Abstract:
One embodiment of a long-distance synchronous bus includes a sending unit and a receiving unit. The sending unit and receiving unit are configured to use credit-based handshaking signals to regulate data flow between themselves. The receiving unit includes a skid buffer for storing data packets received from the sending unit. The sending unit transmits one data packet to the receiving unit for each credit in possession and consumes one credit for each such transmitted data packet. The receiving unit transmits one credit to the sending unit for each data packet that is read out of the skid buffer. In another embodiment, transmitted data may be broadcast to multiple receiving units by routing the data from the sending unit to the multiple receiving units and maintaining separate credit-based handshaking signals for each receiving unit.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   Embodiments of the present invention relate generally to integrated circuit design and more specifically to a long-distance synchronous bus. 
   2. Description of the Related Art 
   Complex integrated circuit designs are typically comprised of a plurality of logic blocks. As is often the case, a first logic block transmits data to a second logic block. There are several established methods of data flow control that are used in cases when the second logic block cannot always accept the data from the first logic block. One such method uses a valid and a busy signal. The valid signal is transmitted with the data and, when asserted, indicates to the second block that the data arriving at the second block is valid (i.e., should be accepted). The busy signal is asserted by the second logic block to indicate to the first logic block the second logic block cannot currently accept data. The first logic block responds by not transmitting any additional data until the busy signal is de-asserted. 
   For this method of flow control to work properly, there cannot be any excessive latency in the timing of the busy and valid signals. For example, assuming a synchronous system, the busy signal must be received by the first logic block within the same clock cycle as the data being currently being transmitted. If the busy signal is not received within the same clock cycle, then the data may be lost since the first logic block ends up sending the data when the second logic block is not accepting data. The timing requirements of the valid and busy signals limit the distance that the first logic block may transmit data to the second logic block to approximately the distance that a signal propagates during one-half a clock cycle. 
   In view of flow control signal timing requirements, one or more retiming stages may be used between logic blocks to increase the distance data can be transmitted. Generally, a retiming stage receives a data packet and a corresponding valid signal from a first logic block and then transmits both to a second logic block so long as the second logic block has not sent a busy signal to the first logic block. In a synchronous system, the signals and data packet are typically received by flops clocked by a clock signal. Since the valid and busy signals are timing critical signals, the retiming stage includes additional logic to decouple the valid and busy signals from the first logic block and the second logic block. The decoupling allows a busy signal sent from the second logic block to be detected so that a data packet transmitted by the first logic block can be held until the busy signal has cleared. 
   One drawback to this approach is that the additional logic adds propagation delays to the valid and busy signals. As is well-known, the distance that a signal travels between a first logic block and a second logic block during a clock cycle may be determined using a timing budget, where the sum of the propagation delays of the signal through system elements, such as logic components, and the propagation delay of the signal in the wire(s) between the first and second logic blocks is equal to the clock period. When a retiming stage disposed between the first and second logic blocks includes additional logic, the propagation delays through the additional logic reduce the amount of time left in the clock cycle for the signal to propagate through the wire(s) between the first and second logic blocks. Thus, the signal does is unable to travel as far during each clock cycle. Since the distance traveled is reduced, more retiming stages may be required between the first logic block and the second logic block, leading to increased design complexity and retiming stage area requirements. 
   Another drawback is that a logic designer may try to anticipate timing problems caused by long data paths between logic blocks by inserting retiming stages into the initial hardware design. However, if a retiming stage is not needed in the final design, then the overall design is burdened with excess logic, which unnecessarily consumes die area. 
   These drawbacks are exacerbated in cases where a sending logic block transmits data to more than one receiving logic block. 
   As the foregoing illustrates, what is needed in the art is a more flexible bus design that implements simpler retiming stages. 
   SUMMARY OF THE INVENTION 
   One embodiment of the present invention is system for transmitting data over a bus. The system comprises a sending unit that includes a sender controller, and a receiving unit that includes a receiver controller. The sender controller is configured to receive a credit transmitted by the receiver controller and, in response to receiving the credit, the sending unit is configured to transmit a data packet to the receiving unit. 
