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'[PIC]: PIC16F877 programming & RA4 / T0CKl questio'
2001\07\04@021302 by Jeff DeMaagd

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Can anyone tell me why RA4 is an "open drain" output when all the other
PORTA pins are "full output drivers"?  It took me a while to figure that one
out, for a while I thought I had configured timer0 wrong.

Also, it takes a long time for the Picstart Plus to program the entire code
space of the 16F877.  Is there a workaround?  Am I doing something wrong
with my serial port?  Can I set my entire project and the programmer to
PICF874 so it only programs the first 4k?  I don't want to  break my parts
just trying it.  I am only using a few hundred bytes at the moment while
experimenting with several code ideas.

I am runing Windows NT 4 and I've tried setting the serial port to the max
possible rate.  It seems like it can only program about 32 bytes a second,
so it programs in over four minutes.

Jeff

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2001\07\04@083800 by Patrick J

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From: "Jeff DeMaagd"
> Also, it takes a long time for the Picstart Plus to program the entire code
> space of the 16F877

Jeff, there is a halfway workaround. Enalble the picstartplus, and then
go into the menu:
Picstartplus -> program/verify and change end address to say 0200h but
as soon as that window is closed the setting goes away so you haveto redo
that or leave it open. Programming only takes a few secs so I think its worth
it !

/PJ

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2001\07\11@145014 by uter van ooijen & floortje hanneman

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> Can anyone tell me why RA4 is an "open drain" output when all the other
> PORTA pins are "full output drivers"?  It took me a while to figure that
one
> out, for a while I thought I had configured timer0 wrong.

The 'normal' PIC pins have a protection diode to Vcc, so you can not use 'm
to switch a voltage greater than Vcc. (not even using a simulated open-drain
by switching the direction). So with such a pin it would not be possible for
me to create my programmer (which switches the Vpp using a PIC pin - RA4 of
course). So uChip included - for my joy - one pin without the upper
protection diode, that can withstand a voltage up to 14V. I guess the upper
driver FET would not like being reverse-biased, so they had to make it open
drain. In't it great that I had such an impact even before I knew what a PIC
was?

Wouter

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2001\07\16@074541 by Wollenberg, Frank

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On Wednesday, July 11, 2001 6:40 PM, Wouter [wfspamKILLspamXS4ALL.NL] wrote
>
> The 'normal' PIC pins have a protection diode to Vcc, so you
> can not use 'm
> to switch a voltage greater than Vcc. (not even using a
> simulated open-drain
> by switching the direction). So with such a pin it would not
> be possible for
> me to create my programmer (which switches the Vpp using a
> PIC pin - RA4 of
> course). So uChip included - for my joy - one pin without the upper
> protection diode, that can withstand a voltage up to 14V. I
> guess the upper
> driver FET would not like being reverse-biased, so they had
> to make it open
> drain. In't it great that I had such an impact even before I
> knew what a PIC was?
>
If i remember correctly, Microchip has revised the specification in the
errata.
RA4 can withstand up to 10V.
Correct me if i'm wrong.

Frank

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2001\07\16@162318 by mike

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On Mon, 16 Jul 2001 13:45:05 +0200, you wrote:

{Quote hidden}

RA4 does not have a diode to Vdd, but I've seen at least one mention
somewhere of this being used for factory test, so using it above Vdd
could be risky. This may  only apply to older devices - I think it was
mentioned in a very old C5x data sheet.

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2001\07\16@164804 by Spehro Pefhany

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At 01:45 PM 7/16/01 +0200, you wrote:

>If i remember correctly, Microchip has revised the specification in the
>errata. RA4 can withstand up to 10V.
>Correct me if i'm wrong.

Not even. The current 16F877 data sheet (DS30292C) shows an ABSOLUTE
MAXIMUM rating on RA4 of 8.5V (page 149). This was also in the
"Preliminary" spec's in the previous data sheet (DS30292B) on
page 151.

Best regards,

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