[time-nuts] Re: electronics question or how not to fry my raspberry pi

[David Taylor]

On 28/01/2022 19:41, folkert wrote:
> Hi,
[]
> The RPI doesn't like 5v on its GPIO pins.
> So I wonder:
> - can I feed the picdiv 5v on its GPIO pin while giving it a 3.3v
>    voltage so that it outputs 3.3v as well to the rpi pins?
> - or should I use a voltage divider? I was thinking of a 4.7k ohm and
>    8.2k ohm resistor giving slightly less than 3.2v - will that work? or
>    will that attenuate the signal too much? The 50 ohm bnc cable between
>    the amplifier and the rpi is 3m long. Anything else I should be aware
>    of?
> 
> 
> Regards,
> 
> Folkert van Heusden
> PD9FVH

Folkert,

In similar circumstances I've used a resistive divider and it's worked exactly 
as expected.

For such short cables there are unlikely to be any other effects you need to 
take care of.  You might like to see what the voltage levels are if you 
terminate the 50-ohm cable with a 47-ohm resistor - it /may/ drop to 2.5V 
peak-to-peak.

I would simply try it and see.

Of course, if the idea is to send PPS to the RPi getting the appropriate GPS 
devices would be my preferred solution!

Oh, just looked, I would expect to see a square-wave, not the near sine-wave 
your 'scope trace shows.  Is it nearer to square with a much faster timebase?

Cheers,
David
-- 
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