rs232 power up failure

csanders · Joined: 23 Apr 2005 Posts: 16

I've got an 18F8720 mcu that when is first powered up, the hardware uart is not receiving any characters, but when the mcu is reset, begins receiving data as expected? Anybody have any ideas for the cause of this malfunction?

PCM programmer · Joined: 06 Sep 2003 Posts: 21708

Do you have the BROWNOUT fuse enabled ? If not, add it.
Set the brownout voltage to a value appropriate for your Vdd voltage.
Example: If your Vdd is +5v, then use the BORV42 fuse, which sets
the brownout voltage to +4.2v.

csanders · Joined: 23 Apr 2005 Posts: 16

I did not have those fuses set to begin with, but after adding them I am getting the same results.

#fuses WDT, HS, MCU, PUT, NOLVP, WDT128, NODEBUG, BROWNOUT, BORV42

Do you know of anything else that could be causing this?

PCM programmer · Joined: 06 Sep 2003 Posts: 21708

The first thing I would do, is to get rid of the Watchdog timer while
testing this problem. Change the fuse to NOWDT.

Also, do you have a pullup resistor on the MCLR pin ?

There is an errata that says there's a possible reset problem with this PIC.
http://ww1.microchip.com/downloads/en/DeviceDoc/80172c.pdf
Does your problem only affect the UART receiver, or does it affect your
entire program ?

Can you post an extremely short test program that demonstrates the
problem ? This would be a program that would echo incoming chars
from the PC, back to the terminal window on the PC.