   One advantage of the disclosed system is that the sending unit is configured to transmit a data packet to the receiving unit only when the sender controller receives a credit from the receiving unit. Each credit indicates that a skid buffer included in the receiving unit has space available for an additional data packet, so the sending unit never transmits a data packet when the receiving unit does not have the capacity to handle an additional data packet. Thus, with the configuration of the present invention, the skid buffer does not overflow. Unlike the prior art approach, the architecture of the present invention does not rely on busy signals to indicate that the receiving unit is unable to accept additional data packets. Consequently, simpler retiming stages may reside in between the sending unit and the receiving unit since the retiming stages do not have to include logic for detecting busy signals transmitted by the receiving unit. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     So that the manner in which the above recited features of the present invention can be understood in detail, a more particular description of the invention, briefly summarized above, may be had by reference to embodiments, some of which are illustrated in the appended drawings. It is to be noted, however, that the appended drawings illustrate only typical embodiments of this invention and are therefore not to be considered limiting of its scope, for the invention may admit to other equally effective embodiments. 
       FIG. 1A  is a conceptual diagram of a long-distance synchronous bus, according to one embodiment of the invention; 
       FIG. 1B  is a conceptual diagram of the sending unit of  FIG. 1A , according on one embodiment of the invention; 
       FIG. 1C  is a conceptual diagram of the receiving unit of  FIG. 1A , according to one embodiment of the invention; 
       FIG. 2  is a flowchart of method steps for controlling data transmissions from the sending unit, according to one embodiment of the invention; 
       FIG. 3  is a flowchart of method steps for controlling credit transmissions from the receiving unit, according to one embodiment of the invention; 
       FIG. 4  is a conceptual diagram of the receiving unit of  FIG. 1A , according to an alternative embodiment of the invention; 
       FIG. 5  is a flowchart of method steps for controlling credit transmissions from the receiving unit of  FIG. 4 , according to an alternative embodiment of the invention; 
       FIG. 6  is a conceptual diagram a long-distance synchronous bus, according to one embodiment of the invention; 
       FIG. 7  is a conceptual diagram of a long-distance synchronous bus configured to transfer data to multiple receiving units, according to one embodiment of the invention; and 
       FIG. 8  is a flowchart of method steps for transmitting data to multiple receiving units, according to one embodiment of the invention. 
   

   DETAILED DESCRIPTION 
     FIG. 1A  is a conceptual diagram of a long-distance synchronous bus  100 , according to one embodiment of the invention. As shown, the long-distance synchronous bus  100  includes, without limitation, a sending unit  110  and a receiving unit  150 . The sending unit  110  includes a sender controller  130  and is configured to transmit data packets to the receiving unit  150  only when the sender controller  130  has an inventory of credits. A credit indicates to the sender controller  130  that the receiving unit  150  has the capacity to accept an additional data packet. The sender controller  130  “consumes” a credit for each data packet transmitted to the receiving unit  150 . Thus, the sender controller  130  is configured to track the credits received from the receiving unit  150  and consumed when data packets are transmitted to the receiving unit  150 . In addition, for each transmitted data packet, the sender controller  130  is configured to send a corresponding valid signal to the receiving unit  150 . As used herein, a “data packet” may be any fixed amount of data such as a bit, byte, word or the like. 
   The receiving unit  150  includes a skid buffer  155  and a receiver controller  160 . When a valid signal transmitted by the sender controller  130  is asserted, the receiving unit  150  receives the data packet corresponding to that valid signal and stores it temporarily in the skid buffer  155 . The receiver controller  160  monitors the skid buffer  155  and releases one credit through the credit for each data packet that is read out of the skid buffer  155 . The released credit is transmitted to the sending unit  110  and indicates that there is room in the skid buffer  155  to accommodate an additional data packet. 
     FIG. 1B  is a conceptual diagram of the sending unit  110  of  FIG. 1A , according on one embodiment of the invention. As previously described, the sender controller  130  controls the transmission of data packets to the receiving unit  150 . The sender controller  130  includes, without limitation, a credit counter  135  that is configured to track credits. The credit counter  135  is incremented for each credit received from the receiving unit  150  and is decremented for each data packet transmitted to the receiving unit  150 . As previously explained herein, each credit indicates that the skid buffer  155  has memory space available for one additional data packet. Therefore, the sender controller  130  is configured not to transmit data packets to the receiving unit  150  unless the credit counter  135  is greater than zero. 
