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PICList Thread
'in-circuit assumptions'
1998\10\21@151105 by jim nestor

picon face
Fortunately I have lots of hair to pull out cause this problem is driving
me crazy!

I am trying to get going with in-circuit programming of the PIC 16F84.

I have tried using an ITU Pic-1 and a couple of different protoboards.

First of all, the Pic-1 works fine in regular programming.

I have made some assumptions which may not be correct.

1)  in-circuit means that the Pic is actually in a target circuit, such as
a protoboard
2) the target circuit is complete with xtal or resonator and power source
such as a 5v regulator and a 9v battery
3) the target circuit can actually execute the code in the Pic when not
being programmed

Another key assumption is that MCLR is connected to the Pic-1s VPP line,
and capable of being pulled low for reset and then to +12-14v for
programming by the Pic-1. I have tried a couple of variations on the MCLR
theme. the first was to isolate MCLR completely from the target board VDD
(+5v) supply during programming. The second was to use a diode (IN914)  and
resistor (10k) from +5v to MCLR and connect the Pic-1 VPP on the other side
of the diode so it could pull MCLR high or low without effecting the +5v to
the rest of the Pic.

Of course, I connected the Pic-1's lines to RB6 & RB7 and GND. I powered
the target circuit with a regulated supply and the Pic-1 with it AC wall
wart supply. It actually seemed to work, programming the 16F84 on the
target board.
the program would not run until I disconnected the lines from the Pic-1.

I tried unsucessfully to use a toggle switch to connect/disconnect thePic-1
lines. After that, it wouldn't program in-circuit any longer.
Throughout these tests, the Pic-1 to target board connections have been
less than 6" long and connected via crimpon connectors over header pins. I
tried WireWrap and very short leads but it didn't seem to matter.

I guess I'm most confused by the fact that the process sometimes works,
sometimes does not.

This was supposed to be simple. Am I missing something here?

Any suggestions would be appreciated. I can always stick with swapping
chips between programmer and target board. At a minimum, I need to get a
ZIF socket in the target board. BTW, I tried plugging the ZIF socket into a
machined pin 18-pic socket but it doesn't fit well, there isn't room to
solder the pins, and it keeps poppoing out of the socket on the target board.

I hate to give up, but my wife says I'm getting cranky.


1998\10\21@164130 by Russell McMahon

picon face
A possible source of problems:
Make sure that your target circuit does not load the Isp lines down
so much that they can't reach proper levels.
Also ensure that you programming voltage is high enough at the PIC
(as I recall the specified voltage is 13 volts). Generally MCLR may
be connected to a highish value resistor (say 10K) to your reset
circuitry which allows Vpp to be applied with an additional  load of
only 1ma or so, which most programmers will source without effort.

{Original Message removed}

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