Werner Soekoe wrote: >I wonder if anyone could adivse me. I made a PIC16F84 circuit using all 13 >I/O pins to drive 13 LEDs, but the LED connected to RA4 doesn't light up. It >appears that the pin does provide a signal, though. When I read up some more >in the PIC16F84 datasheet, I came accross a paragraph which said that all RA >pins are CMOS drivers, except RA4 which is an "open-drain" output. Is this >why it can't deliver current? What does the "open-drain" mean? RA4 can pull down to ground but has no high-side transistor like the other pins. You can still use it to drive LEDs by hanging the LED & resistor between RA4 and +5V supply. (anode to +5V) This means that the logic is now inverted; 1 = off and 0 = on but that is easily catered for on the software side of things. Regards...