I'm working on a design that has a keypad, a 16F1847 and a MAX3232 for
RS232 conversion.  The idea is to have the PIC sleep until a key is pressed
and then have the circuit transmit an RS232 string depending on which key
on the keypad has been pressed.  I started with just the keypad and the PIC
with a flashing LED to indicate which key had been pressed.  I have the
keypad connected to the PIC on portb.  All worked quite well with the PIC
consuming 10 ua in sleep.  When a key was pressed the PIC woke up, flashed
the LED the requisite amount of times and then went back to sleep.

I then added a MAX3232 to the circuit and interface the RS232 side of this
chip to a PC.  I had the MAX3232 powered from one of the pins on the PIC so
I could turn it off except when it was needed to transmit data.  I'm
bit-banging the serial data because the EUSART pins are on portb where the
keypad is connected.

When I hook up the power connection on the MAX3232 everything still works
fine, with a power consumption of about 10 ua except when I'm pressing a
key.  But when I hook up the TTL side to the MAX3232, the idle current
jumps to 80 ua and after a keypress, the PIC doesn't go back to sleep.  The
PIC datasheet says that one must be careful during sleep not to have I/O
pins floating and I'm guessing that by having the MAX3232 powered down, I'm
essentially allowing those two connections to float and this is causing the
problem.  But keeping the power on to the MAX3232 would defeat the purpose
of putting the PIC to sleep.

By experimenting with the MAX3232 connections one at a time, I've been able
to determine that it is the connecting the PIC receive data line that
causes the idle current consumption to rise from 10 to 80 ua and it is
connecting the PIC transmit data line that causes the PIC to not go back to
sleep.

Anybody have any thoughts on how to get around this?

Thanks in advance,

John
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