Vpp and Vdd timing for some PICs

[Ron]

Hello,

When programming PIC like 16F84 and 16F876, is it possible to apply
Vpp (13V) and Vdd (5V) at the *same time* and still remain within
Microchip specifications?
I have to ask because I have a hard time interpreting Microchip data.

Thanks!

 

[Jan Panteltje]

Hello,

When programming PIC like 16F84 and 16F876, is it possible to apply
Vpp (13V) and Vdd (5V) at the *same time* and still remain within
Microchip specifications?
I have to ask because I have a hard time interpreting Microchip data.

Thanks!

Yes you have to have Vdd at +5V, and then raise Vpp from zero to +13V
to start the programming cycle.
Vpp is also !MCLR, so keep it at 5V in normal operation, 0V for reset.

For some flow chart see for example:
39589b.pdf
(enter it in www.google.com)
JP

 

[Activ8]

Then get used to interpreting it 'cause you'll want to interpret
errata, too.

Yes you have to have Vdd at +5V, and then raise Vpp from zero to +13V
to start the programming cycle.

But you can't let /mclr hang at 5V for long or the chip will be
incrementing it's program counter and you'l start programming
somewhere in the memory map.

There's some errata on programming. www.picklist.com is a good place
to ask about these gotchas. Go there, read the page, read the FAQ,
then sign up.

 

[Luhan Monat]

Hello,

When programming PIC like 16F84 and 16F876, is it possible to apply
Vpp (13V) and Vdd (5V) at the *same time* and still remain within
Microchip specifications?
I have to ask because I have a hard time interpreting Microchip data.

Thanks!

Hi,

My experience is that it is sometimes *necessary* to raise both lines at
once. This is the case where the PIC was programmed for the internal
oscillator and no external MCLR. Holding MCLR low on power up has no
effect, the processor starts running, and (often) executes instructions
that make programming fail.

I had to make a special jig to 'recover' from this condition. Now, I
try never to specify the internal oscillator without enabling the
extenal MCLR.

 

[Ron]

Hi,

My experience is that it is sometimes *necessary* to raise both lines at
once. This is the case where the PIC was programmed for the internal
oscillator and no external MCLR. Holding MCLR low on power up has no
effect, the processor starts running, and (often) executes instructions
that make programming fail.

I had to make a special jig to 'recover' from this condition. Now, I
try never to specify the internal oscillator without enabling the
extenal MCLR.

You probably mean PICs that can have the MCLR/Vpp pin programmed as an
I/O pin like the 16F628 or 12F629. In that case, yes, the chip will
start its program as soonn as it goes out of reset state, no matter
what you put on the MCLR line, the reset being internal. The Vpp setup
time seems to be at least 2uS, according to Microchip. I deal with
that just by adding a small cap (100nF) to Vdd line, its rise time is
then slower (1V/10uS in my programmer) than Vpp which have picofarads
capacitance in its line.

Ron

 

[Wouter van Ooijen]

My experience is that it is sometimes *necessary* to raise both lines at

once. This is the case where the PIC was programmed for the internal
oscillator and no external MCLR. Holding MCLR low on power up has no
effect, the processor starts running, and (often) executes instructions
that make programming fail.

For the PICs fore which this is the case it is specified clearly in
the programmiong specificxation.


Wouter van Ooijen

-- ------------------------------------
http://www.voti.nl
PICmicro chips, programmers, consulting

 

[Wouter van Ooijen]

When programming PIC like 16F84 and 16F876, is it possible to apply

Vpp (13V) and Vdd (5V) at the *same time* and still remain within
Microchip specifications?

AFAIK not.

Note: there are some other PIC types that require a Vpp-before-Vdd
sequence!


Wouter van Ooijen

-- ------------------------------------
http://www.voti.nl
PICmicro chips, programmers, consulting