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PICList Thread
'In-circuit programming (again)'
1997\05\08@130830 by obo (Ingenieria Fotonica)

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Hello to all,
I need to program a 16c84 part soldered to the PCB, and I've built two
homemade programmers: a "ludipipo" for the serial port and a "topic" for
the parallel port.
None of them are able to program the pic in the circuit, but they work
fine with a "off-circuit" PIC.  I've read in this List some things to try:

  - Stop the PIC oscillator grounding the OSC1 pin.
  - Shield the cables to the PIC.
  - Cables as short as possible.

My questions is: It's really needed to stop the PIC oscillator? What else
can I try?
Thanks in advance.


-----------------------------------------------------------------------
|         Adolfo Cobo Garcia  -  UNIVERSIDAD DE CANTABRIA               |
|    E.T.S.I.I. y Telecomunicacion, Grupo de Ingenieria Fotonica        |
|        Avda. Los Castros s/n  E-39005  Santander  SPAIN               |
|  Tfno. +34-42-201539  Fax +34-42-201873  Email: spam_OUTacoboTakeThisOuTspamteisa.unican.es |
-----------------------------------------------------------------------

1997\05\09@112852 by mbhstdj

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Adolfo Cobo Garcia wrote:

>  I need to program a 16c84 part soldered to the PCB, and I've built two
> homemade programmers: a "ludipipo" for the serial port and a "topic" for
> the parallel port.
>  None of them are able to program the pic in the circuit, but they work
> fine with a "off-circuit" PIC.

What circuitry is connected to RB6 and RB7?  As explained in the
hardware.txt file that came with the TOPIC info this programmer can
only drive a load of around 20-30kohms when the PIC is in-circuit
(based on the assumption that the parallel port uses open-collector
drivers).  Also how do you ensure /MCLR is raised to VPP?  With both
LUDIPIPO and TOPIC you will have problems if you don't isolate the
normal connection to VDD using a diode (the TOPIC schematic shows
the sort of thing).  You asked whether it is really necessary to stop the
oscillator while programming in-circuit.  Using the TOPIC software
I've not found this to be necessary using either an RC (4k7, 22pF) or
an XT (4MHz) oscillator (which are the only configurations I've
actually tested).  My guess is your problems are due to lack of drive
to RB6/RB7, poor control of /MCLR or using a excessively long cable to
connect the programmer to your board.  Have you looked at the programmer
described in Microchip Application note AN589 (or one of the clones)
as this has active drive of RB6/RB7?

David
--
http://www.man.ac.uk/~mbhstdj

1997\05\09@190255 by Tony Matthews

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Adolfo Cobo (Ingenieria Fotonica) wrote:
>
>  Hello to all,
>  I need to program a 16c84 part soldered to the PCB, and I've built two
> homemade programmers: a "ludipipo" for the serial port and a "topic" for
> the parallel port.
>  None of them are able to program the pic in the circuit, but they work
> fine with a "off-circuit" PIC.  I've read in this List some things to try:
>
>    - Stop the PIC oscillator grounding the OSC1 pin.
>    - Shield the cables to the PIC.
>    - Cables as short as possible.
>
>  My questions is: It's really needed to stop the PIC oscillator? What else
> can I try?
>  Thanks in advance.
>
I had to isolate the programming pins with a switch even though the
circuits the pins were connected to were relatively high in impedance I
suspect rise time problems due to capacitive loading.Afterwords it
occured to me that I probably could have built a cmos buffer into a dip
clip and succeded but I did'nt try it.There's no doubt others on the
list with more usable advice but I just could'nt help myself..Tony M.

1997\05\09@201543 by Dennis Kegley

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I think the app notes say something about some isolation resistors which
would take care of capacitive loading.

                                               Sincerely,
                                               Dennis Kegley
*******************************************************************************
A foolish constinecy is the hobgoblin of little minds.
                                               Ralph Waldo Emerson
*******************************************************************************

1997\05\12@163848 by Byon Garrabrant
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>  I need to program a 16c84 part soldered to the PCB, and I've built two
> homemade programmers: a "ludipipo" for the serial port and a "topic" for
> the parallel port.
>  None of them are able to program the pic in the circuit, but they work
> fine with a "off-circuit" PIC.  I've read in this List some things to try:
>
>    - Stop the PIC oscillator grounding the OSC1 pin.
>    - Shield the cables to the PIC.
>    - Cables as short as possible.
>
>  My questions is: It's really needed to stop the PIC oscillator? What else
> can I try?
>  Thanks in advance.

 I have used my Tait programmer to program 16F84s in-circuit (well,
in bread/pref board) without modification.  I DID NOT stop the
ceramic resonator, and I am using a short non-shielded cable.  I even
use the switched programmer power to power the device.  However, my
circuit does not yet use RB6 and RB7, so I can dedicate them to
programming.    If there is interest, I can share information on my
setup further.  I am very happy with my Tait programmer, it let me
start experimenting with PICs for very little cost!

Byon
________________________________________
Byon Garrabrant  N6BG  .....byonKILLspamspam@spam@netcom.com

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