Maximum voltage into PIC pins

manxman · Post by **manxman** » Mon Nov 02, 2009 3:03 pm

I am using an 18F252 in a project which counts pulses into RA4 (T0CK1) and RC0 (T1CK1). On my board VDD = 5V.

What is the maximum voltage of the pulses that I can apply to these pins? Table 22.2 on the datasheet suggests that it is only VDD (5V) but I am not sure I fully understand the specification.

rmteo · Post by **rmteo** » Mon Nov 02, 2009 3:14 pm

Looking under electrical specifications (Absolute Maximum Ratings), this is typical for most PICs:

Voltage on any pin with respect to VSS (except VDD and MCLR) ................................ -0.3V to (VDD + 0.3V)

NOTICE: Stresses above those listed under Absolute Maximum Ratings may cause permanent damage to the device. This is a stress rating only and functional operation of the device at those or any other conditions above those indicated in the operation listings of this specification is not implied. Exposure to maximum rating conditions for extended periods may affect device reliability.

Francis · Post by **Francis** » Mon Nov 02, 2009 5:26 pm

Agreed.

For higher V inputs you normally use a potential divider.
But, with care, you can input higher Vs to an Input via a high-ish value resistor as the clamp diodes will limit it. But this must be used with great care - I don't know if the clamping diodes are used on the pins you want to use - check data sheet.

manxman · Post by **manxman** » Wed Nov 04, 2009 10:00 am

Thanks Francis and rmteo. It seems that the safest solution is for me to ensure that the pulses are constrained to no more than VDD+0.3V (=5.3V). This will make for a more reliable product.

Doj · Post by **Doj** » Sat Nov 07, 2009 12:46 am

Manxman,
The reason for this rating is down to the protection diodes on every pin of the micro.
There is a diode junction from every pin to supply voltage that act as conductors for naughty electrons who might want to cause mischief given half a chance.
Without these little friends any little surge of excess voltage or static would just get soaked up in the peripheral and likely destroy it.
With them the unwanted current is directed into the low impedance power supply path and dissipated.

The diodes have a conducting knee of 0.3v which why the voltage is maximum supply+0.3v.

If you put more than 0.3v above supply the diode will conduct the current to supply until is exceeds its specification of a few milliamps, and then go pop as you might expect.

I have never known any manufacturer be happy with allowing the input to include this value.
If a supply is 5v then input pins would be much happier with 4.75v as their maximum so they will never get any chance of being stressed.

It is also worth noting that TTL and Schottky inputs have different levels for low and high input voltages, some ports are TTL and some are Schottky just to make it interesting.

manxman · Post by **manxman** » Mon Nov 09, 2009 11:51 am

Cheers Doj. Thanks for your helpful and detailed clarification.

As it happens, the 2.5MHz pulses which I am counting peak at around 4.5V so I am safely avoiding those naughty electrons.

Actually, I was'nt aware that fundamental electrons had personalities. Presumably, protons chum up with electrons and neutrons generally avoid the crowd, while none of them like to socialise with antiparticles!