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'[PIC]:Change of LVP status'
2001\06\24@133525 by =?iso-8859-1?Q?=C5ke_Neehr?=

picon face
Hello!

In Microchips directions of programming
for PIC16F87x  devices can You read
following:

"To disable low voltage ICSP mode, the
LVP bit must be programmed to'0'.
This must be done while entered with high
voltage entry mode (LVP bit= 1).
RB3 is now a general purpose I/O pin."

I don´t understand exactly what they say,
therefore I´m asking the autority on the
PIC-list before I destroy the chip,
Could it be like that:
If the chip is programmed with LVP ON
from the factory so must the pin RB3
beconnected to VDD (+5V) under the
programming before it can change from
LVP on to LVP OFF?

Please help me!

Regards
ÅkeN

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2001\06\24@143410 by Byron A Jeff

face picon face
On Sun, Jun 24, 2001 at 07:29:37PM +0200, Åke Neehr wrote:
> Hello!

Hi. I thought I just answered this question less than a week ago.
{Quote hidden}

It's very unlikely that you'll destroy the chip in any case.

> Could it be like that:
> If the chip is programmed with LVP ON
> from the factory so must the pin RB3
> beconnected to VDD (+5V) under the
> programming before it can change from
> LVP on to LVP OFF?

No. Here are the rules:

1) The chip can always be programmed in HVP mode. Always.
2) The chip can be programmed in LVP mode only if the LVP config bit is set.
3) The LVP config bit can only be changed in HVP mode. LVP mode cannot change    the LVP config bit. This prevents an LVP programmer from making the chip    not programmable with said LVP programmer.
4) When LVP config bit is set, RB3 must be held low except during LVP    programming. So that means that RB3 must be held low during program    execution and that RB3 is unavailable during execution. It's the only    design mistake I see in the whole process. LVP should have been assigned to    a end pin of a port instead of a middle pin.
5) There is an errata sheet that stipulates that RB3 should be held low
  during HVP programming even if the LVP config bit is off. It implies that    the chip could be put into a weird mode if RB3 is high (or floating) when    trying to program in HVP mode. Essentially the chip tries to go into LVP
  programming mode before it makes it to HVP mode.

  You can find that errata sheet here:

  http://www.microchip.com/Download/lit/suppdoc/errata/80076b.pdf

  As a precaution I always tie RB3 to ground through a resistor even if I'm
  building a HVP programmer for 16F87X parts.


That's it. That's all there the rules for HVP/LVP programming on 16F87X parts.

So here are the issues in answer to your specific question.

1) If it's an LVP programmer (setting RB3 to +5 as you propose) you cannot change the LVP config bit according to rule #2 above.

2) If it's an HVP programmer, you should ground RB3 through a resistor (10k recommended in the errata sheet) so you would not set it to +5. An HVP
programmer would be able to change the LVP config bit.

So there is no way to change the LVP config bit if RB3 is set to +5.

Hope this helps,

BAJ

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