VPP/RA4 voltage confusion.

[jmgelba]

I have a schematic where the VPP terminal of a connector could be exposed to anywhere from 15V to 42V so I need to use a zener to limit that to within the maximum VPP. I was thinking a 12V zever as VPP max is 13.25V.

However the issue is, I need to use that pin as an I/O when not programming and it appears according to the manual that when used as I/O, RA4 can only see a max of 8.5V. I am going to guess then that VPP must be above 8.5V?

http://ww1.microchip.com/downloads/e...Doc/39605F.pdf

Chapter 22, page 237.

[jmgelba]

One must RTFM more thoroughly!

I'll ask an additional question then. It appears you can have the pin configured as an I/O and use it in the VPP range but the manual says it is not recommended. Has anyone created a design and had experience with RA4 seeing high voltages?

[Ramius]

Hi Jmgelba,
I think you might have a small confusion going on? VPP is part of MCLR/ RA5 not RA4. MCLR typically has a 4.7k resistor to 5.0v. As for voltages other than 5.0v such as your 15 to 42v I'd probably use a 5.0v voltage regulator IC or an optical isolator to ensure the VPP does not see anything higher than 5.0v. Best, Ed

[jmgelba]

Ed,
VPP is programming voltage and must be higher than 5 volts. For the 18F1320 I'm using it must be 9.0V minimum for programming mode.
RA4 was a typo and should have read RA5.

I have a space issue so I cannot use a dedicated programming header. I have to use the connector that also supplies the power, typically 15 - 42V and has the sense pins, which are either pulled to ground or pulled to the supply. Thus the programming pins on the PIC must be protected from the high voltages. All but VPP are protected by 5.0V zeners and resistors. VPP must have a 9.0V or higher zener so that it will enter programming mode when required.

Can this pin see the programming voltage when I dont want it to enter programming mode?

[Ramius]

Hi!
According to the same spec sheet you referenced earlier it states " This pin maybe kept in this range at times other than programming. But it is not recommended." Note 2 on page 254. You are talking about being able to supply a voltage at 10 ma over a wide voltage range so again I would be back to suggesting an optical isolator or 7812 unless you know the zener you plan to use will tolerate more than 10 ma. Best, Ed!