     FIG. 1C  is a conceptual diagram of the receiving unit  150  of  FIG. 1A , according to one embodiment of the invention. The receiving unit  150  includes, without limitation, a receiver controller  160  and the skid buffer  155 . As previously described, the skid buffer  155  is configured to temporarily store data packets that are received from the sending unit  110 . The receiver controller  160  includes a release credit counter  165  that is configured to track the number of credits yet to be released to the sending unit  110 . The release credit counter  165  is incremented for each data packet read out of the skid buffer  155  and is decremented for each credit transmitted to the sending unit  110 . As described in further detail herein, the receiver controller  160  is configured to transmit credits to the sending unit  110  until the release credit counter  165  equals zero. 
   As persons skilled in the art will recognize, since the sending unit  110  is configured to transmit a data packet to the receiving unit  150  only when the credit counter  135  indicates that there is an available credit (i.e., the count is greater than zero) and each credit indicates that the skid buffer  155  has space available for an additional data packet, the skid buffer  155  never overflows. Further, the sending unit  110  never transmits a data packet when the receiving unit  150  does not have the capacity to handle an additional data packet. Thus, unlike the prior art approach, the architecture of the present invention does not rely on busy signals to indicate that the receiving unit  150  is unable to accept additional data packets. As described in further detail herein, one advantage of the present invention is that the retiming stages between a sending unit and a receiving unit can be simplified since they do not need to include the additional logic required to detect a transmitted busy signal as required of the prior art retiming stages. 
     FIG. 2  is a flowchart of method steps for controlling data transmissions from the sending unit  110 , according to one embodiment of the invention. Persons skilled in the art will recognize that any system configured to perform the method steps in any order is within the scope of the invention. 
   The method begins in step  202 , where the credit counter  135  is initialized to one. Since the credit counter  135  is initialized to one, the sending unit  110  is able to transmit one data packet to the receiving unit  150  when the data is available for transmission. In step  204 , the sender controller  130  determines if a credit has been received from the receiving unit  150 . If a credit has not been received, then the method proceeds to step  208 . If, however, a credit has been received, then in step  206 , the credit counter  135  is incremented. Again, the credit counter  135  tracks the inventory of credits available to the sender controller  130 . 
   In step  208 , the sender controller  130  determines if there is a data packet available to transmit. If there is no data packet available, then the method returns to step  204 . If, on the other hand, there is a data packet available, then in step  210 , the sender controller  130  determines if the credit counter  135  is greater than zero. If the credit counter  135  is not greater than zero, the sending unit  110  has no credits available, indicating that the receiving unit  150  is currently unable to accept an additional data packet. Therefore, in step  212 , the data packet and its corresponding valid signal are not transmitted. If, however, the credit counter  135  is greater than zero, then in step  214 , the sending unit  110  and the sender controller  130  transmit the data packet and the corresponding valid signal to the receiving unit  150 . In step  216 , the credit counter is decremented since a data packet was transmitted. The method then returns to step  204 . 
   The method of  FIG. 2  advantageously configures the sending unit  110  such that data packets are only transmitted when the sender controller  130  is certain that the data can be stored in the skid buffer  155  of the receiving unit  150 . The credit counter  135  enables the sender controller  130  to track the available space within the skid buffer  155 . 
     FIG. 3  is a flowchart of method steps for controlling credit transmissions from the receiving unit  150 , according to one embodiment of the invention. Persons skilled in the art will recognize that any system configured to perform the method steps in any order is within the scope of the invention. 
   The method begins with step  302 , where the receiver controller  160  waits for a first data packet and corresponding valid signal to be transmitted by the sending unit  110 . In step  304 , if the receiver controller  160  does not receive the first data packet and valid signal, then the method returns to step  302 . If, however, the receiver controller  160  receives the first data packet and valid signal, then in step  306 , the first data packet is stored in the skid buffer  155 . Next, in step  308 , the release credit counter  165  is initialized to a count of N−1, where N is the maximum number of data packets that the skid buffer  155  is configured to store. For example, if the skid buffer can accommodate four data packets, then N would equal four. As set forth below, the initialization of the release credit counter  165  enables the receiving unit  150  to transmit N−1 credits to the sending unit  110 , which would indicate to the sending unit  110  that there is N−1 packets worth of space available in the skid buffer  155 . 
