Abstract:
An integrated circuit (IC) with programmable circuitry having programmable functions and programmable interconnections. The IC further includes: a first module having an output with a first fixed data width or first variable data width; a second module having an input with a second fixed data width or a second variable data width; and a data width converter receiving data from the output of the first module and sending the data to the input of the second module, the data width converter configured to convert data from the first fixed data width or first variable data width to the second fixed data width or the second variable data width.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to an integrated circuit (IC) having circuitry with programmable functions and programmable interconnections. More specifically, the present invention relates to a method and apparatus for converting to and from variable-width data paths. 
   2. Related Art 
   In the past, multi-gigabit transceivers (MGTs) have not been included on programmable logic devices (PLDs) for various reasons, where a PLD is any IC which has programmable functions and programmable interconnections. However, commonly owned, copending U.S. patent application Ser. No. 10/090,250 filed on Mar. 1, 2002 entitled “High Speed Configurable Transceiver Architecture” by Suresh M. Menon et al., describes the manner in which MGTs can be included on a PLD, such as a field programmable gate array (FPGA). It would therefore be desirable to optimize the data paths between the core logic of a PLD and the MGTs located on the PLD. 
   PLD commonly includes one or more data paths, or collections of digital signals routed through the system during processing. The size of a collection, called the “data width” or “data path width” herein, depends on a number of factors. One factor in determining the data path width is the significance of the signals (i.e., the information that the signals represent, and the format of the signals). Another factor is the required speed of operation of the design. Yet another factor is the size constraints introduced by the design. Other factors may also possibly affect the data path width. 
   In some cases, it may be desirable to modify the width of a data path at some point in the design, changing the extent to which data is propagated in parallel. This may be necessary, for example, because of: different operating speeds in different portions of the design, or different constraints on the data width in different portions of the design. It may also be beneficial for this data width modification to be programmable and to be done dynamically. 
   It would therefore be desirable to have a PLD capable of implementing a variable-width data path. 
   SUMMARY 
   The present invention provides a method and system for converting data on a first bus of a first fixed or variable width to data on a second bus of a second fixed or variable width. An exemplary embodiment of the present invention includes: an integrated circuit (IC) with programmable circuitry having programmable functions and programmable interconnections. The IC further includes: a first module having an output with a first fixed data width or first variable data width; a second module having an input with a second fixed data width or a second variable data width; and a data width converter receiving data from the output of the first module and sending the data to the input of the second module, the data width converter configured to convert data from the first fixed data width or first variable data width to the second fixed data width or the second variable data width, where the first fixed data width is not equal to the second fixed data width. 
   An embodiment of the present invention provides an integrated circuit (IC) including: programmable circuitry having programmable functions and programmable interconnections, where the programmable circuitry includes a first transmit port having a first fixed data width or a first variable data width, and a first receive port having a second fixed data width or a second variable data width; a transceiver with a second transmit port having a third fixed data width or a third variable data width, and a second receive port having a fourth fixed data width or a fourth variable data width; a transmit converter coupling the first transmit port of the programmable circuitry and the second receive port of the transceiver, where the transmit converter is operably configured to convert the first fixed data width to the fourth variable data width, the first variable data width to the fourth fixed data width, or the first variable data width to the fourth variable data width; and a receive converter coupling the first receive port of the programmable circuitry and the second transmit port of the transceiver. The IC may also have the receive converter operably configured to convert the third fixed data width to the second variable data width, the third variable data width to the second fixed data width, or the third variable data width to the second variable data width. 
   Further, in another embodiment, the transmit converter couples the first transmit port of the programmable circuitry and the second receive port of the transceiver, where the transmit converter is operably configured to convert the first fixed data width to the fourth fixed data width; and the receive converter couples the first receive port of the programmable circuitry and the second transmit port of the transceiver. The receive converter operably configured to convert the fourth fixed data width to the second fixed data width. 
   The present invention will be more fully understood in view of the following description and drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a block diagram of a programmable logic device in accordance with one embodiment of the present invention. 
       FIG. 2-1  is a block diagram of a multi-gigabit transceiver and variable-width interface in accordance with one embodiment of the present invention. 
       FIG. 2-2  is a block diagram of another embodiment of the present invention; 
       FIG. 2-3  is a block diagram of yet another embodiment of the present invention; 
       FIGS. 3A ,  3 B,  3 C and  3 D show the clock waveforms (CLK 1248 ) used to control variable-width 1-bit, 2-bit, 4-bit and 8-bit data paths, respectively, as well as the clock waveform (CLK 2 ) used to control fixed-width 2-bit data paths in accordance with one embodiment of the present invention. 
       FIG. 4  is a circuit diagram of a transmit variable-width interface in accordance with one embodiment of the present invention. 
       FIG. 5  is a circuit diagram of a transmit width control circuit used to control the transmit variable-width interface of  FIG. 4 , in accordance with one embodiment of the present invention. 
       FIG. 6  is a waveform diagram illustrating the relationship between the CLK 2  signal, the CLK 1248  signal and a delayed CLK 1248  signal (CLK 1248 D), which is enabled when an 8-bit variable-width data path is selected in accordance with one embodiment of the present invention. 
       FIGS. 7A ,  7 B,  7 C, and  7 D are waveform diagrams illustrating the timing of the transmit variable-width interface of  FIG. 4  for 1-bit, 2-bit, 4-bit and 8-bit data paths, respectively, in accordance with one embodiment of the present invention. 
       FIG. 8  is a circuit diagram of a receive variable-width interface in accordance with one embodiment of the present invention. 
       FIG. 9  is a circuit diagram of a receive width control circuit used to control the receive variable-width interface of  FIG. 8 , in accordance with one embodiment of the present invention. 
       FIGS. 10A ,  10 B,  10 C, and  10 D are waveform diagrams illustrating the timing of the receive variable-width interface of  FIG. 8  for 1-bit, 2-bit, 4-bit and 8-bit data paths, respectively, in accordance with one embodiment of the present invention. 
       FIG. 11  is a waveform diagram of three clock signals (CLK — A, CLK — B and CLK — C) used to control the 8-bit wide data path of the receive variable-width interface of  FIG. 8  in accordance with one embodiment of the present invention. 
   

