Internal or External Diode Clamps

[abrand]

I am interfacing 5V TTL logic to a 3.3v device. The 3.3v device is not 5V tolerant but does have internal clamping diodes. Is is safe to use those diodes as long as I have a series resistor to limit the current (datasheet says 20mA max)? Or is it better to use external diodes as long as they are configured to turn on before the internal network?

I know they have the internal diode network to protect the I/O pins, but wasn't sure if it is "good practice" to use them for this purpose or not?


Thanks,
Alex

 

[H.E.Tjelum]

Hi

If I was You, I would use a simple voltage divider. This asures that the voltage is 3.3V to the input, with no problems.
I suggest a 1k8 resistor in series with a 3k3 resistor. This gives You a current of apprx. 1 mA. There are other solutions, but this is a very simple, cheap and well working solution.

Friendly regards
H.E.Tjelum

 

[Einar M]

Mr. Tjelum's idea is good, but it is usually not a good idea to turn on the input diodes. It is done tho, with a max current well below 20 mA. This can make for a slow interface so isn't popular.