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

Steve Lenham steve at bendentech.co.uk
Thu Feb 25 18:39:10 CET 2021


Thanks, chaps. It's useful to get a second opinion.

I think I may struggle to get 5m of coax inside a RAM cart, so will 
probably stick with the little RC if I ever need to build one of these 
in anger (which I probably won't - it was just a little experiment to 
prove the concept at this stage).

Cheers,

Steve L.
Benden Sound Technology


On 25/02/2021 16:36, Richie Burnett wrote:
> All seems perfectly reasonably to me Steve.
> 
> For smaller delays you can put little wiggles in the PCB trace or send the signal down a short length of transmission line. But you'd still need several metres of coax to nudge it back by just 25ns!
> 
> The slow rise time resulting from the RC circuit is not a problem in itself, but can cause problems in the presence of noise if the receiving device is fast enough to register the additional clock edges.
> 
> To be honest I've seen more problems with lightening-fast clock edges due to signal reflections. So I'd take a clock line with controlled dv/dt any day of the week :-)
> 
> -Richie,
> 
> Sent from my Xperia SP on O2
> 
> ---- Steve Lenham 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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