   DETAILED DESCRIPTION 
     FIG. 1  is a block diagram of a programmable logic device (PLD)  100  in accordance with one embodiment of the present invention. In the described embodiment, PLD  100  is a field programmable gate array (FPGA) that includes select I/O blocks (labeled I/O), digital clock managers (labeled DCM) and multi-gigabit transceivers (labeled MGT) located around the perimeter of the device. Each MGT includes a full-duplex differential data channel, such as channel  115 . PLD  100  also includes core block  150 , which includes an array of configurable logic blocks (CLBs), programmable routing circuitry, and optional embedded hardwire circuitry, for example, processor block  130 , in the described embodiment. Variable-width interface circuits (labeled VWIF) are located between each of the MGTs and core block  150 . Select I/O blocks I/O, digital clock managers DCM and core block  150  are well known to those of ordinary skill in the art. These conventional elements of PLD  100  are described in detail in “Virtex™-II Platform FPGA Handbook”, December 2000, pages 33–75, and in the Virtex II Pro™ Platform FPGA Handbook, October 2002, available from Xilinx Inc., 2100 Logic Drive, San Jose, Calif. 95124. 
   PLDs, such as FPGAs, have not previously included multi-gigabit transceivers or variable-width interfaces. As described in more detail below, some of the variable-width interfaces (VWIFs) enables a data path between core block  150  and the corresponding MGT to have a selectable data path width. For example, variable-width interface VWIF  111  enable data paths to core block  150  having widths of N, 2N, 4N or 8N, where N is an integer. Both the transmit and receive data paths between VWIF  111  and MGT  110  have widths equal to M, where M is an integer. In the examples described below, M is equal to 2N, although this is not necessary. 
   In other embodiments of the present invention, the VWIFs may connect the core block  150  to I/Os, e.g.,  122  and  124 , and/or a processor block  130  to one or more CLBs  132  in the core block  150 . VWIF  124  has data paths to/from core block  150  having variable widths of N, 2N, 4N or 8N and data paths to/from I/O  122  having fixed data width M. VWIF  128  has data paths to/from core block  150  having variable widths of N, 2N, 4N or 8N and data paths to/from I/O  122  having variable widths of N, 2N, 4N or 8N. VWIF  134  has data paths to/from CLBs  132  in core block  150  having variable widths of N, 2N, 4N or 8N and data paths to/from processor block  130 , embedded in core block  150 , having fixed data width M. In yet other embodiments processor block  130  may be replaced by a digital signal processor (DSP), a Block random access memory (BRAM), RAM, non-volatile memory, one or more CLBs, and application specific integrated circuit or other hardwired circuitry. 
     FIG. 2-1  is a block diagram illustrating multi-gigabit transceiver  110  and variable-width interface  111  in accordance with one embodiment of the present invention. MGT  110  includes a physical media access (PMA) sublayer  201 , which includes a serializer/deserializer (SERDES)  211  having a 20-bit wide serializer data input port  212 , a 1-bit wide serializer data output port  213 , a 1-bit wide deserializer data input port  214 , and a 20-bit wide deserializer data output port  215 . MGT  110  also includes a physical coding sublayer (PCS)  202 , which includes transmit processing block  221  and receive processing block  222  coupled to the 20-bit wide serializer data input port  212  and the 20-bit wide deserializer data output port  215 , respectively. Although MGTs have not previously been included on programmable logic devices, the various elements of MGTs are well known to those of ordinary skill in the art. 
   Transmit processing block  221  includes a 16-bit wide transmit data input bus  231 , and receive processing block  222  includes a 16-bit wide receive data output bus  232 . Thus, in the described embodiment, M is equal to 16. The widths of transmit data input bus  231  and receive data output bus  232  are fixed in the described embodiment. Transmit data input bus  231  and receive data output bus  232  are coupled to variable-width interface  111 . More specifically, transmit data input bus  231  is coupled to transmit variable-width interface  241 , and receive data output bus  232  is coupled to receive variable-width interface  242 . Both transmit variable-width interface  241  and receive variable-width interface  242  are coupled to the programmable interconnect resources  250  of core block  150 . 
   In accordance with one embodiment, variable-width interface  111  supports a variable-width transmit data path  251 , which is created from programmable interconnect resources  250 , having a width of 8-bits, 16-bits, 32-bits or 64-bits. Similarly, variable-width interface  111  supports a variable-width receive data path  252 , which is created from programmable interconnect resources  250 , having a width of 8-bits, 16-bits, 32-bits or 64-bits. Thus, in the described embodiment, N is equal to 8. The variable-width data paths  251 – 252  can be controlled to have the same width, or different widths, in different embodiments of the present invention. Advantageously, the variable-width data paths  251 – 252  can have a smaller width, the same width, or a wider width with respect to the width of data paths  231 – 232 . This provides flexibility in operating PLD  100 . 
   Simplified representations of transmit variable-width interface  241  and receive variable-width interface  242  will now be described in more detail. As described above, M is equal to 16 and N is equal to 8 in the example illustrated by  FIG. 2-1 . However, the following simplified examples describe a transmit variable-width interface and a receive variable-width interface having a width M equal to two and a width N equal to 1. Given these examples, one of ordinary skill can easily expand these interfaces to create larger interfaces, such as the one defined by  FIG. 2-1 . With N equal to 1, variable-width data paths  251 – 252  can have widths equal to 1-bit, 2-bits, 4-bits and 8-bits. With M equal to 2, fixed-width data paths  231 – 232  have widths equal to 2-bits.  FIGS. 3A ,  3 B,  3 C and  3 D show the clock waveforms (CLK 1248 ) used to control the variable-width 1-bit, 2-bit, 4-bit and 8-bit data paths, respectively, as well as the clock waveform (CLK 2 ) used to control the fixed-width 2-bit data paths within variable-width interface  111 . The waveforms shown in  FIGS. 3K–3D  indicate not only the relative frequencies of the two clock signals CLK 2  and CLK 1248 , but also their phase relationship. 
   The described design assumes that all flip-flops (described below) in transmit variable-width interface  241  and receive variable-width interface  242  are positive edge triggered. The described design also assumes that in order to eliminate flip-flop hold time as a critical design issue, it is required that rising (positive) edges of the CLK 2  and CLK 1248  signals are not aligned. The latter requirement is met by defining the clock waveforms CLK 2  and CLK 1248  such that the rising edges of the slower clock signal are aligned with falling edges of the faster clock signal. In the case of the 2-bit data path ( FIG. 3B ), either clock signal CLK 2  or clock signal CLK 1248  may be regarded as the “faster” or “slower” clock signal for the purpose of this requirement. 
