Re: Perplexing 64 addressing finding

From: Francesco Messineo <>
Date: Wed, 9 Aug 2017 09:38:37 +0200
Message-ID: <>
according to the datasheets, HCT08 is a few ns SLOWER than an LS08.
Are all unused inputs of the '08 tied to a supply rail?
Frank  IZ8DWF

On Wed, Aug 9, 2017 at 6:49 AM, Jim Brain <> wrote:
> Trying to help some folks with issues related to an EasyFlash-derived
> design, I have been debugging the circuit and I'm momentarily stumped on the
> issue.
> The design is exactly the same as the stock EF1 design, with 1 change.
> Instead of 2 FLASH ROMs (one in $8000 and one at $a000), I put a single
> FLASH ROM on the PCB and wired ROMH and ROML through a 74'08 to !CE
> With all HCT parts, the unit will not flash on non 250469 boards.
> With all HCT parts but the '08 (74ls08), the same unit will flash correctly
> on non 250469 boards (at least a few I have tried).
> SO, my hyposthesis was that the ROMH and ROML signals need some settling
> time before being applied to !CE.
> However, when I remove the '08 from the circuit and hardwire the !CE line of
> the FLASH ROM to ROML, the unit works on non 250469 boards.  That would
> indicate either 1 of two things:
> Faster than HCT (no latency) is required (or lots of LS latency)
> The interaction of the ROMH and ROML signals is causing glitches.
> I don't see the glitches yet, but I continue to test.  What I wonder is how
> other systems handled sharing a ROM between ROMH and ROML.  THe '08 seemed a
> logical choice (no pun intended), but possibly some other idea is better.  I
> tried gating the CE signal with PHI2, but that did not help.
> Any thoughts are appreciated.
> Jim
> --
> Jim Brain

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Received on 2017-08-09 08:00:07

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