; inputting a first signal to a first CML latch of the series; latching an output signal of a second CML latch of the series, the second CML latch being connected at a point in the series downstream from the first CML latch; and determining whether the output signal changes in accordance with a change in the first signal. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a schematic diagram illustrating a current mode logic (“CML”) latch in accordance with the prior art. 
       FIG. 2  is a schematic diagram illustrating a CML latch circuit in accordance with a first embodiment of the invention. 
       FIG. 3  is a schematic diagram illustrating a CML latch circuit in accordance with a particular embodiment of the invention in which active devices include n-type field effect transistors (“NFETs”). 
       FIG. 4  is a schematic diagram illustrating a variation of the CML latch circuit shown in  FIG. 3 , in which active load devices are utilized in place of resistors. 
       FIG. 5A  is a schematic diagram illustrating a CML latch circuit in accordance with a particular embodiment of the invention in which active devices include p-type field effect transistors (“NFETs”). 
       FIG. 5B  is a schematic diagram illustrating a variation of the CML latch circuit shown in  FIG. 5A , in which active load devices are utilized in place of resistors. 
       FIG. 6A  is a schematic diagram illustrating a CML latch circuit in accordance with a particular embodiment of the invention in which active devices include npn type bipolar transistors. 
       FIG. 6B  is a schematic diagram illustrating a variation of the CML latch circuit shown in  FIG. 6A , in which active load devices are utilized in place of resistors. 
       FIG. 6C  is a schematic diagram illustrating a CML latch circuit in accordance with a particular embodiment of the invention in which active devices include pnp type bipolar transistors. 
       FIG. 6D  is a schematic diagram illustrating a variation of the CML latch circuit shown in  FIG. 6C , in which active load devices are utilized in place of resistors. 
       FIG. 7  is a block and schematic diagram illustrating a serializer circuit including a plurality of CML latches in accordance with an embodiment of the invention, as operated in a normal operational mode. 
       FIG. 8  is a block and schematic diagram illustrating operation of a serializer circuit in a test mode, the serializer circuit being arranged in accordance with the embodiment of the invention illustrated in  FIG. 7 . 
   

