The logic to light the characters appears in the bottom half of the simulation window. The top half of the simulation window implements a modulo-12 counter. In other words, the counter increments up to 11 then resets. This counter is used to determine which character we should output to the 7-segment display. The truth table for the design can be found in the Design Spreadsheet.
Turn all pins OFF, keep the clock running and watch the rolling text on the 7-segment display. Or turn pin 3 ON and manually select a letter using pins 4 to 7.
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| # | Input | Output | Bidirectional |
|---|---|---|---|
| 0 | segment a | ||
| 1 | reset (sync) | segment b | |
| 2 | segment c | ||
| 3 | clock override | segment d | |
| 4 | clock bit 3 | segment e | |
| 5 | clock bit 2 | segment f | |
| 6 | clock bit 1 | segment g | |
| 7 | clock bit 0 |