Abstract:
A Data Throttling method duplicates the full-speed transmission of data so that it appears to be transmitting at a 10 Mhz rate. Additional storage elements and multiplexers are added along the data path but this completely eliminates undesirable complexity in the clock tree. In a two-bit application, data is received and transmitted two bits at a time, and yet the output 10 Mhz data rate is maintained. For an even ratio between the system clock rate and the 10 Mhz clock signal rate, bit 0  is transmitted for half the time and bit 1  is transmitted for the other half of the time. But if the full-speed clock rate is an odd multiple of 10 Mhz, then there will be a “split cycle” including one bit 0  and one bit 1.

Description:
BACKGROUND OF THE INVENTION 
     The present invention is related to data transmission, and, more particularly, to transmitting data at a rate slower than a system clock signal speed during link initialization in conformance with a SpaceWire application. 
     SpaceWire is the common name associated with the European Cooperation For Space Standardization Specification ECSS-E-50-12A. According to this specification the data rate on the wire must be slowed down to 10 Mhz during link initialization. Data Throttling is a way to meet this requirement without having to adjust the transmit clocks. 
     Prior auto-detect art has been developed that involves on-chip generation of a 10 Mhz clock, multiplexing with the full speed clock to create a qualified clock, and distribution of this clock on a dedicated clock network. This method proved to be unworkable when used in an FPGA implementation and when coupled with an additional requirement to drive this qualified clock off-chip. Data Throttling removes the need to create the derived clock and also the need to drive this clock off chip. The present invention therefore makes the implementation of a SpaceWire protocol handler on an FPGA more easily realizable. It can also be carried over to ASIC implementations. 
     An example of a prior art auto-detect circuit  100  is shown in  FIG. 1 . Circuit  100  includes a multiplexer  102 , a D-type flip-flop  104 , and a transmit buffer  106 . The D-input of the D-type flip-flop receives input Transmit data. The two inputs of multiplexer  102  receives a normal Transmit clock and a slower 10 Mhz clock used during link initialization. The output of multiplexer  102  is coupled to the clock input of flip-flop  104 . The output of the multiplexer provides one of the two possible clock signals in the “Txq_clk” signal depending upon the state of the multiplexer control signal (“Link_run”). Note that in  FIG. 1 , circuit  100  includes a multiplexer  102  in the clock path, complicating the clock tree and making FPGA implementation difficult. 
     What is desired is a circuit and method for providing two clock signals for Data Throttling in a SpaceWire application without the necessity of multiplexing the clock signal that is suitable for use in single or multiple bit applications. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to duplicate the full-speed transmission of data so that it appears to be transmitting at a 10 Mhz rate. For example a logic one at 100 Mhz is sent ten times to appear to be a logic one at 10 Mhz. Implementing the approach of the present invention involves adding additional storage elements and multiplexers along the data path but completely eliminates undesirable complexity in the clock tree. A slight complication is addressed by the present invention for multiple-bit applications. In a two-bit application, data is received and transmitted two bits at a time, and yet the output 10 Mhz data rate must be maintained. In a “normal” situation in which there is an even ratio between the system clock rate and the 10 Mhz clock signal rate, Bit 0  is transmitted for half the time and bit 1  is transmitted for the other half of the time. But if the full-speed clock rate is an odd multiple of 10 Mhz, then there will be a “split cycle” according to the present invention. For example if the full-speed clock rate is 150 Mhz, and the data to be sent is a “1-0” data pair, then, according to the present invention, there are seven transmissions of a “1-1” data pair, one transmission of a “1-0” data pair, and seven transmissions of a “0-0” data pair. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The aforementioned and other features and objects of the present invention and the manner of attaining them will become more apparent and the invention itself will be best understood by reference to the following description of a preferred embodiment taken in conjunction with the accompanying drawings, wherein: 
         FIG. 1  is a transmit circuit for providing data at two different data rates for a SpaceWire application using a multiplexed clock signal according to the prior art; 
         FIG. 2  is a Data Throttling circuit for a single-bit application suitable for use in a SpaceWire application according to the present invention; 
         FIG. 3  is a timing diagram associated with the circuit of  FIG. 2 ; 
         FIG. 4  is a Data Throttling circuit for a two-bit application suitable for use in a SpaceWire application according to the present invention; 
         FIGS. 5 and 6  are timing diagrams associated with the circuit of  FIG. 4 ; 
         FIG. 7  is a Data Throttling circuit for a four-bit application suitable for use in a SpaceWire application according to the present invention; and 
