[time-nuts] Question concerning failure and value of HP 5371A

buehl buehl at superlink.net
Sun Oct 16 11:10:42 EDT 2005


Some more info regarding use of Xilinx FPGA for counting.

I recently completed a design using Xilinx Spartan II FPGA.  This FPGA is 
only specified for maximum 200 MHz internal clock.
My skills in this area are very limited, and this is my first for FPGA 
design.  When finished, it counted reliably to above 210 MHz.  Input pulses 
about 2 nS wide.  So the performance of the Xilinx parts are excellent.

This was a pre-set counter:  Enter desired count number into registers/ 
memory, and the counter would output pulse when this count 
finished.  Counting could be triggered or could automatically begin 
again.  Therefore the OUTPUT could be every 10 pulses, up to every 
999,999,999 pulses, with resolution of 1 pulse.

Although this was not interfaced with time reference, circuit could be 
modified to compare with a reference.

I have a plan to do the next one with higher speed Xilinx, and also know 
how I will be able to get counting at twice the internal clock rate of the 
FPGA.  This would make the FPGA with minor additional circuitry able to 
count at greater than 500 MHz.

Just as other time_nuts cautioned, the problem is handling the input 
signals at this frequency.  Even at the 200 MHz rate, the very high 
transition time of the input pulses required that the circuit be "clean" at 
1 GHz.  You really need to design the input circuit first, so that the 
input signal is properly "conditioned" to provide clean data pulses to the 
FPGA.  This was my biggest mistake on the first design.  The FPGA counted 
fine if driven directly from a generator, but amplifying the input signals 
to adjust gain and trigger threshold was a VERY big design task.

As you preceed, I would be glad to give you my comments on your work.

Tom Buehl       buehl at superlink.net.

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