   DETAILED DESCRIPTION 
   A dual purpose current mode logic (“CML”) latch circuit in accordance with an embodiment of the invention includes a CML latch and a mode control circuit. The CML latch is operable to receive a pair of differential input data signals and a pair of differential clock signals and is operable to generate at least one output signal in accordance with the states of the pair of differential input data signals. A mode control signal is applied to the mode control circuit which determines whether the CML latch operates as a latch or as a buffer. Thus, when the clock signal is present, the CML latch can be operated in a normal operational mode to generate and latch the output signal in accordance with the differential input data signals that are applied to it at a timing determined by the clock signal. On the other hand, when the clock signal is not present, a mode control signal can be activated for operating the CML latch circuit in a test mode. In such test mode, the CML latch generates the output signal in accordance with the state of the differential input data signals but operates like a buffer instead of a latch. When the CML operates as a buffer, the output signal changes whenever the states of the differential input data signals change. 
   Each of the CML latches described herein in accordance with the various embodiments of the invention has dual operational modes. In the first operational mode in which the clock signal is supplied to the CML latch, the CML latch latches the output signal at a timing determined by the state of the clock signal. In the second operational mode, the CML latch operates as a buffer during a test mode when the clock signal normally supplied thereto is absent. In this way, during production testing, the CML latch can be operated as a buffer while testing electrical continuity of circuits which include the dual mode CML latch. For example, the CML latch can be operated as a buffer during wafer-level or chip-level production testing prior to packaging the chip when it is technologically forbidding or inconvenient to supply a high switching frequency clock signal to the chip. 
     FIG. 2  is device-level schematic diagram illustrating a CML latch circuit  210  in accordance with an embodiment of the invention. As shown in  FIG. 2 , the CML latch circuit  210  includes a CML latch  100  such as that shown and described above with reference to  FIG. 1 . The CML latch circuit also includes a mode control circuit  201  including a mode control device  202  which is connected in parallel with the first tail device  110  of the CML latch. In the particular example shown in  FIG. 2 , the mode control device  202  is connected between a first node  200  and a second node  220 , the second node  220  being directly connected to a current source  114 . In such way, the mode control device provides an alternative path for the flow of current between the input devices  102 ,  104  and the current source  114 , thus eliminating the need for a first differential clock signal CP to be provided to the first tail device. During a normal operational mode, an LTEST signal at the input to the mode control circuit is held inactive such that differential clock signals CP and CN provided to inputs of the first and second tail devices  110  and  112  control the operation of the CML latch  100 . The second clock signal CN has the same clock frequency and the same voltage levels as the clock signal CP but is an inverted version of clock signal CP. While in the normal operational mode, during a first phase of a cycle of the differential clock signals, a first differential clock signal CP is active and a second differential clock signal CN is inactive. During the first phase of the differential clock cycle, the CML latch  100  begins to generate the output signals ZN and ZP in accordance with the signals AP and AN applied to input devices  102 ,  104 . Subsequently, during a second phase of the differential clock cycle, the first differential clock signal CP is inactive and the second differential clock signal CN is active. During the second phase, the cross-coupled devices  106 ,  108  amplify the difference between the output signals ZN and ZP at that time and latch the states of the output signals until the first phase of the next differential clock cycle begins. 
   During a test mode of operation, the first and second differential clock signals are not supplied to the CML latch as input to the first and second tail devices  110 ,  112 . Instead, the voltage or current at the second differential clock signal input CN at the input to the second tail device  112  is held constant at a level to maintain tail device  112  active. In addition, an active level is supplied at the LTEST signal input to the mode control circuit  201 . The voltage or current at the input to the first differential clock signal input CP can be either left to float or held constant in an inactive state. Under such conditions, the CML latch generates differential output signals ZP and ZN according to the states of the differential input signals AN and AP, respectively. It is not necessary for the differential clock signals CP and CN to be provided to the CML latch  100  at that time. The output signals ZP and ZN generated by the CML latch  100  change in accordance with the input signals AN and AP as quickly as the input devices  102 ,  104  and the cross-coupled devices  106 ,  108  are able to amplify the input signals AN and AP. Thus, during the first operational mode, the CML latch circuit  210  operates as a latch timed in accordance with the pair of differential clock signals CP and CN. Otherwise, during the test mode, the CML latch  210  operates as a buffer when the LTEST signal and a voltage or current provided at the CN input to the second tail device are held in the active state. 
     FIG. 3  illustrates a CML latch circuit  310  in accordance with a particular embodiment of the invention. This embodiment is the same as that described above with respect to  FIG. 2 , except that the mode control device  301 , the input devices  302 ,  304 , the cross-coupled devices  306 ,  308 , and the tail devices  312  and  314  are specified to be n-type field effect transistors (“NFETs”). The input signals, output signals and operation of the CML latch circuit  310  are the same as that described above with respect to  FIG. 2 . 
     FIG. 4  illustrates a CML latch circuit  330  according to a variation of the above-described CML latch circuit ( 310 ;  FIG. 3 ). In such CML latch circuit  330 , p-type field effect transistors (“PFETs”) function as active load devices  320 ,  322 , having drain terminals connected in conductive paths to drain terminals of the NFET input devices  302 ,  304 . A bias voltage VB applied to the gates of the active load devices  302 ,  304  controls the conductivity of the load devices, and hence, the voltage drop across each of them according to the voltages of the respective input signals AP and AN applied to the input devices. The bias voltage can be held constant or modulated according to the operating conditions of the CML latch and the chip on which it is implemented. In a particular embodiment, the bias voltage VB is generated in accordance with a stable reference voltage such as a bandgap voltage and is applied to the load devices  320 ,  322  through a current mirroring arrangement. In such manner, the bias voltage may compensate for variations in the manufacturing process that affect the particular chip as well as changes in the operating environment such as temperature and operating loads. 
     FIG. 5A  illustrates a CML latch circuit  510  according to further variation of the above-described CML latch circuit  210 . In this variation, p-type field effect transistors (“PFETs”) are utilized as the mode control device  501 , the input devices  502 ,  504 , the cross-coupled devices  506 ,  508 , and the tail devices  512  and  514 . In contrast to the embodiment illustrated in  FIG. 2 , in this embodiment the clock signal CN is input to the tail device  512  which sources current to the input devices  502 ,  504 . Clock signal CP is input to the tail device  514  which sources current to the cross-coupled devices  506 ,  508 . Operation of the CML latch circuit  510  is the same as that described above with respect to  FIG. 2 , noting that the input data signals AP, AN, the clock signals CN and CP and the /LTEST signal input thereto are active when at a lower voltage level rather than when at a higher voltage level. 