         FIG. 8  is a timing diagram associated with the circuit of  FIG. 7 . 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring now to  FIG. 2  a basic one-bit Data Throttling circuit  200  is shown, including bit generation logic  202  for receiving a throttle signal coupled to a flip-flop  204  for receiving a CLK signal and providing a signal bit “bit 0 ” data output. In an “open throttle” condition in which the throttle control signal is set to a logic zero, the output data changes for every CLK cycle. In a “closed throttle” condition current data is held until the throttle control signal is released. No other logic or programming support is required for the simple one-bit data throttling shown in  FIG. 1 , other than the throttle control waveform and the CLK clock signal. 
     Referring now to  FIG. 3 , a timing diagram is shown for the Data Throttling circuit  200  of  FIG. 2 . The timing diagram includes the CLK clock signal, and a representative throttle signal “t”. Note that when the throttle signal “t” is low, the output data changes state on every leading edge of the CLK signal. Alternatively, when the throttle signal “t” is high, the output data remains in the same data state and does not provide new data until the throttle signal “t” goes low. 
     The present invention can be extended to two-bit Data Throttling. In order to handle multiple-bit Data Throttling, the length of the throttle time must be known in terms of the system clock. For example, the throttle length or throttle rate (“TR”) may be specified as two, three, four, or more clock cycles. Then, Data Throttling replicates each bit for that consecutive number of clock cycles as is explained in further detail below with reference to the timing diagram of  FIG. 5 . 
     Referring now to  FIG. 4  a two-bit Data Throttling circuit  400  is shown, including bit 0  generation logic  402  and bit 1  generation logic  404 . Logic blocks  402  and  404  each receive the throttle signal. The outputs of logic blocks  402  and  404  are respectively coupled to the inputs of flip-flops  406  and  408 . The clock input of flip-flops  406  and  408  each receive the same CLK signal. Flip-flop  406  provides a signal bit “bit 0 ”, and flip-flop  408  provides a signal bit “bit 1 ”. A first multiplexer  410  receives the “bit 0 ” and “bit 1 ” signal bits, a “muxb 0 ” control signal, and provides a “b0_out” data output signal. A second multiplexer  412  receives the “bit 0 ” and “bit 1 ” signal bits, a “muxb 1 ” control signal, and provides a “b1_out” data output signal. In an “open throttle” condition in which the throttle control signal is set to a logic zero, the output data changes for every CLK cycle. New data is generated only when the throttle control signal is at a logic zero level. Additional logic circuitry is required to generate the multiplexer control signals. As in the previous one-bit application, data is held when the throttle control signal is set to a logic one level, as is explained in further detail below. 
     Referring now to  FIG. 5 , a timing diagram is shown for the Data Throttling circuit  400  of  FIG. 4 . The timing diagram includes the CLK clock signal, a representative throttle signal “t”, and the throttle rate “TR” value. For a throttle rate of two, data pair “C-D” is distributed over two clock cycles, which provides output data of two data pairs “C-C” and “D-D”. For a throttle rate of three, data pair “E-F” is distributed over three clock cycles, which provides output data of three data pairs “E-E”, “E-F”, and “F-F”. For a throttle rate of four, data pair “G-H” is distributed over four clock cycles, which provides output data of four data pairs “G-G”, “G-G”, “H-H”, and “H-H”. Note that in the timing diagram of  FIG. 5 , the throttle signal “t” may or may not be a 50% duty cycle signal. In the example of  FIG. 5 , the throttle signal “t” has a duty cycle of greater than 50%. When the throttle signal is low in the two-bit application, input data is provided directly to the output at the normal clock rate (not shown in  FIG. 5 ). 
     Referring now to  FIG. 6 , a further timing diagram is shown for the Data Throttling circuit  400  of  FIG. 4 . For the two-bit Data Throttling application, two additional timing signals are required to establish the multiplexer control signals. A throttle phase signal “tp” is derived from the throttle signal “t” and the “TR” register or input. The throttle phase is asserted for the last one-half of the throttle cycle. If the TR is an odd number, then the throttle phase is asserted for the last one-half of the throttle cycle plus an additional clock cycle as shown in  FIG. 6 . A split cycle signal “sp” is derived from the throttle phase signal and the “TR” register or input. If TR is odd, then “sp” is asserted for the first cycle of the throttle phase signal as shown in  FIG. 6 . The timing diagram of  FIG. 6  includes the CLK clock signal, and a representative throttle signal “t” having a TR of five, the throttle phase signal, the split cycle signal, the input data, and the output data. Note in  FIG. 6  that the input data pair “A-B” is distributed over five clock cycles corresponding to the TR of five, and provides output data of five data pairs “A-A”, “A-A”, a “split” data pair “A-B”, “B-B”, and “B-B”. 
     The truth table for generating the muxb 0  signal is shown below it Table One. The muxb 0  signal can be easily derived from the output data state allowed to pass through the corresponding multiplexer  410  (“b0_out”). 
     
