The '628 errata mentions that when LVP is enabled, MCLR is MCLR whether or
not you want it to be (ie: regardless of the CONFIG value).

See DS80073F - page 9, item 5.

Bob Ammerman
RAm Systems

----- Original Message -----
From: "Kyrre Aalerud" <kreature@C2I.NET>
To: <PICLIST@MITVMA.MIT.EDU>
Sent: Thursday, November 07, 2002 4:24 PM
Subject: [PIC]: Wierd thing about the 16F628 ???


> Hi all!  I've been off the list for quite some time now and it's nice to
be
> back :-)
>
> To the point...
>
> There was something wierd the orther day about my 16F628 based
prototype...
> I could simply not get the thing to execute code!
> After going through all the docs again I still hadn't found what was wrong
> so I did something that shouldn't have solved the issue according to
specs.
> I tied MCLR to VDD *even* if it was already tied to VDD internally.  (Yes,
I
> had disabled it because I didn't need it, and I was using LVP ICSP.)
> Now, everything worked as it was supposed to?
>
> It doesn't say anything in the docs or programming guide that even when
you
> disable MCLR, you need top pull it high during startup to avoin entering
> programming mode...  But that was what was happening!
> Either that, or some limbo-state occurred.  I'm not sure about this as I
> measured on the pinns while in this state, and some of them had 1.8v 2.1v
> and 3.2v on them, and they were actually not attached to anything???
> (Floating)
>
> I had previously made up a SCHAER compatible programmer that used MCLR av
> VDD control so that it would be compatible with LVP.    I did this based
on
> the info in the programming doc where it shows how to enter programming
> mode.  Now I have adjusted the programmer a little more and so it will now
> do LVP ICSP with the circuit wired with MCLR high or pulled high.  It even
> works with ic-prog :-)
>
> Oh'well...  Turned out that a wierd little glitch stopped me dead for many
> hours...
>
> I'm posting this here, now, because I want to know if I misinterpreted
> anything in the manuals or if others may have similar problems.  I have
> already searched general usenet and the piclist archives without finding
> anything, and so this might be helpful to others in the future...
>
> Oh, and if anyone is interested, I might post a schematic and PCB of the
> little programmer i made on my homepage...  It uses 3-wire only +VDD/VSS
and
> does LVP mode only.  The whole thing fits in the DB25 connector and
attaches
> to the LPT-port.  (This might beling in a EE: but I'm to lazy to write 2
> posts...)
>
> Just for the search engines in the future:
> On the PIC16F628, MCLR have to be pulled high during startup, regardless
of
> wether it is enabled or not to avoid entering programming-mode or some
other
> limbo-state.
>
> --
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>
>

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