> From:         Daren X[SMTP:attesaman@HOTMAIL.COM]
> Sent:         Monday, January 05, 2004 5:43 PM
> To:   PICLIST@MITVMA.MIT.EDU
> Subject:      [PIC:] VIHH on MCLR doesnt put PIC in programming mode

> Hi Everyone,

> Here is some feedback from the replies I have got so far.

> The voltage on the VPP pin was 11.9v, I removed a 100ohm resistor on the
> programmer to increase it to 12.9v, still no difference.

> I have grounded the CLKIN pin which stops it from executing code,
> reading from the PIC still doesn't work.

> The pic is an 877, not 877A

> The programmer works when the PIC is in its own target board, but not in
> my target veroboard.

> The PIC is definitely running when MCLR is at 12v as the LCD displays
> the intro message and rs232 communications begin.

> I have RB4-7 set as inputs, also have pull-ups enabled on those pins for
> input buttons (which aren't connected at the moment).

> I can look into minimising any ringing on the clock and data lines, but
> I don't think there is a point until I can get this MCLR problem sorted.

> Regards,
> Daren

First, have you measured the Vpp voltage at the chip? There may be
a bad connection between the programmer and the MCLR pin.

If that is not the problem, there may be a capacitor on your board
from ground to MCLR. *This is not good.* There is a maximum rise time
(typically 1 microsecond) for Vpp to rise. If the rise time is too long,
for example slowed down by a "reset" capacitor on the MCLR line,
the chip may not enter programming mode before it begins to
execute code. Once execution begins, programming mode is
impossible. This is mentioned in some of the programming
documents.

John Power

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