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Altera_Forum's avatar
Altera_Forum
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15 years ago

Large fast counters (MaxII)

I'm trying to create a large fast long variable delay. My obvious approach is a 17-bit counter that loads when the trigger arrives and counts down to zero when it stops. Unfortunately even with a -3 speed MAXII, I can't make this work for more than 8-bits with a 250MHz clock.

If I were doing this in 74 series logic, I'd use enable lookahead on parts such as 74163 but I'm not sure this approach is possible in VHDL on a MAXII.

Is this the right sort of approach, or are there better ways of achieving the desired effect?

4 Replies

  • Altera_Forum's avatar
    Altera_Forum
    Icon for Honored Contributor rankHonored Contributor
    15 years ago

    --- Quote Start ---

    Try chaining small counters together.

    --- Quote End ---

    To get to 17-bit equivalent with 8-bit counters, I'd need 512 of them, so this isn't viable.

    ===

    I've realised that carry lookahead won't work in the same way as on a 74163 because the logic is ram lookup and therefore any input change could cause a glitch defeating the lookaheads purpose of a longer setting time.

    My current thought is to divide the 250MHz clock by 4 (synced at 250MHz to the trigger input) and use this 62.5MHz clock to do 15-bits of the delay plus about three-bits of 250MHz delay.

    Is it possible to connect an internally generated signal to a global clock line? If so how?