   In a manner like that shown and described above with respect to  FIG. 4 , active load devices  520 ,  522  can also be utilized in the CML latch circuit  530  ( FIG. 5B ) in place of the resistors R 1 , R 2  ( FIG. 5A ). 
     FIG. 6A  illustrates a CML latch  610  circuit according to yet another variation in which each of the mode control device  601 , input devices  602 ,  604  and cross-coupled devices  606 ,  608  and tail devices  612 ,  614  of a are implemented as npn-type bipolar transistors. Operation is the same or similar to that described above with respect to the NFET embodiment  310  illustrated in  FIG. 3 . In a CML latch circuit  620  ( FIG. 6B ) according to a variation of the embodiment shown in  FIG. 6A , pnp type active load devices  625 ,  627  or other appropriate active load devices are utilized in place of the load resistors R 1  and R 2 . 
     FIG. 6C  illustrates a CML latch circuit  630  according to a further variation in which each of the mode control device  631 , input devices  632 ,  634  and cross-coupled devices  646 ,  648  and tail devices  642 ,  644  are implemented as pnp-type bipolar transistors. Operation is similar if not functionally nearly the same as that described above with respect to the PFET embodiment  510  illustrated in  FIG. 5A . In a CML latch circuit according to a further variation  650  ( FIG. 6D ), npn type active load devices  535 ,  537  or other appropriate active load devices are utilized in place of the load resistors R 1  and R 2 . 
     FIG. 7  is a block and schematic diagram illustrating a serializer circuit  700  in accordance with a further embodiment of the invention. The serializer circuit incorporates CML latch circuits in accordance with any one or more of the embodiments described above with respect to  FIGS. 2 through 6D . Specifically, each of the flip-flops and latches in the serializer circuit  700  includes a CML latch according to one of the above-described embodiments. The serializer circuit  700  is used to convert a stream of parallel data into a serial data stream, such as for the purpose of transmitting data over a serial data transmission link. One bit data signals D 0 , D 1 , D 2  and D 3  are input to respective ones of the flip-flops  710 ,  711 ,  712  and  713 , each of the flip-flops including a CML latch in accordance with one of the embodiments described above with reference to  FIGS. 2 through 6D . The output of certain flip-flops  711  and  713  are input to latches  714 , which themselves are CML latch circuits having a structure and operating in accordance with one of the embodiments described above with reference to  FIGS. 2 through 6D . 
   In an example of operation, a full rate clock signal (C 1 ) including a pair of differential clock signals is input to the serializer  700  at a frequency of 6.4 GHz and is buffered and supplied to the serializer circuit as the differential pair of clock signals  742 . A synchronous divider  724  divides that clock frequency in half to 3.2 GHz for input as a differential pair of clock signals to second stage flip-flops  718 , latch  720  and as a select signal to multiplexer  722 . In addition, the synchronous divider  724  outputs another pair of differential clock signals  746  at a divided down clock frequency of 1.6 GHz. This pair of differential clock signals provides the CP and CN clock inputs to first stage flip-flops  710 ,  711 ,  712 , and  713 . A clock converter circuit  726  converts the divided down differential clock signal  746  to a single-ended clock signal  750  at the same frequency (1.6 GHz) for output to rail-to-rail logic circuits on the chip, for example CMOS logic circuits. The rail-to-rail logic circuits utilize the divided down single-ended clock signal  727  for control of sequential logic circuits, including logic circuits which produce the input data signals D 0 , D 1 , D 2  and D 3 . The data signals D 0 , D 1 , D 2  and D 3  preferably are single-ended and the flip-flops  710 ,  711 ,  712  and  713  convert these single-ended data signals to respective pairs of differential signals, 
   As timed by the pair of differential clock signals  746 , the flip-flops  710 ,  711 ,  712 ,  713  latch the input data signals D 0 , D 2 , D 1  and D 3 , respectively, into the serializer  700  as pairs of differential data signals. In addition, the pair of differential clock signals  746  are input as a select signal to the multiplexers  716  in the first stage of the serializer circuit  700 . 
   One of the multiplexers  716 , operated by a clock signal  750 , selects alternating ones of data bits D 0  and  10  D 2  input thereto through flip-flops  710 ,  711  and latch  714 , and another one of the multiplexers  716  selects alternating ones of the data bits D 1  and D 3  input thereto through flip-flops  712 ,  713  and latch  714 . The output of the multiplexers  716  are input through flip-flops  718  and latch  720  to a further multiplexer  722  that operates with the differential pair  744  of clock signals at twice the rate of the clock signal  750  supplied to the multiplexers  716 . Finally, the data output by multiplexer  722  is latched by the pair  742  of differential clock signals into a series of serially connected flip-flops  730 ,  732 ,  734 ,  736  and  738  at the final (undivided) clock rate to obtain the serialized data signal. 
   Each of the flip-flops  732 ,  734 ,  736 , and  738  in the series includes two latches so as to produce two outputs, each output as a pair of differential signals. Each of the outputs of the flip-flops is delayed by 0.5 cycles of the differential clock in relation to one other output of the series of flip-flops, except for the output Z 0  of flip-flop  732  which is the first output in the series. Thus, output Z 05  is delayed by 0.5 cycles of the differential clock in relation to output Z 0  and output Z 1  is delayed by 0.5 cycles of the differential clock in relation to output Z 05 , and so on among all the outputs of the flip-flops  732 . In such way, the outputs Z 0 , Z 05 , Z 1 , Z 15 , Z 2 , Z 25 , Z 3  and Z 35  of the flip-flops are taps of a tapped delay line. These taps are provided to a finite impulse response (“FIR’) transmitter, which in turn, is used to shape the serialized data stream signal for transmission over a serial data transmission link (not shown). 
   During a particular mode of operation, a demultiplexer  740  also receives outputs Y 0 , Y 1 , Y 2  and Y 3  of the flip-flops  732 ,  734 ,  736  and  738 , respectively, these preferably being the same signals as provided at the outputs Z 0 , Z 1 , Z 2  and Z 3 . The demultiplexer is operable to output four bits of parallel data at the original parallel clock signal rate (1.6 GHz) as wrap data during a particular test mode. 
   During a test mode of the chip for performing continuity testing, the LSSD test signal is activated to each of the CML latches of the serializer circuit, in a manner as shown in  FIG. 2 , for example. Referring to  FIG. 8 , as a result, each of the CML latches, including each of the flip-flops and latches in the serializer circuit  700  now operates as a buffer or as a pair of series-connected buffers instead of a flip-flop or a latch. Each of the flip-flops  710 ,  711 ,  712 , and  713  and latches  714  operates as a buffer. Each of the flip-flops  730 ,  732 ,  734 ,  736  and  738  operates as a pair of series-connected buffers. 
   During the test mode of operation, a latch, preferably a “level sensitive scan device” (LSSD) latch  765 , also referred to as a “shift register latch” (SRL), provides a data bit signal at the input to the serializer circuit  700 . In place of the clock signal, a pair of select signals LSSDS 0  and LSSDS 1  are input to the serializer for selecting a particular one of the digital bits D 0 , D 1 , D 2  or D 3  to be passed between the input and the output of the serializer circuit. After modification by logic  770  and/or the converter circuit  726 , these select signals LSSDS 0  and LSSDS 1  are applied to the select inputs of the multiplexers  716  and  720 . Thus, the LSSDS0 and LSSDS1 signals control the selection of signals through the multiplexers  716  and  720 . Specifically, the digital bit that is selected by the multiplexers appears at the flip-flop  730  in accordance with the following truth table: 
   
