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Back feeding LM9076S-5.0 when ICSP PIC

Hello. I'm in the middle of the ICSP, In circuit serial programming headache for my PIC processor application board. I need the ICSP to update firmware on the fly while the surface mounted PIC is already on the board, so it's not possible to take it out of the board and program it. I use the excellent LM9076S-5.0 voltage regulator to power the PIC and some surrounding components. When I hook up my external PIC programmer it will feed the PIC with +5V, this will of course even reach the output of the LM9076S and my question is: Can this regulator tolerate back feeding voltage from the pic programmer? Or must I always program the PIC when the application is already powered up by normal means, and don't use the pic programmer supply at all. Thanks for your advice.

Reply to
powerampfreak
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Usually regulators don't like back feeding voltages. For my PIC circuits I use this wiring:

PIC programmer VDD: open programmer VPP connected to PIC VPP programmer GND connected to circuit GND programmer PGD connected to PIC PGD programmer PGC connected to PIC PGC circuit VDD with a diode and 10 k connected to PIC VPP (for MCLR)

--
Frank Buss, http://www.frank-buss.de
piano and more: http://www.youtube.com/user/frankbuss
Reply to
Frank Buss

If you are using the PICKIT2 or PICKIT3 or ICD2 or ICD3, you can turn OFF the VCC supplied by the programmer.

Check under Programmer -> Settings -> Power.

Or you can RTFM.

hamilton

Reply to
hamilton

Thanks for your advice - BUT, I now power up my application board first, (through the LM9076S-5.0), then I try to apply the pic programmer but then it's impossible to program the pic. Is it really possible to have the +5V on to the pic a good while before applying the programming voltage to MCLR = 13VDC. It looks my pic as soon recieving power (of course) begin to execute the firmware code and doesn't want to be re-programmed after.

Reply to
powerampfreak

This regulator is rated for inputs more negative than ground, so it's not likely that the unpowered input to the regulator (the filter capacitor acting as a low impedance) will cause any destructive current to flow. Lacking input power, the regulator is most likely turned OFF (in a high impedance state). Connect your programmer with confidence (assuming, of course, that the programmer power is sufficient to run the board).

Reply to
whit3rd

You'll have to contact National about it. But don't hold your breath, the usual answer will be "no, not allowed to backfeed".

--
Regards, Joerg

http://www.analogconsultants.com/
Reply to
Joerg

Yes, it is possible to re-program the PIC all the time.

Are you sure the firmware is running? What is your config for the MCLR pin and how did you connect it? If your config is this:

#pragma config MCLRE = ON

then you have to connect it to your 5V supply (with a diode and resistor), and from MPLAB click the symbol "Bring target MCLR to Vdd" after programming to start the program, if you don't want to unplug the programmer all the time.

Some time ago I had problems with my old PICstart programmer and long wires. But with PICkit2 and pin headers near the CPU no problems so far.

--
Frank Buss, http://www.frank-buss.de
piano and more: http://www.youtube.com/user/frankbuss
Reply to
Frank Buss

Ok, I guess RTFM is not in your vocabulary.

If you connect the 6 pins of the programmer directly to the pins of PICxx with out the suggested isolation resistors and diode, you would be correct.

If you add the 4 resisters and one diode, there would not be a problem.

As you have not specified which PIC you are using, I can only suggest you RTFM for the chip you have chosen.

hamilton

Reply to
hamilton

To help complete your library:

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Reply to
hamilton

As far as my experience goes some PICs are programmed using a slightly higher voltage (5.5V IIRC) and verified at lower voltages to make sure the flash always works as expected. The rest of the circuitry should allow for that. Applying external power could cause problems.

--
Failure does not prove something is impossible, failure simply
indicates you are not using the right tools...
nico@nctdevpuntnl (punt=.)
--------------------------------------------------------------
Reply to
Nico Coesel

Probably- it's equivalant to having a giant output capacitor at power-down and there's no limit on the output capacitor size. Usually they're okay if the voltage is less than 6V so it won't break down a Vbe junction.

Best regards, Spehro Pefhany

--
"it's the network..."                          "The Journey is the reward"
speff@interlog.com             Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog  Info for designers:  http://www.speff.com
Reply to
Spehro Pefhany

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