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5V -> 3V3 for 1PPS

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jailbird:
I know this is a stupid newbie question, sorry.

So I picked up a FE-5680A GPS Discipline module from Tindie.  The 1PPS output on it is 5V, while the GPIO inputs on the Raspberry Pi are 3V3.  I need to make them happily coexist.

Normally I'd use a level shifter, but I'm concerned on what effect that would have on the signal (eg, delay it, make it inconsistent, etc).  I'm thinking maybe a pair of diodes and resistors would maybe make more sense?

I'm sure this is a common issue, just wondering what people have used and recommend.

Thanks!

nctnico:
What is the output impedance of the 1PPS? Chances are it is 50 Ohms so a simple resistor divider will do the trick. If you are going to attach it to a GPIO anything you put in between won't have any consequence for the timing accuracy. Likely you'll be looking at micro-second timing accuracy if you are lucky.

jailbird:
It is indeed 50 \$\Omega\$, sorry for not mentioning that!

Thanks for the help!

MagicSmoker:
Oh, there's a really simple solution to this sort of problem which won't affect timing at all: put a Schottky diode in between the two devices with its cathode connected to the 5V output of the GPSDO and a pullup resistor (100R to 330R - not critical) from its anode to 3.3V.

nctnico:
No. Never ever use supply rails to dump overvoltage into. Regulators only source and they don't sink so if you push more current into the rail than is being consumed you'll push up the supply rail.
In this case a simple resistive divider will do just fine. A 100 Ohm resistor will load the 50 Ohm output enough to get into the 3.3V ball-park.

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