   In step  310 , the receiver controller  160  determines if the release credit counter  165  is greater than zero. If the release credit counter  165  is not greater than zero, then no credits are released and transmitted to the sending unit  110 , and the method proceeds to step  316 . If, however, the release credit counter  165  is greater than zero, then there are one or more credits that may be released and transmitted to the sending unit  110 , and the method proceeds to step  312 . In step  312 , a credit is released to the sending unit  110 . In step  314 , the release credit counter  165  is decremented, and the method proceeds to step  316 . 
   In step  316 , the receiver controller  160  determines if a data packet has been read out of the skid buffer  155 . A data packet may be read out of the skid buffer  155  by a processor, a hardware unit such as a logic block, or the like. If a data packet has not been read out of the skid buffer  155 , then the method returns to step  310 . If, on the other hand, a data packet has been read out of the skid buffer  155 , then the skid buffer  155  has memory space available for an additional data packet. Therefore, in step  318 , the receiver controller  160  increments the release credit counter  165 . The method then returns to step  310 . 
   As persons skilled in the art will recognize, beginning with step  310 , the operation of the release credit counter  165  is decoupled from the storing of data packets in the skid buffer  155 . The release credit counter  165  is incremented only when a data packet is read out of the skid buffer  155  and decremented only when a credit is released to the sending unit  110 . As previously explained herein, the manner in which credits are released and transmitted to the sending unit  110  prevents the skid buffer  155  from overflowing. 
   Because to the initialization steps of  302 ,  304 ,  306  and  308 , the sending unit  110  may be designed without any knowledge of the size of the skid buffer  155  in the receiving unit  150 . As soon as the receiving unit  150  receives the first data packet from the sending unit  110 , the receiver controller  160  sets the release credit counter  165  to N−1. The N−1 credits released from the receiving unit  150 , in addition to the credit used to initialize the credit counter  135 , equals the total number of data packets, N, that may be stored in the skid buffer  155  at any given time. Hence, the same sending unit design may be used with different types receiving units having skid buffers of varying sizes. 
   Another advantage of the present invention is that, without retiming stages, the sending unit  110  is able to transmit data packets greater distances relative to the prior art design. Unlike the valid and busy signals of the prior art design, the valid and credit signals are not required to be generated and sent within the same clock cycle since the sending unit  110  never transmits a data packet unless there is an inventory of credits in the credit counter  135 . Consequently, without retiming stages, the valid signal has a full clock cycle to propagate to the receiving unit  150 , and, likewise, the credit has a full clock cycle to propagate to the sending unit  110 . Thus, the sending unit  110  and the receiving unit  150  may be spaced farther apart relative to prior art designs. 
     FIG. 4  is a conceptual diagram of the receiver  400  of  FIG. 1A , according to an alternative embodiment of the invention. The receiver  400  includes, without limitation, the receiver controller  160 , a skid buffer  402 , a read pointer  404 , a write pointer  406  and a credit pointer  408 . In this embodiment, the skid buffer  402  includes four locations where data packets may be stored. Other embodiments may include more or less than four such locations. The receiver controller  160  accesses the contents of the skid buffer  402  using the read pointer  404  and the write pointer  406 . Data is read from the skid buffer  402  by reading from the location indicated by the read pointer  404 . Similarly, data is stored in the skid buffer  402  by writing to the location indicated by the write pointer  406 . After data is written to or read from the skid buffer  402 , the associated pointer is advanced to the next skid buffer location. For example, after data is read from the skid buffer location indicated by the read pointer  404 , the read pointer  404  is advanced to the next location of the skid buffer  402 , as illustrated by the arrows in  FIG. 4 . Likewise, after data is written to the skid buffer location indicated by the write pointer  406 , the write pointer  406  is advanced to the next location of the skid buffer  402 , as also illustrated by the arrows in  FIG. 4 . 
   The credit pointer  408  is used to determine if credits should be released and transmitted to the sending unit  110 . If the credit pointer  408  is not pointing to the same location as the read pointer  404 , then a credit is released and the credit pointer  408  advanced to the next skid buffer location. This process is described in greater detail in  FIG. 5 . 
     FIG. 5  is a flowchart of method steps for controlling credit transmissions from the receiving unit of  FIG. 4 , according to an alternative embodiment of the invention. Persons skilled in the art will recognize that any system configured to perform the method steps in any order is within the scope of the invention. 
