The design uses in it's core a series of modified 2-bit-up-down-counters that allow for setting a custom maximum value. Carry information is only provided if this is intended by the operating mode. The mode can is selected from individual inputs by a mode selection module. The output of the counters is serialized and encoded for the 7-segment-display.
For each digit used (due to limitations that's only 2) a push button in active high configuration is used. A third push button for the refreshing of limits is also required. Further more, 3 switches are used to determine the operating mode. On the output side, for each digit an 8-bit-shift-register and a 7-segment-display is used, as the output value is serialized. For the shift clock two inverse outputs are provided, in case a shift register with both a load and output input. In that case, the shift_clk is used for loading and not_shift_clk is used for output. The design was made with the register HC595 in mind, but will work with other 8-bit shift registers.
| # | Input | Output | Bidirectional |
|---|---|---|---|
| 0 | Button 0 In | Digit 0 Out | Up-Down-Select In |
| 1 | Button 1 In | Digit 1 Out | Set-Carry In |
| 2 | Set-Max In | ||
| 3 | Refresh-Limits In | ||
| 4 | |||
| 5 | |||
| 6 | Shift-Clk Out | ||
| 7 | Not-Shift-Clk Out |