   An alternative embodiment of  FIG. 2-1  is shown in  FIG. 2-2 . The MGT  110  of  FIG. 2-1  is replaced by a transceiver  312  having a SERDES circuit  314 . The transceiver  312  is any conventional transceiver as known to one of ordinary skill in the arts. There is a 1-bit serial input and a 1-bit serial output into/out of transceiver  312  as shown by label  310 . Data bus or data path  324  has a width of M 1  bits. Data bus or data path  326  has a width of M 2  bits. M 1  and M 2  are positive integers. Data buses  324  and  326  may have fixed or variable widths and may be created using programmable interconnect resources. Data bus  324  is coupled to transmit (TX) variable-width interface (I/F)  320  (similar to TX variable-width I/F  241 ), and data bus  326  is coupled to receive (RX) variable-width interface (I/F)  322  (similar to RX variable-width I/F  242 ). Both transmit variable-width interface  320  and receive variable-width interface  322  are coupled to core block  150  via the programmable interconnect resources that create data bus  332  with width of N 1  bits and data bus  334  with width of N 2  bits, where N 1  and N 2  are positive integers. 
   In one example of the alternative embodiment, N 1  equals N 2 , and N 1  has a width selectable from a group having widths of 8-bits, 16-bits, 32-bits, and 64-bits. M 1  equals M 2 , and M 1  has a width selectable from a group having widths of 16-bits and 32-bits. The TX Variable Width I/F  320  and the RX Variable Width I/F  322  may be included in core block  150 , and data buses  324  and/or  326  may be created from programmable interconnect resources to be either  16  or  32  bits. In other embodiments, M 1 , M 2 , N 1  and N 2  have various combinations of positive integers and fixed or variable data widths. In an alternative embodiment the TX Variable Width I/F  320  and the RX Variable Width I/F  322  may be hardwired circuitry. In yet another embodiment, the TX Variable Width I/F  320  and the RX Variable Width I/F  322  may be combined into one module, buses  324  and  326  may be combined into a bi-directional bus, and buses  332  and  334  may be combined into another bi-directional bus. 
   Yet another embodiment of the present invention is shown in  FIG. 2-3 . Two modules  340  and  360  are connected together by data width converters  348  and  350 . The modules  340  and  360  and data width converters  348  and  350  are located on an integrated circuit having programmable logic and programmable interconnections. Modules  340  and  360 , for example, may include CLBs, serdes circuitry, a transceiver, an I/O module, an embedded microprocessor core, a hardwired digital signal processing core, or other programmable and/or hardwired circuitry. The data width converters  348  and  350  may be hardwired or formed using programmable logic. In addition the data width converters  348  and  350  may be combined into one data width converter circuit. Also buses  346  and  348  may be combined into one bi-directional bus and buses  352  and  354  may be combined into another bi-directional bus. The data width converter, e.g.,  348  or  350 , receives a first fixed or variable data width and converts the data width to a second fixed or variable data width. The data width conversion circuitry used has been explained earlier with reference to  FIGS. 2-1  and  2 - 2 . 
   Module  340  has an input port IN  342  and an output port OUT  344 . For illustration purposes, let module  340  be an embedded microprocessor such as in Virtex II Pro™ FPGA from Xilinx Corp. of San Jose, CA. Bus  346  of width M 3  may be an input data bus into IN  342 , and bus  348  of width M 4  may be an output address bus from OUT  344  of the microprocessor. Module  360  in this example is part of the FPGA&#39;s programmable logic fabric, which may include a block random access memory (BRAM). Module  360  has output port OUT  362  and input port IN  364 , which may represent the address (IN  364 ) to the BRAM and the data (OUT  362 ) retrieved from the address. IN  364  receives the address from bus  354  of width N 4  and OUT  362  sends data to bus  352  of width N 3 . N 3 , N 4 , M 3 , and M 4  are positive integers. 
   Data width converter  350  receives the address on bus  348  of width M 4  and converts it to an address on bus  354  of width N 4 . The address, in this example, on bus  348  is typically a fixed data width, although in cases of other types of modules, bus  348  can be of a fixed or variable data width. The address on bus  354  is of a fixed or variable data width. Module  360  receives the address and retrieves the data at the address from BRAM. The data is then put on bus  352  of fixed or variable width N 3  and then converted to typically a fixed width M 3  on bus  346  by data converter  348  in order to be used by the microprocessor in module  340 . Again bus  346  may be fixed or variable for other types of modules  340 . 
   In a programmable FPGA environment, the clock waveforms defined in  FIGS. 3A–3D  may be generated without additional external components using a single digital clock manager DCM ( FIG. 1 ) located on PLD  100 . Each DCM is similar in functionality to a phase-locked loop (PLL). 
   Transmit Interface 
     FIG. 4  is a circuit diagram of a transmit variable-width interface  400  in accordance with one embodiment of the present invention. This interface  400  roughly corresponds with transmit variable-width interface  241  illustrated in  FIG. 2-1 . Transmit variable-width interface  400  includes flip-flops A 00 –A 7 , multiplexers MO–M 1 , flip-flops B 0 –B 1  and half-cycle delay  401 . Flip-flops A 00 –A 01  receive input data signal D[0], and flip-flops A 7 –A 1  receive input data signals D[7:1], respectively, from a data path corresponding to variable-width data path  251 . Flip-flops A 00 –A 7  are clocked by the CLK 1248  signal, and provide output data signals D 00 –D 7 , respectively. Multiplexer M 0  receives data values D 00 , D 2 , D 4  and D 6  on the “00”, “01”, “11” and “10” input terminals, respectively. Multiplexer M 0  is controlled by control signals S 1  and S 0 . Multiplexer M 1  receives data values D 01 , D 1 , D 3 , D 5  and D 7  on the “100”, “000”, “001”, “011” and “010” input terminals, respectively. Multiplexer M 1  is controlled by control signals S 2 , S 1  and S 0 . Multiplexers M 0  and M 1  route data signals to flip-flops BO and B 1 , respectively. Flip-flops B 0  and B 1  are clocked in response to the CLK 2  signal, and provide the output signals P 0  and P 1 , respectively. 
     FIG. 5  is a circuit diagram of a transmit width control circuit  500  used to control transmit variable-width interface  400  of  FIG. 4 . Transmit width control circuit  500  generates the control signals required to operate transmit variable-width interface  400 . Transmit width control circuit  500  includes OR gates  501 – 502 , AND gates  511 – 514  and inverters  521 – 522 , which are configured as illustrated to generate the enable signals E 4   —   7 , E 2   —   3 , E 1 , E 01 , and E 00  and the select signals S 2 , S 1  and S 0 . 
   The data inputs to the transmit variable-width interface  400  include D[7:0] (for the 8-bit data path), D[3:0] (for the 4-bit data path), D[1:0] (for the 2-bit data path), and D[0] (for the 1-bit data path). The clock inputs to transmit variable-width interface  400  include the CLK 1248  clock signal (for the input variable-width data path), and the CLK 2  signal (for the output 2-bit data path). The control inputs to interface  400  include width control signals X 1 , X 2 , X 4 , and X 8  (for variable data-width selection). One and only one of width control signals X 1 , X 2 , X 4  or X 8  is set to a logic high (“1”) value, thereby identifying the selected data path width as 1-bit, 2-bits, 4-bits or 8-bits, respectively. Although the X 2  control signal is not directly used in the described example, it is understood that this control signal X 2  can be used in other variations. Transmit variable-width interface  400  provides a 2-bit output signal P[1:0]. 
   Transmit variable-width interface  400  and control circuit  500  operate as follows. First, the user determines the desired width of the data path into interface  400 . The values of the width control signals X 1 , X 2 , X 4  and X 8 ; the CLK 1248  signal; and the input data values are then determined by this desired width. Table 1 below summarizes the values of the width control signals, the CLK 1248  signal, and the input data values for the selected widths of 1-bit, 2-bits, 4-bits and 8-bits. 
   
