Off Topic: V=3.6V on PIC pins, no Vdd?

arrow · Joined: 17 May 2005 Posts: 213

Hi

I have a battery application where the PIC communictes with a chip using SPI.
At times the device might connected to an external power (via a USB) and the chip communicates to a different SPI device (not on my board). I am plannign that my on board PIC will not be powered in this instance.

Is this a problem? i.e. the SPI lines (and hence the PIC SPI pins) will be receiving some Voltage, but the PIC will not be powered?
(The data sheet says that I cannot power input pins by more than Vdd+0.3, in my case that would be 0.3V?)

If it is a problem then how could I solve it?
Thank you
a.

libor · Joined: 14 Dec 2004 Posts: 288 Location: Hungary

Yes, this is a problem, almost all PIC I/O pins have internal protective clamping diodes (againt ESD) tied GND and VCC. So your PIC (and all your board as well) might get 'powered' (if enough current) or otherwise misbehave from a signal going into a I/O pin of a powered-off PIC. This may cause many problems, can also stress the outside device's output drivers or the built-in clamping diodes in your PIC over their maximum current limit, and such detroying the chip whichever is weaker.

I use series resistors on all input pins (those coming from outside a board), this makes such an effect harmless limiting the current. Even with the resistors I sometimes observe power-on problems with a partially alive PIC (eg. a PIC in a sleeping/brownout/unknown state, powered by uA-s coming via an input pin), a proper manual/or chip reset helps a lot in this case.

One option for a more robust solution would be including a separate driver IC with an enable/disable/shutdown pin. Depends on your application.

arrow · Joined: 17 May 2005 Posts: 213

Hi Libor

Thank you for your reply.

So you just put resistors in line with the PIC input pins from the device outside the board? What value of R's do you use?
and will this effect SPI mode at all?

I appreciate your help.
a.

libor · Joined: 14 Dec 2004 Posts: 288 Location: Hungary

I use 1K - 3.3K resistors. During the effect in question it would stress the participant chips to a theoretical max. of 5mA (at 5 volts, and even lower at 3.6V), so you can avoid permanently harming your equipment. (check the max. output power of your outside SPI device output) A proper reset strategy might be needed anyway.
This low value would not affect SPI operation, with the low-impedance input of a PIC you can safely use anything up to 10K also, I think.