Thanks to Roman, David and other people to introduce me with this matter.

My experience using the Picstart Plus for ICSP applied on the PIC16F873
is following:

The five standard ICSP signals (Vdd, GND, MCLR, RB6, and RB7) are
connected from the board to the programmer. Power Vdd from programmer is
separated from Vcc on the board with a Schottky diode.  MCLR has pull-up
10K via a diode (1N4148) and Cap 0.01uF to the GND.

1. Picstart Plus (Firmware V.2.01.00)

Programmer always uses Vihh (13V) for programming and does not control
pin RB3.  I thought    LVP enable  implies low voltage programming mode
but it only determines LVP config bit.  For this mode in the PIC16F87X
Spec is noted:
 The high voltage programming mode is always available, regardless of the
state of the LVP bit, by applying Vihh to the MCLR pin.

2. LVP Enable programming (LVP config. bit is set)

RB6 and RB7 are set as output. Programming works w/ or w/o pull-up or
pull-down resistors.
Programmer does not care for RB3, but it can not be used in application.
I set it as output and pull down with 10K. If I leave it to float,
despite it is set as output, PIC will be very sensitive to any
disturbances. Touching with finger pushes PIC somewhere (program modes?)
after it passes reset and recovers to work. In this case RB6, RB7 are set
as output with 33K pull-ups?  WDT is disabled.

3. LVP Disable (LVP bit is cleared)

I have no any problem with this mode. RB3 is used by my application. RB6
and RB7 could be used but I don t need them. They are set as output with
pull-up 33K, and 1K toward input.

4. Conclusion

- High voltage programming mode is very convenient. LVP bit can be
cleared or set how much time you want but once it is cleared only high
voltage programming mode is available. Changing LVP bit actually has only
effect on the application possibility to use or not use RB3 pin.
- If you do not really need LVP mode, disable it.  You will get one pin
more and less worry about unwilling program/verify mode.
- LVP disable mode does not release you from possibility to go into
unwilling program mode. Spikes from solenoid in my application sometime
push PIC into undefined mode because of high sensitivity of the MCLR
pin.  I can not apply a 5V transzorb (Vihh=13V), but a Cap.  0.01uF can
help by now.  Good protection of the MCLR pin has also to meet driving
capability of the programmer.

Regards,
Vlado Kecman

--
http://www.piclist.com#nomail Going offline? Don't AutoReply us!
use mailto:listserv@mitvma.mit.edu?body=SET%20PICList%20DIGEST