Many thanks, Matt. There are some days when I get stuck on really simple concepts that=20 always have been clear in my mind. I guess this is one of those days . Yeah - I've been doing the old RC-Time timer for decades with no=20 problems. This time, its my very-often-used technique where I=20 multiplex a configuration jumper (input) on top of a output driver or=20 LED. But it's an old project that I'm doing a fairly major re-write=20 on and this caught my eye. Any other day of the week: it probably wouldn't have been a=20 problem. But today it had me worried. Many thanks for your clear description. dwayne At 01:31 PM 12/6/2013, Matt Pobursky wrote: >Think about this -- if read port register instructions affected the output >bit latch states then all port reads would result in unpredictable=20 >port operation. > >If you look at the block diagram of the PIC IO port structure in any=20 >of the data >sheets, you'll see that the only way an output bit latch is changed is wit= h >a write operation. > >I've done this numerous times where I set the port bit low, set pin to >output to discharge a capacitor, set pin back to input which allows a the >cap charge, read and wait for a logic 1 state (RC timing on one pin). >Alternate discharging and timing by setting or clearing the TRIS bit, neve= r >have to change the logic 0 on the port bit latch as it's persistent. I >suspect you are doing something very similar to this. --=20 Dwayne Reid Trinity Electronics Systems Ltd Edmonton, AB, CANADA (780) 489-3199 voice (780) 487-6397 fax www.trinity-electronics.com Custom Electronics Design and Manufacturing --=20 http://www.piclist.com/techref/piclist PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist .