.

                           FPGA Decoder

       How to decode the input SMT data stream for FPGA simulation


                          STR, 21 April 1999



      Assuming an input file has been generated, with the specifications
     described in the " Addendum to D0Note 3599 ", here is the description
     of a simple decoder.

     Let's say that we just want to read out the input data stream and
     count how many chips and how many channels have been readout.
     The input data stream, for now, should contain only one HDI,that belongs
     to a specific Sequencer.
      Let's remind here that the input data stream is composed of words
      8 bit long.


   step 1 - INITIALIZE Sequencer_Number and HDI_Number and counters:

                 Sequencer_Number = 0
                 HDI_number = 0
                chip_count = 0
                 channel_count = 0

    step 2 - read first two words on input file :

             - read first word -
             // this gives the sequencer number
                ________________________
                |0 |0 |0 |0 |0 |1 |0 |1 |
                ------------------------
              in this case the Sequencer number is 9
              - save it in Sequencer_Number

              - read second word -
              //  the 3 LSB ( less significant bits) of this word
                  give the HDI number
                 _________________________
                 |X |X |X |X |X |0 |1 |0 |
                 ------------------------
                 n.b. - the X stands for any number, we disregard this part
                        for now
                 in this case the HDI number is 4
               - save it in HDI_Number

     step 3 -   start the read loop :
               // from this point on, the data structure contains a regular
                  sequence of 'odd words' and 'even words'. The odd words
                  contain control bits and addresses, and the even words
                  contain data ( channel content )
		  
		
                - read in 'next word' and check on the 2 MSB(most significant
                  bits):

                  if the 2 MSB = "1 0" // it is a new chip
                     ________________________
                     |1 |0 |0 |0 |0 |0 |1 |1 |
                     ------------------------

                   then   - read chip number ( in the 6 LSB) of the same word
                            in this case chip number is 5.
                          - save this value in local variable chip_number
                          - increment counter chip_count
                          - check that next word is
                            ________________________
                            |0 |0 |0 |0 |0 |0 |0 |0 |
                            ------------------------
                          IF it is not, send a 'fatal error' and EXIT

                   if the 2 MSB = "0 0" // it is a channel address

                      ________________________
                      |0 |0 |  |  |  |1 |0 |0 |
                      ------------------------

                    then  -  read the channel address . In this case it is 8.
                          -  save it in local variable channel_address
                          -  increment counter channel_count
                          -  read next word
                          -  save its content in local variable channel_content
  
                   if the 2 MSB = "1 1" // this is the end of data, or HDI's
end

                       ________________________
                       |1 |1 |  |  |  |  |  |  |
                       ------------------------

                   then  - print " end of HDI "
                         - print counters : Sequencer_Number = 9
                                            HDI_Number = 4
                                            chip_count=
                                            channel_count =





  Remark -

     Local variables channel_address and channel_content will constitute
     the input for the "Cluster Algorithm" processor. In this specific
     program they are of no use.
     The other local variable chip_number also is of no use at present.
     (We keep it and probably want to save it as information of the hardware
     address in case this will need to be forwarded to L3).