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

MTG grant at musictechnologiesgroup.com
Fri Feb 26 16:22:47 CET 2021


You can buy single gates in SMD. Not sure about every possible gate, but 
I wouldn't be surprised. Hand-soldering-wise they are easy to work with. 
You could drop one on a tiny (SIP?) adapter PCB if you really wanted to. 
Or maybe do an SMD version of the gate(s) lined up as you mentioned 
originally.

GB

On 2/25/2021 9:39 AM, Steve Lenham wrote:
> 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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