     
       
             
             
             
             
             
             
             
             
           
         
             
                 
               TABLE 1 
             
             
                 
                 
             
             
                 
               Width 
               X8 
               X4 
               X2 
               X1 
               CLK1248 
               Data 
             
             
                 
                 
             
           
           
             
                 
               1-bit 
               0 
               0 
               0 
               1 
               FIG. 3A 
               D[0] 
             
             
                 
               2-bits 
               0 
               0 
               1 
               0 
               FIG. 3B 
               D[1:0] 
             
             
                 
               4-bits 
               0 
               1 
               0 
               0 
               FIG. 3C 
               D[3:0] 
             
             
                 
               8-bits 
               1 
               0 
               0 
               0 
               FIG. 3D 
               D[7:0] 
             
             
                 
                 
             
           
        
       
     
   
   The CLK 1248 D clock signal is generated as follows. Half cycle delay flip-flop  401  includes a clock terminal coupled to receive the CLK 2  signal, a data input terminal coupled to receive the CLK 1248  signal, and an enable terminal coupled to receive the X 8  width control signal. If the X 8  width control signal has a logic “0” value (i.e., during 1-bit, 2-bit and 4-bit operation), then the CLK 1248 D signal is held at a reset value of “0”. However, if the X 8  width control signal has a logic “1” value, then flip-flop  401  is enabled. In this case, delay flip-flop  401  causes the CLK 1248 D signal to lag the CLK 1248  signal by one half cycle of the CLK 2  signal.  FIG. 6  is a waveform diagram illustrating the relationship between the CLK 2 , CLK 1248  and CLK 1248 D signals when the X 8  width control signal has a logic “1” value. 
   The various widths of transmit variable-width interface  400  will now be described in detail. 
   1-Bit Data Path 
   When transmit variable-width interface  400  is configured to have a 1-bit width, the X 8 , X 4 , X 2 , X 1  signals have values of (0,0,0,1) as illustrated in Table 1. In this case, transmit width control circuit  500  generates enable signals E 4   —   7 , E 2   —   3 , E 1 , E 01  and E 00 , and select signals S 2 , S 1  and S 0  as illustrated in Table 2. Note that the symbol “#” identifies the inverse of a signal. Also note that the enable signals are labeled to identify the flip-flops A 00 –A 7  ( FIG. 4 ) that they enable. Thus, enable signal E 4   —   7  enables flip-flops A 4 –A 7 , enable signal E 2   —   3  enables flip-flops A 2 –A 3 , enable signal E 1  enables flip-flop A 1 , enable signal E 01  enables flip-flop A 01 , and enable signal E 00  enables flip-flop A 00 . 
   
     
       
             
             
             
             
             
             
             
             
           
         
             
               TABLE 2 
             
             
                 
             
             
               E4 — 7 
               E2 — 3 
               E1 
               E01 
               E00 
               S2 
               S1 
               S0 
             
             
                 
             
           
           
             
               0 
               0 
               0 
               CLK2 
               CLK2# 
               1 
               0 
               0 
             
             
                 
             
           
        
       
     
   
   Turning to  FIG. 4 , these enable and select values have the following effect in transmit variable-width interface  400 . The logic “0” enable signals E 4   —   7 , E 2   —   3  and E 1  disable flip-flops A 1 –A 7 . Enable signals E 01  and E 00  alternately enable flip-flops A 00  and A 01  during alternate half-cycles of the CLK 2  signal. Each time that flip-flop A 01  is enabled, a rising edge of the CLK 1248  signal causes the applied 1-bit data value D[0] to be latched into flip-flop A 01 , and provided as data signal D 01 . The data signal D 01  is applied to the “100” input terminal of multiplexer M 1 . Data signal D 01  is routed through multiplexer M 1  to flip-flop B 1  in response to select signals S 2 , S 1 , S 0 , which have a value of (1,0,0). 
   Similarly, each time that flip-flop A 00  is enabled, a rising edge of the CLK 1248  signal causes the applied 1-bit data value D[0] to be latched into flip-flop A 00 , and provided as output signal D 00 . Data signal D 00  is applied to the “00” input terminal of multiplexer M 0 . Data signal D 00  is routed through multiplexer M 0  to flip-flop B 0  in response to select signals S 1  and S 0 , which have a value of (0,0). 
   Flip-flops B 0  and B 1  are clocked in response to the rising edges of the CLK 2  signal, thereby providing the data signals D 00  and D 01  as output signals P 0  and P 1 , respectively. The timing of transmit variable-width interface  400  for a 1-bit data path is illustrated in FIG.  7 A. Note that the offset between the rising edges of the CLK 1248  and the CLK 2  signals (which is equal to half the period of the CLK 1248  clock signal) allows the interface  400  to exhibit adequate set-up and hold times even if the CLK 1248  and CLK 2  signals exhibit small amounts of skew. 
   2-Bit Data Path 
   When transmit variable-width interface  400  is configured to have a 2-bit width, the X 8 , X 4 , X 2 , X 1  signals have values of (0,0,1,0) as illustrated in Table 1. In this case, width control circuit  500  generates enable signals E 4   —   7 , E 2   —   3 , E 1 , E 01  and E 00 , and select signals S 2 , S 1  and S 0  as illustrated in Table 3. 
   
