Peter L. Peres email (remove spam text)
On Mon, 2 Nov 1998, [iso-8859-1] André Malafaya Baptista wrote:
> There are several schematics for PIC16C84 programmer on the Internet. Most
> of them use the TXD line from the serial port to obtain +12V (mainly the
> ones that use COM84) which they wire to the MCLR (Vpp) pin via a resistor.
> I'm intrigued on how they do this. In order to get +12V on TXD, one must
> continuously transmit zeroes which I think is impossible (because of the
> STOP bit which is -12V). If TXD ever get negative, I think that would ruin
> the 16C84, or not?
Yes, however there is a way to do this by setting the BREAK condition in
the serial transmitter. Look up a NS16550[A] datasheet / register level
programming. The 12V method works only for the 84 as far as I can tell.
> Other thing:
> to verify the contents of program memory, they use CTS to read from the
> 16C84. But what can be read is TTL levels 0 and 5 volts which are
> incompatible with RS232 levels of -12 and +12 volts. I can't understand how
> does this work either? What will be read by the serial port when there's a
> zero voltage or 5V at the PIC data output?
Most serial receivers are slightly assymetrical to accomodate just this
case and to give the receiver a defined state if not connected to
anything. They thereby violate RS232 level standards slightly, but it is
for a good cause.
In reply to: <C007F153AE4BD111943600608C30C9936E1895@SENT5>
See also: www.piclist.com/techref/microchip/devprogs.htm?key=programmer
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