I have no idea, but... Quote from Microchip ERRATA sheet for rev A1 12c5XX silicon: If INTRC mode is selected, and GP5 = output, pin will become INPUT after exec. of SLEEP instruction. If XT/LP selected, GP4 will become and INPUT after exec. of SLEEP. Maybe this is relevant, and just hasn't been picked up by MC in the specific case you mentioned. (You didn't specify a PIN number) ALSO, page 28 of the 12c508 book says that the TRIS register is set to --11 1111 after wake-up, reset, WDT etc. This means that your IO pin goes HIGH Z, and the PC's pull-up/down (your guess what the manufacturer put in there) can cause your problem. ALSO, just a thought, check that you write the desired value into the GPIO register BEFORE you set the TRIS register. Otherwise you may get a (very) short LOW pulse just before you set it high. Hope it helped. At 10:46 AM 1/27/97 +0000, you wrote: >I'm using a 12C508 with one of it's output pins tied directly to the RxD >pin on a PC's printer port (i.e. 0..5V levels rather than true RS232 -- >but works without problems since minimal cable lengths). >Even though I set this output pin to 1 within 10us after reset, I still >get s on the PC every time the PIC resets on the WDT (at 9600bps). >Why? > >Frank > >------------------------------------------------------------------------ >Frank A. Vorstenbosch Phone: +44-181-941 7899 >Electronics & Software Engineer Mobile: +44-976-430 569 >Eidos Technologies Ltd., Hampton, Middx Fax: +44-181-941 7895 > > Jan van der Watt [Consultants never charge too much - they work too litlle !]