Problem with Atmel CPLD

Discussion in 'The Projects Forum' started by Michau, Jun 20, 2012.

  1. Michau

    Thread Starter New Member

    Jun 20, 2012
    Hello all!

    I am designing a CPLD-based replacement of the Signetics 82S100 chip, which is an early PLA chip used in many Commodore computers. These old chips often fail and are no longer availabe for purchase.

    I have chosen the Atmel ATF1502AS CPLD chip as the replacement, because it is 5V-powered as the original. I programmed it using the same logical equations as the original chips using WinCUPL. I designed a PLCC44 to DIL28 adapter for this purpose.

    The chip reads correctly (when placed in the adapter) in a PROM burner/reader instead of the Signetics chip (i.e it produces the same truth table). The problem is, it refuses to work in the original environment in Commodore computer, interfacing with TTL and NMOS chips. It behaves as if it was powered down, or as if all outputs were high impedance. Simply put, the outputs do not react to inputs whatsoever, but are left dangling somewhere around 0.7V.

    In was able to solve this in one of my Commodore machines by placing a resistor between power and one of the chip outputs. Mysteriously, the whole chip suddenly started working! I have no idea why this modification had such an effect. Unfortunately, in another Commodore machine this did not help and the chip is still not working at all.

    I would be very grateful for any clues as why such behaviour might be exhibited, and what I should do to correctly utilize the chips.