       
         
               
               
               
               
               
             
           
               
                   
                 TABLE ONE 
               
               
                   
                   
               
               
                   
                 Throttle 
                 Throttle Phase 
                 Split Cycle 
                 B0 Data Out 
               
               
                   
                   
               
             
             
               
                   
                 0 
                 0 
                 0 
                 B0 
               
               
                   
                 0 
                 0 
                 1 
                 B0 
               
               
                   
                 0 
                 1 
                 0 
                 B0 
               
               
                   
                 0 
                 1 
                 1 
                 B0 
               
               
                   
                 1 
                 0 
                 0 
                 B0 
               
               
                   
                 1 
                 0 
                 1 
                 B0 
               
               
                   
                 1 
                 1 
                 0 
                 B1 
               
               
                   
                 1 
                 1 
                 1 
                 B0 
               
               
                   
                   
               
             
          
         
       
     
     Similarly, the truth table for generating the muxb 1  signal is shown below it Table Two. The muxb 1  signal can be easily derived from the output data state allowed to pass through the corresponding multiplexer  412  (“b1_out”). 
     
       
         
               
               
               
               
               
             
           
               
                   
                 TABLE TWO 
               
               
                   
                   
               
               
                   
                 Throttle 
                 Throttle Phase 
                 Split Cycle 
                 B1 Data Out 
               
               
                   
                   
               
             
             
               
                   
                 0 
                 0 
                 0 
                 B1 
               
               
                   
                 0 
                 0 
                 1 
                 B1 
               
               
                   
                 0 
                 1 
                 0 
                 B1 
               
               
                   
                 0 
                 1 
                 1 
                 B1 
               
               
                   
                 1 
                 0 
                 0 
                 B0 
               
               
                   
                 1 
                 0 
                 1 
                 B0 
               
               
                   
                 1 
                 1 
                 0 
                 B1 
               
               
                   
                 1 
                 1 
                 1 
                 B0 
               
               
                   
                   
               
             
          
         
       
     
     If desired, the Data Throttling circuit and method of the present invention can be extended to four-bit or even higher-bit applications. Referring now to  FIG. 7  a four-bit Data Throttling circuit  700  is shown, including bit 0  generation logic  702 , bit 1  generation logic  704 , bit 2  generation logic  706 , and bit 3  generation logic  708 . Logic blocks  702 - 708  each receive the throttle signal. The outputs of logic blocks  702 - 708  are respectively coupled to the inputs of flip-flops  710 ,  712 ,  714 , and  716 . The clock input of flip-flops  710 - 716  each receive the same CLK signal. Flip-flop  710  provides the signal bit “bit 0 ”, flip-flop  712  provides the signal bit “bit 1 ”, flip-flop  714  provides the signal bit “bit 2 ”, and flip-flop  716  provides the signal bit “bit 3 ”. A first multiplexer  718  receives all four signal bits, a “muxb 0 ” control signal, and provides a “b0_out” data output signal. A second multiplexer  720  receives all four signal bits, a “muxb 1 ” control signal, and provides a “b1_out” data output signal. A third multiplexer  722  receives all four signal bits, a “muxb 2 ” control signal, and provides a “b2_out” data output signal. A fourth multiplexer  724  receives all four signal bits, a “muxb 3 ” control signal, and provides a “b3_out” data output signal. 
     Referring now to  FIG. 8 , a timing diagram is shown for the Data Throttling circuit  700  of  FIG. 7 . The timing diagram includes the CLK clock signal, a representative throttle signal “t”, and the throttle rate “TR” value. For a throttle rate of two, data word “A-BC-D” is distributed over two clock cycles, which provides output data of two data words “A-A-B-B” and “C-C-D-D”. For a throttle rate of three, data word “E-F-G-H” is distributed over three clock cycles, which provides output data of three data words “E-E-E-F”, “F-F-G-G”, and “G-H-H-H”. For a throttle rate of four, data word “I-J-K-L” is distributed over four clock cycles, which provides output data of four data words “I-I-I-I”, “J-J-J-J”, “K-K-K-K”, and “L-L-L-L”. 
     The multiplexer control signals can be derived in a manner similar to that described above with respect to the two-bit application. In particular, additional multiplexer signals are required as explained in greater detail below. 
     As stated previously when extending this invention to multiple bits, the throttle rate must be known along with the number of bits involved. These two values are then used to determine how many times a bit must be seen on the outputs to achieve the desired throttle effect and to create the corresponding multiplexer control signal timing. Specifically the following equation:
 
TO(throttle output)= TR /#bits
 
is used for this purpose, where TR must be &gt;=1. TR=1 is the minimum and is really the same as no throttling. In that case each throttle bit gets put out on one output for one cycle. Also note that TR can be programmed as a value in a register set.
 