     
       
             
             
             
             
             
             
             
             
             
           
         
             
                 
               TABLE 3 
             
             
                 
                 
             
             
                 
               E4 — 7 
               E2 — 3 
               E1 
               E01 
               E00 
               S2 
               S1 
               S0 
             
             
                 
                 
             
           
           
             
                 
               0 
               0 
               1 
               0 
               1 
               0 
               0 
               0 
             
             
                 
                 
             
           
        
       
     
   
   Turning to  FIG. 4 , these enable and select values have the following effect in transmit variable-width interface  400 . The logic “0” enable signals E 4   —   7 , E 2   —   3 , and E 01  disable flip-flops A 01  and A 2 –A 7 . Enable signals E 1  and E 00  enable flip-flops A 1  and A 00 , respectively. Each rising edge of the CLK 1248  signal causes the bits D[1] and D[0] of the applied 2-bit data value D[1:0] to be latched into flip-flops A 1  and A 00 , and provided as data signals D 1  and D 00 , respectively. The data signal D 1  is applied to the “000” input terminal of multiplexer M 1 . Data signal D 1  is routed through multiplexer M 1  to flip-flop B 1  in response to select signals S 2 , S 1 , S 0 , which have a value of (0,0,0). 
   Similarly, data signal D 00  is applied to the “00” input terminal of multiplexer MO. Data signal D 00  is routed through multiplexer M 0  to flip-flop B 0  in response to select signals S 1  and S 0 , which have a value of (0,0). 
   Flip-flops B 0  and B 1  are clocked in response to the rising edges of the CLK 2  signal, thereby providing the data signals D 1  and D 00  as output signals P 0  and P 1 , respectively. The timing of transmit variable-width interface  400  for a 2-bit data path is illustrated in  FIG. 7B . Note that the offset between the rising edges of the CLK 1248  and the CLK 2  signals (which is equal to half the period of the CLK 1248  clock signal) allows the interface  400  to exhibit adequate set-up and hold times even if the CLK 1248  and CLK 2  signals exhibit small amounts of skew. 
   4-Bit Data Path 
   When transmit variable-width interface  400  is configured to have a 4-bit width, the X 8 , X 4 , X 2 , X 1  signals have values of (0,1,0,0) as illustrated in Table 1. In this case, width control circuit  500  generates enable signals E 4   —   7 , E 2   —   3 , E 1 , E 01  and E 00 , and select signals S 2 , S 1  and S 0  as illustrated in Table 4. 
   
     
       
             
             
             
             
             
             
             
             
           
         
             
               TABLE 4 
             
             
                 
             
             
               E4 — 7 
               E2 — 3 
               E1 
               E01 
               E00 
               S2 
               S1 
               S0 
             
             
                 
             
           
           
             
               0 
               1 
               1 
               0 
               1 
               0 
               0 
               CLK1248 
             
             
                 
             
           
        
       
     
   
   Turning to  FIG. 4 , these enable and select values have the following effect in transmit variable-width interface  400 . The logic “0” enable signals E 4   —   7  and E 01  disable flip-flops A 01  and A 4 –A 7 . Enable signals E 2   —   3 , E 1  and E 00  enable flip-flops A 3 , A 2 , A 1  and A 00 . Each rising edge of the CLK 1248  signal causes the bits D[3], D[2], D[1] and D[0] of the applied 4-bit data value D[3:0] to be latched into flip-flops A 3 , A 2 , A 1 , and A 00 , and provided as data signals D 3 , D 2 , D 1  and D 00 , respectively. The data signals D 3  and D 1  are applied to the “001” and “000” input terminals of multiplexer M 1 . The data signals D 2  and D 00  are applied to the “01” and “00” input terminals of multiplexer M 0 . 
   When the CLK 1248  signal has a value of “1”, data signals D 3  and D 2  are routed through multiplexers M 1  and M 0 , respectively, to flip-flops B 1  and B 0 , respectively, in response to select signals S 2 , S 1 , S 0 , which have a value of (0,0,1). 
   When the CLK 1248  signal has a value of “0”, data signals D 1  and D 00  are routed through multiplexers M 1  and M 0 , respectively, to flip-flops B 1  and B 0 , respectively, in response to select signals S 2 , S 1 , S 0 , which have a value of (0,0,0). 
   Flip-flops B 0  and B 1  are clocked in response to the rising edges of the CLK 2  signal, thereby providing the data signals D 3  and D 2  as output signals P 0  and P 1 , respectively, in response to a rising edge of the CLK 2  signal, and providing the data signals D 1  and D 00  as output signals P 0  and P 1 , respectively, in response to the next rising edge of the CLK 2  signal. The timing of transmit variable-width interface  400  for a 4-bit data path is illustrated in  FIG. 7C . Note that the offset between the rising edges of the CLK 1248  and the CLK 2  signals (which is equal to one quarter of the period of the CLK 1248  clock signal) allows the interface  400  to exhibit adequate set-up and hold times even if the CLK 1248  and CLK 2  signals exhibit small amounts of skew. 
   8-Bit Data Path 
   When transmit variable-width interface  400  is configured to have an 8-bit width, the X 8 , X 4 , X 2 , X 1  signals have values of (1,0,0,0) as illustrated in Table 1. In this case, width control circuit  500  generates enable signals E 4   —   7 , E 2   —   3 , E 1 , E 01  and E 00 , and select signals S 2 , S 1  and S 0  as illustrated in Table 5. 
   
     
       
             
             
             
             
             
             
             
             
           
         
             
               TABLE 5 
             
             
                 
             
             
               E4 — 7 
               E2 — 3 
               E1 
               E01 
               E00 
               S2 
               S1 
               S0 
             
             
                 
             
           
           
             
               1 
               1 
               1 
               0 
               1 
               0 
               CLK1248 
               CLK1248D 
             
             
                 
             
           
        
       
     
   
