voltage over V+ on input pin

Does anyone know if it is adequate to clamp an input pin on an 08M to V+ with a 1N914 diode if the input to the pin will be up to 12V with a series current limiting resistor of say 1400 ohms. I.E. current limited to about 5 ma with v+ being 5v and input to current limiting resistor of 12v. Thus the input pin might be 0.7v above the V+ being clamped to (voltage across a conducting Si diode). I think this would be about the same as clamping an inductive load with a diode, but exceeds the figure I have seen that an input pin shouldn't go over V+ plus 0.3v. Will the internal clamping of the chip protect it in this case? Any suggestions?

Bob Shaw VE3SUY


Technical Support
Staff member
Based on that being the way the download Serial In circuit works (+/-12V) you should be okay with a 22K. You can rely on the internal diodes to do the clamping to V+, except on Pin 3 of an 08/08M/14M where an external diode is needed because there isn't an internal one.
5mA is a bit high but should be OK if you NEED to use a resistor that low.
Otherwise, it is generally accepted to limit the current to 2mA or less.
As stated by Hippy, using 22k (for Vin=12 and Vcc=5) has been proved to work very reliably as per the download circuit. That limits the current to about 0.3mA which is more than enough to drive the input at the frequencies required for a download.
For much higher frequencies it would be worth dropping down to 10k or even 4k7.

Something to be careful about. If your (clamped) input current exceeds the total 5v load current and the 5v comes from a regulator, the entire supply rail will go up because there is nowhere for the current to go. Most regulators can only source current. They will not clamp the supply rail down to their specified output voltage.

Edited by - beaniebots on 30/08/2007 08:36:11