[16F871] Power on RA4 before power on vdd?

[Buzzz]

Hello folks!

I am using a PIC16F871, with a voltage regulator (with power switch) for Vdd. I want to use RA4 as open drain output connected to a buzzer (BZ) which is powered directly from battery. Will I fry my 871 when Vbatt is connected to the buzzer, but no Vdd is present at the PIC? Or will it start buzzing when Vdd is not present?
In other words: What will happen when the switch is opened in this schematic:

F871
----
    |        ____
RA4 |-------|_BZ_|--------------------------      
    |                                      |   
Vdd |------\ -----------+5V ---regulator -------- +7.2V
    |     switch
----
  |
GND

Thanks for your help!

[Melanie]

No pin on a PIC should be at a higher potential than Vdd, and certainly NOT highter than the max rating in the Datasheet (which is usually around 5.5v). Some PICs are rated to run to 12v. Check the Datasheet.

[Buzzz]

I have been looking around, and it seems like I am allowed to put a higher voltage on pin RA4. Copy-paste from datasheet:

Absolute Maximum Ratings †

Voltage on any pin with respect to VSS (except VDD, MCLR. and RA4) ... -0.3V to (VDD + 0.3V)
Voltage on VDD with respect to VSS ................... -0.3 to +7.5V
Voltage on MCLR with respect to VSS (Note 2) ........0 to +13.25V
Voltage on RA4 with respect to Vss .......................0 to +8.5V

I am using the PIC at 5V, so I thought that voltage on any pin would be Vdd + 0.3V, so that would be 5.3V max.
The line about RA4 simply says 0 to +8.5V, so I assume that I am allowed to sink 25mA's max at 8.5V max since I am using RA4 as an Open Drain output.

So this would leave me with the problem that I don't know whether I am allowed to connect the buzzer directly to 7.2V, and whether it will fry my PIC whenn Vdd is switched off? Also, am I getting the first part right?

[Acetronics2]

Hi, Buzz

Why not use a stupid N Mosfet ( 2N7000 or BS 170 ) to switch your Buzzer ???
It would provide all the insulation you need ...

just invert your output level ( better for Power-on response of the Buzzer... hmmmm ? ) or use a P-Mosfet ( BS 250 ) and buzzer between Drain and ground ...

Alain

PS : 13v ... doesn't make you remember something about programming your PIC ??? ... jam at sight !!!

[Buzzz]

Yes, a stupid mosfet or even a cheap ass transistor could be the solution. Problem is, that I already have a PCB, and even with redesign really no space for extra components, not even a TO92...
So that's why I would like to fix this with just a piece of wire between the (now unused) pin RA4 and the buzzer.

[Acetronics2]

Never soldered some components on your copper tracks and played with the cutter ??? ...
Time to learn !!!

Alain

PS: why not put your "piece of wire" to the 5v instead of 7v2 ??? just a track to cut ...
5v Buzzers also exist !!!