Ronald Dekker put together an LED clock using a PIC 16F84 microcontroller. Some people would wonder why a $5.00 clock from the store couldn’t be used. It could be, but where would the fun be in that?

“The 16F84 processor used here is a little bit out dated and no longer in production. I used it because of the simple fact that I had it lying around. The more modern 16F628 is pin compatible and will perform equally well. The 16F84 has too few I/Os to address all the anode and cathode drivers directly (Fig. 3). A 74HC563 8 bits latch was used to extend the number of outputs by another 8 bits at the expense of a latch enable signal taken from port A. The eight outputs from the latch, together with the lowest 4 bits of port B from the PIC drive address the cathode drivers. Since only one of the anode rows is switched on at any given moment, a HEF4028 1-of-10 decoder could be used to activate one of the six rows using only 3 bits (A0…A2). These bits are connected to the higher 4 bits of port B. To prevent unwanted illumination of LEDs during the multiplexing of data to the 74HC563, I/O bit A3 from port A is used to completely disable all the anode drivers. ”