3.3V PICAXE Supply & 5.0V peripherals

M

mortifyu

New Member
Hello Gurus,

Hope you are all safe and well.

The scenario…

* PICAXE running at 3.3V supply.
* Separate peripheral with a digital output 5V

Is it going to be a problem (damaging to the PICAXE) to connect ANY external device/IC with a digital output of 0v (LOW), 5v (HIGH) directly to a PICAXE input when the PICAXE has a supply voltage of 3.3v?

This question comes because the datasheets indicate I/O pin input threshold to be VCC +0.3v. However, as we all know PICAXE IC’s are 5v compatible.

My thoughts is that I don’t expect it to be a problem other than if a READADC was performed, in which case I would expect to see 255 returned until the input voltage dropped below 3.3v. Otherwise 5V input to a pin would simply return a 1 or HIGH without issue.

What do the Guru’s say/think?

As always, thanks in advance guys/gals.


P.S. Hippy: A personal thank you to all your assistance and contributions over quite a few years. You are highly respected. Best of luck in all your future endeavours friend.


Regards,
Mort.
 
B

Billo

Senior Member
A PICAXE may be able to run at 5V but that does not mean the I/O is 5V tolerant when powered at 3.3V. 5V on the I/O pins when Vcc is 3.3V can cause back feeding into the device that may damage it and/or cause unexpected behavior.

Why not use a simple voltage divider made with a 1K2 and a 3K3 resistor?
 
A

AllyCat

Senior Member
Hi,

Each of the I/O pins of a PICaxe has two internal "electrostatic voltage" protection diodes, between the pin to ground and to the supply rail. Thus there are three possibilities:

1. 5 volts applied to any PICaxe I/O pin pulls the 3.3 volt rail up to about 4.4 volts. This is often called "Phantom Powering" which can cause a PICaxe to run (quite normally) even with its power supply disconnected. ;)
2. If there is a lot of (external) load on the supply rail then the current drain may pull the pin voltage lower.
3. If the drive to the pin and the load on the supply are both strong, then you may destroy the pin connection, the pin circuitry, the protection diode or maybe even the whole PICaxe chip. ;(

The only pin which doesn't have a diode to the supply rail is the "Input Only" pin (Leg4 on most M2s) so it might be "5 volt tolerant" with a 3.3. volt supply. But the reason it has no diode is because on the "base PIC", pulling this pin voltage higher than the Vdd is intended to put the chip into a fast (re-) Programming mode.

Cheers, Alan.
 
B

Billo

Senior Member
mortifyu said:
This is most certainly an appropriate and known/understood resolution, but brushes aside the question at hand.

Regards,
Mort.
Click to expand...
I answered the question at hand, then provided a valid alternative. Why is this a problem?
 
M

mortifyu

New Member
Billo said:
I answered the question at hand, then provided a valid alternative. Why is this a problem?
Click to expand...
Apologies, I did not mean to appear to disregard your earlier comment and your response is certainly appreciated.

Regards,
Mort.
 
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