     The following examples are for two-bit throttling: 
     With TR=2 cycles, each bit must be seen on the outputs (TO)=TR/#bits= 2/2=1. Therefore, each bit must be seen on the outputs for one full cycle. 
     With TR=3 cycles; TO= 3/2=1.5. Therefore, each bit must be seen on the outputs for 1.5 cycles which means sending the bit on both outputs for one cycle and one of the outputs for another cycle. 
     The following examples are for four-bit throttling: 
     With TR=3 the throttle lasts for three clock cycles. Each bit must be seen on three out of the four outputs during the three cycles, i.e. 1112 2233 3444. 
     With TR=4 each bit must be held on all four outputs for 4/4=one full cycle. 
     With TR=5 each bit must be held on all four outputs for 5/4=1¼th cycle. That is all four outputs for one cycle and one output for one additional cycle. 
     With TR=6 each bit must be held on all four outputs for 6/4=1 2/4th cycle. That is all four outputs for one cycle and two outputs for one additional cycle. 
     With TR=10 each bit must be held on all four outputs for 10/4=2 2/4th cycle. That is all four outputs for two cycles and two outputs for one additional cycle. 
     Based on the TO equation, the multiplexer control signals are designed to select each of the bits to be multiplexed to each of the outputs for the required number of cycles. Standard logic design and minimization techniques can be applied differently to solve the specific multiplexer control implementation in several ways but in general there will be increasing number of signals required for increasing numbers of bits. 
     The following is a multiplexer control example for two-bit throttling: 
     Two signals are needed for each bit (four total) to indicate which (if any) output a bit is being sent out in the present cycle. A value of 2′b00 means that the bit is not sent out either output. A value of 2′b01 would mean it is send out on output0 only. A value of 2′b11 would mean it is sent out on both outputs. In the two bit design described above these four signals are the “B0 and B1” signals for each multiplexer. In this implementation, intermediate signals “split” and “phase” are used to help create the final B0/B1 multiplexer signals. 
     The following is a multiplexer control example for four-bit throttling: 
     Four signals are needed for each bit to be throttled (16 total). A value of 4′b1111 would mean the throttle bit is sent out all four outputs this cycle. A value of 4′b0011 would mean that the throttle bit is sent on outputs 0 and 1 only, etc. Intermediate signals such as indicate ¼, half, and ¾ of the throttle period have expired would be one possible implementation path to help create the four bit value for each throttle bit. 
     In extending to even higher-bit applications, the basic structure of the Data Throttling circuit remains substantially the same. The Data Throttling circuit for multiple-bit applications includes “N” logic generation circuits for receiving a data throttling signal, “N” flip-flops coupled to the logic generation circuits for receiving a clock signal, and “N” cross-coupled multiplexers each having a dedicated control signal. The output data word pattern is easily extended using the one, two, and four bit examples shown herein, and the dedicated control signals are generated by using conventional logic design and minimization techniques. 
     While there have been described above the principles of the present invention in conjunction with specific logic designs and methods of operation, it is to be clearly understood that the foregoing description is made only by way of example and not as a limitation to the scope of the invention. For example, the present invention can be used both for node and router applications. Particularly, it is recognized that the teachings of the foregoing disclosure will suggest other modifications to those persons skilled in the relevant art. Such modifications may involve other features which are already known per se and which may be used instead of or in addition to features already described herein. Although claims have been formulated in this application to particular combinations of features, it should be understood that the scope of the disclosure herein also includes any novel feature or any novel combination of features disclosed either explicitly or implicitly or any generalization or modification thereof which would be apparent to persons skilled in the relevant art, whether or not such relates to the same invention as presently claimed in any claim and whether or not it mitigates any or all of the same technical problems as confronted by the present invention. The applicant hereby reserves the right to formulate new claims to such features and/or combinations of such features during the prosecution of the present application or of any further application derived therefrom.