   The method begins in step  502 , where the receiver controller  160  waits for the first data packet and the corresponding valid signal to be transmitted by the sending unit  110 . If, in step  504 , the receiver controller  160  does not receive the first data packet and valid signal, then the method returns to step  502 . If, however, the receiver controller  160  receives the first data packet and the corresponding valid signal, then in step  505 , the credit pointer  408  is initialized to point to a location corresponding to the N−1 location, where N is the maximum number of data packets that can be stored in the skid buffer  155 . In this example, the skid buffer  402  has four locations where data packets transmitted by the sending unit  110  may be stored. After the first data packet and valid signal are received, the receiver controller  160  initializes the credit pointer  408  to point to the N−1 location, which is location “1” in the example of  FIG. 4 . In step  506 , the received data packet is written to the skid buffer  402 , and the write pointer  406  is advanced to the next skid buffer location. 
   As shown in  FIG. 5 , after step  506 , three processes run concurrently. The first concurrent process controls the release and transmission of credits to the sending unit  110 . This process begins in step  510 , where the receiver controller  160  compares the read pointer  404  to the credit pointer  408 . In step  512 , if the read pointer  404  and the credit pointer  408  are the same (i.e., they point to the same skid buffer location), then the first concurrent process returns to step  510 . If, on the other hand, the read pointer  404  is not the same as the credit pointer  408  (i.e., they point to different skid buffer locations), then credits may be released and transmitted to the sending unit  110 . In step  514 , a credit is released to the sending unit  110 , and the credit pointer  408  is advanced to the next skid buffer location. The first concurrent process then returns to step  510 . In this fashion, credits are released and the credit pointer  408  is advanced, until the credit pointer  408  and the read pointer  404  point to the same skid buffer location. 
   The second concurrent process controls the storing of received data packets in the skid buffer  402 . This process begins in step  520 , where the receiver controller  160  waits to receive a new data packet and corresponding valid signal. In step  522 , the receiver controller  160  determines if a new data packet and valid signal are received. If a new data packet and valid signal are not received, then the second concurrent process returns to step  520 . If, on the other hand, a new data packet and corresponding valid signal are received, then in step  524 , the receiver controller  160  stores the data packet in the skid buffer location indicated by the write pointer  406  and advances the write pointer  406  to the next skid buffer location. The second concurrent process then returns to step  520 . 
   The third concurrent process controls the reading of data packets from the skid buffer  402 . This process begins in step  550 , where the receiver controller  160  compares the read pointer  404  to the write pointer  406 . If in step  552 , the read pointer  404  and the write pointer  406  are pointing to the same skid buffer location, then there are no data packets that need to be read from the skid buffer  402 , and the third concurrent process returns to step  550 . If, however, the read pointer  404  and the write pointer  406  point to different skid buffer locations and the receiver controller  160  also determines that a hardware block or software process can accept the data packet, then a data packet is ready to be read from the skid buffer  402 . In step  554 , the receiver controller  160  reads out the data packet from the skid buffer location indicated by the read pointer  404  and advances the read pointer  404  to the next skid buffer location. The third concurrent process then returns to step  550 . 
   Again, in this alternative embodiment, the processes of storing data packets in the skid buffer  402 , reading those data packets from the skid buffer  402  and releasing and transmitting credits to the sending unit are decoupled from one another. A credit is released and transmitted only when the read pointer and the write pointer are pointing to different locations in the skid buffer, indicating that the skid buffer  402  has memory space available for one or more additional data packets. Since the sending unit transmits data only when it receives a credit from the receiving unit, there is no danger that the sending unit may transmit a data packet when memory space is not available in the skid buffer  402  for an additional data packet. 
     FIG. 6  is a conceptual diagram of a long-distance synchronous bus  600 , according to one embodiment of the invention. The long-distance synchronous bus  600  includes, without limitation, a sending unit  110 , a receiving unit  150  and an optional retiming stage  610 . Each of the sending unit  110 , the receiving unit  150  and the optional retiming stage  610  may include one or more flops  620 . The flop  620  typically is the last element a signal passes through as it leaves a logic block as well as the first element that a signal passes through as it enters a logic block. Those skilled in the art will recognize that flops  620  are simple logic elements used to register one or more signals with respect to a clock signal. 
