vhxs/digital-logic-stuff

Note taking on digital logic design.

I want to build a 24-hour binary clock (hours/minutes/seconds) from scratch. This includes everything from the designing the circuit to the building the casing. I'll track my notes and incremental progress here.

Parts distributors

• Elenco
• Jameco
• Digikey

Integrated circuits

• Dual inline packaging.
• Pinouts to make sense of inputs and outputs.
• TTL vs CMOS. CMOS more sensitive, more expensive, lower power. I have TTL ICs.

TTL family

• 5 volts.
• 7400 NAND
• 7402 NOR
• 7408 AND
• 7432 OR
• 7486 XOR
• 74266 XNOR
• 7447 BCD (binary-coded decimal) to 7seg
• 7474 D flip-flop
• 7475 Bistable latch (don't remember what this is?)
• 74148 Decimal to BCD encoder
• 7476 JK flip-flop. Two flip-flops wired together in a master-slave (unfortunate naming...) configuration
• 7490 decade counter. Saves you the trouble of wiring four JK flip flops sequentially.
• 75690 8-bit counter
• 74283 4-bit full adder

Flip flops

• Set and reset are synchronous and take effect on the next clock cycle.
• Preset and clear are asynchronous and take effect immediately.
• Edge triggered vs level triggered. The former seem to be preferred.

Switches

• Single pole, double throw (SPDT)

References

• Decade counter schematic: https://en.wikipedia.org/wiki/Counter_(digital)#Decade_counter
• 7400 series: https://en.wikipedia.org/wiki/List_of_7400-series_integrated_circuits
• Digital clock from scratch: https://medium.com/@erikvanzijst/a-digital-quartz-clock-from-scratch-a80ec5e427
• Debouncing; https://ece.charlotte.edu/sites/ece.charlotte.edu/files/media/ecgr2156-experiment-2-switch-debouncing.pdf

Design notes for a digital clock displaying hours, minutse, and seconds in binary:

• 5 LEDs for 24 hours, 6 LEDs for 60 minutes, and 6 LEDs for 60 seconds means I'm using 17 LEDs total
• Place them in three rows
• Use an 8-bit counter with clear logic for hours, minutes, seconds, respectively?
• Switch for toggling between two modes: letting the clock run (default) and letting the user configure it
• Two push buttons: one for incrementing the hour and another for incrementing the minute
  • These should do nothing if the user is not configuring the time
• Use adders to increment the state? Or will the user pushing the button function as a clock signal?
  • May need to implement debouncing?

Next steps:

• The circuit above is somewhat complex. I should use a circuit simulator to test it out before ordering more parts.
• I knew how to use NI Multisim at some point, but its free version is very limited in its capabilities.
• LTSpice looks like a free circuit simulator. I should start with a simple combinatorial circuit.
  • The computer architects I know have mentioned this in the past.
  • Based on SPICE? https://en.wikipedia.org/wiki/SPICE