   Turning to  FIG. 4 , these enable and select values have the following effect in transmit variable-width interface  400 . The logic “0” enable signal E 01  disables flip-flop A 01 . The logic “1” enable signals E 4   —   7 , E 2   —   3 , E 1  and E 00  enable flip-flops A 1 –A 7  and A 00 . Each rising edge of the CLK 1248  signal causes the bits D[7], D[6], D[5], D[4], D[3], D[2], D[1] and D[0] of the applied 8-bit data value D[7:0] to be latched into flip-flops A 7 , A 6 , A 5 , A 4 , A 3 , A 2 , A 1 , and A 00 , and provided as data signals D 7 , D 6 , D 5 , D 4 , D 3 , D 2 , D 1  and D 00 , respectively. The data signals D 7 , D 5 , D 3  and D 1  are applied to the “010”, “011”, “001” and “000” input terminals of multiplexer M 1 , respectively. The data signals D 6 , D 4 , D 2  and D 00  are applied to the “10”, “11”, “01” and “00” input terminals of multiplexer M 0 , respectively. 
   The timing of transmit variable-width interface  400  for an 8-bit data path is illustrated in  FIG. 7D . At time T 0 , the rising edge of the CLK 1248  signal causes the data values D[7:0] (i.e., A–H) to be latched into flip-flops A 7 –A 1  and A 00  as data signals D 7 –D 1  and D 00 . Prior to time T 1 , the CLK 1248  signal has a logic “1” value and the CLK 1248 D signal has a logic “0” value. As a result, the S 2 , S 1 , S 0  signals have a value of (0,1,0), thereby routing data signal D 7  (i.e., A) and data signal D 6  (i.e., B) through multiplexers M 1  and M 0 , respectively, to flip-flops B 1  and B 0 , respectively. At time T 1 , the rising edge of the CLK 2  signal causes these data signals A and B to be latched into flip-flops B 1  and B 0 , respectively, and provided as output signals P 1  and P 0 . 
   Just prior to time T 2 , the CLK 1248  signal has a logic “1” value and the CLK 1248 D signal has a logic “1” value. As a result, the S 2 , S 1 , S 0  signals have a value of (0,1,1), thereby routing data signal D 5  (i.e., C) and data signal D 4  (i.e., D) through multiplexers M 1  and M 0 , respectively, to flip-flops B 1  and B 0 , respectively. At time T 2 , the rising edge of the CLK 2  signal causes these data signals C and D to be latched into flip-flops B 1  and B 0 , respectively, and provided as output signals P 1  and P 0 . 
   Just prior to time T 3 , the CLK 1248  signal has a logic “0” value and the CLK 1248 D signal has a logic “1” value. As a result, the S 2 , S 1 , S 0  signals have a value of (0,0,1), thereby routing data signal D 3  (i.e., E) and data signal D 2  (i.e., F) through multiplexers M 1  and M 0 , respectively, to flip-flops B 1  and B 0 , respectively. At time T 3 , the rising edge of the CLK 2  signal causes these data signals E and F to be latched into flip-flops B 1  and B 0 , respectively, and provided as output signals P 1  and P 0 . 
   Just prior to time T 4 , the CLK 1248  signal has a logic “0” value and the CLK 1248 D signal has a logic “0” value. As a result, the S 2 , S 1 , S 0  signals have a value of (0,0,0), thereby routing data signal D 1  (i.e., G) and data signal D 00  (i.e., H) through multiplexers M 1  and M 0 , respectively, to flip-flops B 1  and B 0 , respectively. At time T 4 , the rising edge of the CLK 2  signal causes these data signals G and H to be latched into flip-flops B 1  and B 0 , respectively, and provided as output signals P 1  and P 0 . 
   This process is repeated for the next 8-bit data value (i.e., data signals I-P), as illustrated. Note that the offset between the rising edges of the CLK 1248  and the CLK 2  signals (which is equal to one eighth of the period of the CLK 1248  clock signal) allows transmit variable-width interface  400  to exhibit adequate set-up and hold times even if the CLK 1248  and CLK 2  signals exhibit small amounts of skew. 
   In the foregoing manner, transmit variable-width interface  400  supports variable data widths of 1-bit, 2-bits, 4-bits and 8-bits in core block  150 , and a fixed data width of 2-bits in MGT  110 . 
   Receive Interface 
     FIG. 8  is a circuit diagram of a receive variable-width interface  800  in accordance with one embodiment of the present invention. This interface  800  roughly corresponds with receive variable-width interface  242  illustrated in  FIG. 2-1 . Receive variable-width interface operates in response to clock signals CK 2  and CK 1248 . These clock signals CK 2  and CK 1248  are different signals than the clock signals CLK 2  and CLK 1248  described above. However, for purposes of the present description, clock signals CK 2  and CK 1248  have the same phase relationships as clock signals CLK 2  and CLK 1248 , respectively, illustrated in  FIGS. 3A–3D . 
   Receive variable-width interface  800  includes flip-flops J 2 –J 7 , multiplexer M 2 , flip-flops K 0 –K 7  and half-cycle delay  801 . Flip-flops J 2 , J 4  and J 6  receive input data signal Q[0], and flip-flops J 3 , J 5  and J 7  receive input data signal Q[1], from a data path corresponding to fixed width data path  232  ( FIG. 2-1 ). Flip-flops J 2 –J 7  are clocked by the CK 2  signal, and provide output data signals R 2 –R 7 , respectively. Multiplexer M 2  receives data signals Q 0 , R 2  and R 3  on the “0-”, “10” and “11”, input terminals, respectively. Multiplexer M 2  is controlled by control signals T 1  and T 0 . Multiplexer M 2  routes a data signal R 0  to flip-flop KO. Data signals R 1 –R 7  are provided to flip-flops K 1 –K 7 , respectively. Flip-flops K 7 –K 0  are clocked in response to the CK 1248  signal, and provide the output signals R[7:0], respectively. 
     FIG. 9  is a receive width control circuit  900  used to control receive variable-width interface  800  of  FIG. 8 . Receive width control circuit  900  generates the control signals required to operate receive variable-width interface  800 . Receive width control circuit  900  includes inverters  901 – 903 , AND gates  911 – 914 , and OR gates  921 – 922 , which are configured as illustrated. 
   A 2-bit input data signal Q[1:0] is provided to interface  800 . The data outputs of the variable-width data path include R[7:0] (for the 8-bit data path), R[3:0] (for the 4-bit data path), R[1:0] (for the 2-bit data path), and R[0] (for the 1-bit data path). The clock inputs to receive variable-width interface  800  include the CK 1248  clock signal (for the output variable-width data path), and the CK 2  signal (for the input 2-bit data path). The control inputs to interface  800  include width control signals Y 1 , Y 2 , Y 4 , and Y 8  (for variable data-width selection). One and only one of width control signals Y 1 , Y 2 , Y 4  or Y 8  is set to a logic high (“1”) value, thereby identifying the selected data path width as 1-bit, 2-bits, 4-bits or 8-bits, respectively. 
   Receive variable-width interface  800  and receive width control circuit  900  operate as follows. First, the user determines the desired width of the data path out of interface  800 . The values of the width control signals Y 1 , Y 2 , Y 4  and Y 8 , the CK 1248  signal, and the input data values are then determined by this desired width. Table 6 below summarizes the values of the width control signals, the CK 1248  signal, and the output data values for the selected widths of 1-bit, 2-bits, 4-bits and 8-bits. 
   