   The optional retiming stage  610  may be implemented to increase the distance over which the signals (i.e., the data packets, valid signals and credits) between the sending unit  110  to the receiving unit  150  may be transmitted. For example, by including the optional retiming stage  610  in the signal path between the sending unit  110  and the receiving unit  150 , the distance over which the signals between the sending unit  110  and the receiving unit  150  can be transmitted is increased by approximately the distance those signals propagate in one clock cycle, less flop propagation and setup time. Further, additional optional retiming stage  610  may be included in the signal path between the sending unit  110  and the receiving unit  150  to create an even longer-distance synchronous bus. 
   In addition to the foregoing, the optional retiming stage  610  does not include any additional logic because, unlike prior art retiming stages, the optional retiming stage  610  is not charged with detecting busy signals transmitted by the receiving unit  150 . Since no additional logic is used in the optional retiming stage  610 , the propagation delays associated with the optional retiming stage  610  are minimized, thereby enabling the distances between the sending unit  110  and the optional retiming stage  610 , between any two optional timing stages  610 , and between the optional retiming stage  610  and the receiving unit  150  to be increased. Consequently, fewer overall retiming stages may be needed in the long-distance synchronous bus  600  relative to prior art bus designs. 
   The simplicity of the optional retiming stage  610  allows a physical designer to insert the optional retiming stage  610  into a design at any time. Such flexibility is quite advantageous because timing information related to signal transmissions oftentimes is not available until the design has been placed and/or routed by the physical designer. Since the prior art retiming stages are far more complicated than the optional retiming stage  610 , those retiming stages typically have to be included as part of the initial design. If timing information later indicates that a retiming stage is necessary, then a logic designer usually has to revisit the design, insert the required retiming stage and then the physical designer repeats the place and route cycle. Since the present invention allows a physical designer to add the optional retiming stages  610  on an as-needed basis, design time is reduced since the logic designer does not revisit the overall design. 
   In some embodiments, the number of flops  620  included in the round trip signal path may be used to determine the skid buffer size. Consider the long-distance synchronous bus  600  of  FIG. 6  without the optional retiming stage  610 . In such a case, only four flops are in the round trip signal path—two in the sending unit  110  and two in the receiving unit  150 . If each flop in the credit signal path contains a credit, and simultaneously, each flop in the valid and data path contains a data packet and a corresponding valid signal, then the skid buffer  155  should be sized to receive the two data packets that are in route from the sending unit  110  as well as the two data packets that the sending unit  110  may transmit after receiving the two credits in route from the receiving unit  150 . Thus, the skid buffer  155  should be sized to store four data packets. If the optional retiming stage  610  is used, then there are six flops in the round trip signal path and the skid buffer should be sized to store six data packets. 
     FIG. 7  is a conceptual diagram of a long-distance synchronous bus  700  configured to transfer data to multiple receiving units, according to one embodiment of the invention. The long-distance synchronous bus  700  includes, without limitation, a sending unit  710 , a first receiving unit  720 , a second receiving unit  722 , a third receiving unit  724 , a first optional retiming stage  730 , a second optional retiming stage  732 , and a third optional retiming stage  734 . Although only three receiving units are shown, alternative embodiments may include more or less than three receiving units. The sending unit  710 , the first receiving unit  720 , the second receiving unit  722  and the third receiving unit  724  are configured to transmit and receive data packets, valid signals and credits, as previously described herein. Specifically, the sending unit  710  is configured to transmit data packets and corresponding valid signals to the first receiving unit  720 , the second receiving unit  722  and the third receiving unit  724 , and the first receiving unit  720 , the second receiving unit  722  and the third receiving unit  724  are configured to transmit credits to the sending unit  710 . In one embodiment, the receiving units  720 ,  722 , and  724  have dedicated channels for the valid signals and credits. Only the channels for the data packets are common to all of the units. Each receiving unit  720 ,  722 , and  724  includes a skid buffer (not shown). 
   As also described previously herein, the optional retiming stages  730 ,  732  and  734  are used to increase the distance between the sending unit  710  and the receiving units  720 ,  722 , and  724 . In alternative embodiments, more or less than three optional retiming stages may be used. Unlike the optional retiming stage  610  of  FIG. 6 , the optional retiming stages  730 ,  732  and  734  do not process the credits transmitted by the receiving units  720 ,  722 , and  724 . In alternative embodiments, the optional retiming stages  730 ,  732  and  734  may include flops to process the credits, as required by system timing constraints. 