     
       
             
           
             
             
             
           
         
             
               TABLE 1 
             
           
           
             
                 
             
             
               Bit Selection Truth Table 
             
           
        
         
             
               LSSDS0 
               LSSDS1 
               Bit 
             
             
                 
             
             
               0 
               0 
               D3 
             
             
               0 
               1 
               D1 
             
             
               1 
               0 
               D0 
             
             
               1 
               1 
               D2 
             
             
                 
             
           
        
       
     
   
   Such signal propagates through the flip-flops  732 ,  734 ,  736  and  738  to an output of the serializer circuit through a final buffer  760  as an “observe signal” which is latched by an SRL latch  762 . In addition, outputs of the flip-flops propagate through the demultiplexer  740  and are latched by an SRL latch  764 . 
   While the CML latches are operated as buffers in the test mode, a signal applied as input to the serializer circuit at one of the data bit inputs D 0  through D 3  propagates through the serializer circuit without requiring a clock signal to be present. At that time, the states of the select signals LSSDS 0  and LSSDS 1  determine which of the data bit inputs D 0  through D 3  is passed through to the outputs through buffer  760  as the “observe” signal or through the demultiplexer  740 . 
   While the invention has been described in accordance with certain preferred embodiments thereof, many modifications and enhancements can be made thereto without departing from the true scope and spirit of the invention, which is limited only by the claims appended below.