[sdiy] Nanosecond delays - the clean or dirty method?

Neil Johnson neil.johnson71 at gmail.com
Thu Feb 25 17:16:18 CET 2021


25ns  - that's about 5m of coax?

Neil

On Thu, 25 Feb 2021 at 15:59, Steve Lenham <steve at bendentech.co.uk> wrote:
>
> I need to delay the Chip Enable signal to a RAM IC by a tiny amount -
> around 25ns. I'm building a replacement memory cart for a vintage unit,
> and this is necessary to allow the use of a battery-free F-RAM for
> storage (some F-RAMs have certain timing quirks).
>
> I know that the proper way to do this would be to insert one or more
> pairs of inverters to delay the signal by a few propagation delays, but
> that needs, like, another chip.
>
> The DIRTY way to do it is to add a tiny RC delay. I tried 1K5 and 22pF
> and it worked perfectly, delaying the edge by about 50ns and solving my
> problem.
>
> I'm well aware of the dangers of slow pulse edges and would never
> consider this for something more lengthy without adding a Schmitt
> buffer, but am curious as to whether it is still seen as irredeemably
> bad practice for a tiny nudge such as this. I'm sure I have seen it done
> in various vintage schematics.
>
> Any thoughts would be appeciated!
>
> Cheers,
>
> Steve L.
> Benden Sound Technology
>
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