     
       
             
             
             
             
             
             
             
             
           
         
             
                 
               TABLE 6 
             
             
                 
                 
             
             
                 
               Width 
               Y8 
               Y4 
               Y2 
               Y1 
               CK1248 
               Data 
             
             
                 
                 
             
           
           
             
                 
               1-bit 
               0 
               0 
               0 
               1 
               FIG. 3A 
               R[0] 
             
             
                 
               2-bits 
               0 
               0 
               1 
               0 
               FIG. 3B 
               R[1:0] 
             
             
                 
               4-bits 
               0 
               1 
               0 
               0 
               FIG. 3C 
               R[3:0] 
             
             
                 
               8-bits 
               1 
               0 
               0 
               0 
               FIG. 3D 
               R[7:0] 
             
             
                 
                 
             
           
        
       
     
   
   Half cycle delay flip-flop  801  generates the CK 1248 D clock signal in the same manner as flip-flop  401  (See,  FIG. 6 ). The various widths of receive variable-width interface  800  will now be described in detail. 
   1-Bit Data Path 
   When receive variable-width interface  800  is configured to have a 1-bit output width, the Y 8 , Y 4 , Y 2 , Y 1  signals have values of (0,0,0,1) as illustrated in Table 6. In this case, width control circuit  900  generates enable signals EJ 6   —   7 , EJ 4   —   5 , EJ 2   —   3 , EK 4   —   7 , EK 2   —   3  and EK 1 , and select signals T 1  and T 0  as illustrated in Table 7. The enable signals are labeled to identify the flip-flops J 2 –J 7  and K 0 –K 7  ( FIG. 8 ) that they enable. Thus, enable signal EJ 6   —   7  enables flip-flops J 6  and J 7 , enable signal EJ 4   —   5  enables flip-flops J 4  and J 5 , enable signal EJ 2   —   3  enables flip-flops J 2  and J 3 , enable signal EK 1  enables flip-flop K 1 , enable signal EK 2   —   3  enables flip-flops K 2  and K 3 , and enable signal EK 4   —   7  enables flip-flops K 4 –K 7 . Flip-flop K 0  is always enabled. 
   
     
       
             
             
             
             
             
             
             
             
           
         
             
               TABLE 7 
             
             
                 
             
             
               EJ6 — 7 
               EJ4 — 5 
               EJ2 — 3 
               EK4 — 7 
               EK2 — 3 
               EK1 
               T1 
               T0 
             
             
                 
             
           
           
             
               0 
               0 
               1 
               0 
               0 
               0 
               1 
               CK2 
             
             
                 
             
           
        
       
     
   
   Turning to  FIG. 8 , these enable and select values have the following effect in receive variable-width interface  800 . The logic “0” enable signals EJ 6   —   7 , EJ 4   —   5 , EK 4   —   7 , EK 2   —   3  and EK 1  disable flip-flops J 4 –J 7  and K 1 –K 7 . The logic “1” enable signal EJ 2   —   3  enables flip-flops J 2  and J 3 . The received data signals Q 0  and Q 1  are latched into flip-flops J 2  and J 3  as data signals R 2  and R 3 , respectively, in response to rising edges of the CK 2  signal. Flip-flops J 2  and J 3  then provide these data signals R 2  and R 3  to the “10” and “11” input terminals, respectively, of multiplexer M 2 . The control signals T 1 –T 0  provided to multiplexer M 2  transition between values of “11” and “10” in response to the rising and falling edges of the CK 2  signal (see Table 7). Thus, multiplexer M 2  will route the R 3  data signal, and then the R 2  data signal, to flip-flop K 0  as the data signal R 0 . Flip-flop K 0  latches the R 0  data signal on rising edges of the CK 1248  clock signal, thereby providing the 1-bit R[0] data signal. The timing of receive variable-width interface  800  for a 1-bit data path is illustrated in  FIG. 10A . Note that the offset between the rising edges of the CK 1248  and the CK 2  signals (which is equal to half the period of the CK 1248  clock signal) allows the interface  800  to exhibit adequate set-up and hold times even if the CK 1248  and CK 2  signals exhibit small amounts of skew. 
   2-Bit Data Path 
   When receive variable-width interface  800  is configured to have a 2-bit output width, the Y 8 , Y 4 , Y 2 , Y 1  signals have values of (0,0,1,0) as illustrated in Table 6. In this case, width control circuit  900  generates enable signals EJ 6   —   7 , EJ 4   —   5 , EJ 2   —   3 , EK 4   —   7 , EK 2   —   3 , and EK 1 , and select signals T 1  and T 0  as illustrated in Table 8. 
   
     
       
             
             
             
             
             
             
             
             
           
         
             
               TABLE 8 
             
             
                 
             
             
               EJ6 — 7 
               EJ4 — 5 
               EJ2 — 3 
               EK4 — 7 
               EK2 — 3 
               EK1 
               T1 
               T0 
             
             
                 
             
           
           
             
               0 
               0 
               0 
               0 
               0 
               1 
               0 
               CK2 
             
             
                 
             
           
        
       
     
   
   Turning to  FIG. 8 , these enable and select values have the following effect in receive variable-width interface  800 . The logic “0” enable signals EJ 6   —   7 , EJ 4   —   5 , EJ 2   —   3 , EK 4   —   7 , and EK 2   —   3  disable flip-flops J 2 –J 7  and K 2 –K 7 . The logic “1” enable signal EK 1  enables flip-flop K 1 . The received data signal Q 1  is routed directly to flip-flop K 1  as data signal R 1 , and the received data signal Q 0  is routed to flip-flop K 0  through multiplexer M 2  as data signal R 0 . Note that the logic “0” value of the T 1  select signal causes multiplexer M 2  to route the Q 0  signal, regardless of the state of the CK 2  signal. That is, flip-flops J 2 –J 3  are bypassed in the 2-bit data path. The R 1  and R 0  data signals are latched into flip-flops K 1  and K 0 , respectively, in response to rising edges of the CK 1248  clock signal, and provided as 2-bit output signal R[1:0]. The timing of interface  800  for a 2-bit data path is illustrated in  FIG. 10B . 
   4-Bit Data Path 
   When receive variable-width interface  800  is configured to have a 4-bit output width, the Y 8 , Y 4 , Y 2 , Y 1  signals have values of (0,1,0,0) as illustrated in Table 6. In this case, width control circuit  900  generates enable signals EJ 6   —   7 , EJ 4   —   5 , EJ 2   —   3 , EK 4   —   7 , EK 2   —   3 , and EK 1 , and select signals T 1  and T 0  as illustrated in Table 9. 
   