   The sending unit  710  includes a sender controller  740  that controls the transmission of data packets and valid signals to the receiving units  720 ,  722  and  724 . The sender controller  740  includes a first credit counter  750 , a second credit counter  752  and a third credit counter  754 . Each of the credit counters is configured to track the inventory of credits associated with a given receiving unit. For example, the first credit counter  750  is configured to track the inventory of credits associated with the first receiving unit  720 . In one embodiment, the first credit counter  750 , the second credit counter  752  and the third credit counter  754  function substantially like the credit counter  135  of  FIG. 1B . 
   In one embodiment, the first receiving unit  720 , the second receiving unit  722  and the third receiving unit  724  are configured to function substantially like the receiving unit  150  of  FIG. 1C . Therefore, after each receiving unit  720 ,  722  and  724  receives a first data packet and corresponding valid signal, each receiving unit  720 ,  722  and  724  releases a number of credits to the sending unit  710  corresponding to the size of its respective skid buffer. For example, after the first receiving unit  720  receives a first data packet and valid signal, the first receiving unit  720  releases N−1 credits to the sending unit  710  where N is the number of data packets that may be stored in the skid buffer associated with the first receiving unit  720 . 
   As previously described herein, in order to send a data packet to one of the receiving units  720 ,  722  or  724 , the sender controller  740  must have a credit associated with that particular receiving unit. For example, in order to send a data packet to the second receiving unit  722 , the second credit counter  752  should indicate to the sender controller  740  that at least one unconsumed credit has been received from the second receiving unit  722 . Likewise, if data is being sent to more than one receiving unit, then each of the credit counters corresponding to those receiving units must indicate to the sender controller  740  that at least one unconsumed credit is available for those receiving units. 
   In one embodiment, the number of flops in the round trip signal path of the data packets, valid signals and credits determines the size of the skid buffers associated with the receiving units  720 ,  722  and  724 . Thus, the optional retiming stages  730 ,  732  and  734  affect the sizes of the skid buffers associated with the receiving units  720 ,  722  and  724 . For example, assume that the optional retiming stage  730  includes a flop for the transmitted data packets and valid signals. Then, in the case of the first receiving unit  720 , there would be five flops in the round trip signal path. There would be a first flop in the sending unit  710  as a data packet and corresponding valid signal leave the sending unit  710 ; there would be a second flop in the optional retiming stage  730 ; there would be a third flop in the receiving unit  720  to receive the data packet and valid signal; there would be a fourth flop in the receiving unit  720  as a credit is released and transmitted to the sending unit  710 ; and there would be a fifth flop as the sending unit  710  receives the credit signal. Therefore, the skid buffer associated with the first receiving unit  720  should support five data packets. Since the round trip signal path for the second receiving unit  722  and the third receiving unit  724  would differ from the round trip signal path of the first receiving unit  720 , the sizes of the skid buffers associated with the second receiving unit  722  and the third receiving unit  724  would also differ from the size of the skid buffer associated with the first receiving unit  720 . 
   One advantage of the system disclosed in  FIG. 7  is that it reduces routing congestion. With the approach of the prior art, each receiving unit requires a dedicated channel with which to receive data from the sending unit. As the number of receiving units in the system increases, so does the number of required dedicated channels between the sending unit and the various receiving units. Increasing the number of dedicated channels in and out of the sending unit causes routing congestion. By contrast, the disclosed system reduces the routing congestion by using a common data packet channel that may be routed to each receiver in a daisy chain fashion, thereby reducing the number of dedicated channels in and out of the sending unit. 
     FIG. 8  is a flowchart of method steps for transmitting data to multiple receiving units, according to one embodiment of the invention. Persons skilled in the art will recognize that any system configured to perform the method steps in any order is within the scope of the invention. 
   The method begins with step  802  where the credit counters  750 ,  752  and  754  in the sending unit  710  are initialized to one. Since the credit counters  750 ,  752  and  754  are set to one, the sending unit  710  is able to send one data packet to the receiving units  720 ,  722  and  724  when the data is available. In step  804 , the sender controller  740  determines if any credits were received from any of the receiving units. This is denoted in  FIG. 8  by “credit(n)” where n is the number of any receiving unit. If there are no credits received from any of the receiving units  720 ,  722  and  724 , then the method proceeds to step  808 . If, however, a credit was received from any of receiving units  720 ,  722  or  724 , then in step  806 , the credit counter associated with the receiving unit from which the credit was received is incremented. For example, if the sender controller  740  receives a credit from the receiving unit  720 , then the first credit counter  750  is incremented. The credit counters  750 ,  752  and  754  track the inventory of credits associated with each of the receiving units  720 ,  722  and  724  available to the sender controller  740 . The sender controller  740  is configured to act upon the credit counters  750 ,  752  and  754  separately and independently from one another; therefore, if credits are received from multiple receiving units, then each credit counter associated with a receiving unit from which a credit is received is incremented. 
   In step  808 , the sender controller  740  determines if there is data available to transmit. If there is no data available to transmit, the method returns to step  804 . Otherwise, if there is data available to transmit, then in step  810 , the sender controller  740  selects the receiving units that are to receive the data. The data may be transmitted to one or more of the receiving units  720 ,  722  and  724 . In step  812 , the sender controller  740  determines if the credit counters corresponding to the selected receiving units are greater than zero. Again, the sender unit  710  only transmits data packets to receiving units that have memory space available in their respective skid buffers for additional data packets. Therefore, the sender unit  710  only transmits data packets to the receiving units whose associated credit counters are greater than zero. 
   If the credit counters corresponding to the selected receiving units are greater than zero, then in step  816 , the data packet and corresponding valid signals are transmitted to the selected receiving units. As shown in  FIG. 7 , a valid signal is transmitted to a particular receiving unit only if the sender controller  740  determines that the receiving unit is to receive the transmitted data packet. Thus, if a particular receiving unit is not selected in step  810 , then the sender controller  740  does not transmit a valid signal to that receiving unit and the receiving unit does not receive the transmitted data packet. If, on the other hand, the credit counters corresponding to the selected receiving units are not greater than zero, in step  814 , the data packet and corresponding valid signals are not transmitted to the selected receiving units. The method then returns to step  804 . 
   The sender controller  740  executes step  802  only once to initialize the credit counters in the sending unit  710 . In operation, the sender controller  740  continuously loops through steps  804  through  818  to manage the receipt of credits and the transmission of data packets. Again, the method steps are structured such that the sending unit  710  does not transmit a data packet to a receiving unit unless the receiving unit has memory space available for an additional data packet. Thus, the skid buffers associated with the various receiving units do not overflow. 
   To illustrate broadcasting a data packet to the three receiving units  720 ,  722  and  724 , assume that the method has initialized the credit counters  750 ,  752  and  754  (step  802 ). The sender controller  740  does not receive any additional credits, but does determine that there are data packets available to transmit (steps  804  and  808 ). Next, the sender controller  740  determines that the data packet is to be transmitted to all three receiving units  720 ,  722  and  724  and therefore checks to see whether the three credit counters  750 ,  752  and  754  are greater than zero (steps  810  and  812 ). Since the three credit counters  750 ,  752  and  754  are greater than zero (a credit was received from each of the receiving units upon initialization), the sender unit  710  transmits the data packet and the sender controller  740  asserts the three valid signals to the three receiving units  720 ,  722  and  724 . The sender controller then decrements the credit counters  750 ,  752  and  754  (steps  816  and  818 ). 
   Transmitting data packets to fewer than three receiving units proceeds in a similar manner. For example, if a data packet is to be transmitted only to the first receiving unit  720  and the third receiving unit  724 , the sender controller  740  will only determine if the first credit counter  750  and the third credit counter  754  are greater than zero (step  812 ). If true, then the sending unit  710  transmits the data packet and the sender controller  740  asserts only the first and third valid signals to the first receiving unit  720  and the third receiving unit  724  (step  816 ). The second valid signal is not asserted to the second receiving unit  722  and therefore the second receiving unit  722  does not receive the data packet. 
   While the foregoing is directed to embodiments of the present invention, other and further embodiments of the invention may be devised without departing from the basic scope thereof, and the scope thereof is determined by the claims that follow.