     
       
             
             
             
             
             
             
             
             
           
         
             
               TABLE 9 
             
             
                 
             
             
               EJ6 — 7 
               EJ4 — 5 
               EJ2 — 3 
               EK4 — 7 
               EK2 — 3 
               EK1 
               T1 
               T0 
             
             
                 
             
           
           
             
               0 
               0 
               CK1248# 
               0 
               1 
               1 
               0 
               CK2 
             
             
                 
             
           
        
       
     
   
   Turning to  FIG. 8 , these enable and select values have the following effect in receive variable-width interface  800 . The logic “0” enable signals EJ 6   —   7 , EJ 4   —   5  and EK 4   —   7 , disable flip-flops J 4 –J 7  and K 4 –K 7 . The logic “1” enable signals EK 1  and EK 2   —   3  enable flip-flops K 1 –K 3 . The received data signals Q 1  and Q 0  are latched into flip-flops J 3  and J 2 , respectively, as data signals R 3  and R 2 , respectively, when the CK 1248  signal has a logic low value (CK 1248 #=“1”) and the CK 2  signal has a rising edge. On the same rising edge of the CK 2  signal, the Q 1  and Q 0  data signals transition to represent two new data values. These two new data values propagate directly to flip-flops K 1  and K 0  as data signals R 1  and R 0  well before the next rising edge of the CK 1248  signal. At the next rising edge of the CK 1248  signal, the R 3  and R 2  data values in flip-flops J 3  and J 2  are latched into flip-flops K 3  and K 2 , respectively, and the data values R 1  and R 0  are latched into flip-flops K 1  and K 0 , respectively. These data values are provided at the output terminals of flip-flops K 3 –K 0  as the output data signal R[3:0]. The timing of receive variable-width interface  800  for a 4-bit data path is illustrated in  FIG. 10C . 
   8-Bit Data Path 
   When receive variable-width interface  800  is configured to have a 8-bit output width, the Y 8 , Y 4 , Y 2 , Y 1  signals have values of (1,0,0,0) as illustrated in Table 6. In this case, width control circuit  900  generates enable signals EJ 6   —   7 , EJ 4   —   5 , EJ 2   —   3 , EK 4   —   7 , EK 2   —   3 , and EK 1 , and select signals T 1  and TO as illustrated in Table 10. 
   
     
       
             
             
             
             
             
             
             
             
           
         
             
               TABLE 10 
             
             
                 
             
             
               EJ6 — 7 
               EJ4 — 5 
               EJ2 — 3 
               EK4 — 7 
               EK2 — 3 
               EK1 
               T1 
               T0 
             
             
                 
             
           
           
             
               CLK — A 
               CLK — B 
               CLK — C 
               1 
               1 
               1 
               0 
               CK2 
             
             
                 
             
           
        
       
     
   
   In Table 10, CLK — A is equal to the logical AND of CK 1248 D and CK 1248 ; CLK — B is equal to the logical AND of CK 1248 D and CK 1248 #; and CLK — C is equal to the logical AND of CK 1248 # and CK 1248 D#. These clock signals are illustrated in  FIG. 11 . Turning to  FIG. 8 , these enable and select values have the following effect in receive variable-width interface  800 . The logic “1” enable signals EK 4   —   7 , EK 2   —   3  and EK 1  enable flip-flops K 1 –K 7 . The CLK — A, CLK — B and CLK — C signals sequentially enable flip-flop sets J 6 –J 7 , J 4 -J 5 , and J 2 –J 3 , respectively. Successive rising edges of the CK 2  signal (starting with the second rising edge of the CK 2  signal after a rising edge of the CK 1248  signal) latch data signals Q 1  and Q 0  into: flip-flops J 7  and J 6  (at time T 2  in  FIGS. 10D and 11 ); then flip-flops J 5  and J 4  (at time T 3  in  FIGS. 10D and 11 ); and then flip-flops J 3  and J 2  (at time T 4  in  FIGS. 10D and 11 ). The edge of the CK 2  signal that stores data signals Q 1  and Q 0  into flip-flops J 3  and J 2  also latches new values Q 1  and Q 0 , which propagate directly to flip-flops K 1  and K 0  sufficiently fast to satisfy the setup time requirements of R 1  and R 0 , prior to the next rising edge of the CK 1248  signal. The next rising edge of the CK 1248  signal then stores the data values R 7 –R 0  into flip-flops K 7 –K 0 , which are then provided as output data value R[7:0]. The timing of interface  800  for an 8-bit data path is illustrated in  FIG. 10D . 
   By changing the values of data width selectors Y 1 , Y 2 , Y 4  and Y 8 , interface  800  can be configured to operate using any of several supported data widths. Separate data width selectors may be provided for transmit variable-width interface  400  and receive variable-width interface  800 . In one embodiment using a programmable FPGA environment, interfaces  400  and  800  advantageously avoid the use of programmable resources for the implementation of these interfaces, thereby enabling these interfaces to be implemented in an efficient manner. In another embodiment the programmable resources of the FPGA may be used to allow use of the data-width converters for more applications. 
   Variations on the above implementations are possible. For example, the clock waveforms of  FIGS. 3A–3D  may be defined differently, depending on whether the data paths are positive-edge or negative-edge triggered, and whether it is required to avoid hold-time design issues. 
   The implementation of interface  400  described in connection with  FIGS. 4 and 5  assume that the input data value D[7:0] should be provided directly to flip-flop inputs. If it is permissible to go to flip-flop inputs via minimal logic (i.e., a multiplexer), then flip-flops A 1  and A 01  may be merged into a single flip-flop, with other suitable modifications to the design. Such modifications would include the addition of a multiplexer that provides either the D[1] or D[0] data signal to the merged flip-flop, depending on the configuration of the data path. 
   The implementation described in  FIGS. 8 and 9  makes certain assumptions about propagation delays from the source of Q[1:0]. Different assumptions might lead to not propagating Q[1:0] directly to flip-flops K 1  and K 0  for the 2-bit, 4-bit, and 8-bit data paths, or conversely, to bypassing flip-flops J 2  and J 3  for the 1-bit data path. Similarly, assumptions about propagation delays from P[1:0] in  FIG. 4  could lead to bypassing flip-flops B 1  and/or B 0  in some cases. 
   The implementation in  FIG. 8  defined the enable inputs so that each of flip-flops J 2 –J 7  is written at most once per CK 1248  cycle. An alternative design style would be to organize flip-flops J 2 –J 7  as a shift register, unconditionally loaded (shifted) by each rising edge of CK 2  and periodically loaded into flip-flops K 0 –K 7  by the rising edge of CK 1248 . It is also possible to use a shift register methodology in transmit variable-width interface  400  of  FIG. 4  as well. 
   In addition, interfaces  400  and  800  may be extended to support other data widths, or it may be constrained to support only a subset of the data widths. 
   Although the invention has been described in connection with several embodiments, it is understood that this invention is not limited to the embodiments disclosed, but is capable of various modifications, which would be apparent to a person skilled in the art. Logically equivalent but structurally different implementations are possible. Moreover, other variations in design style or detail may be possible. Thus, the invention is limited only by the following claims.