“This project is 7400 series logic ICs based clock. It keeps time solely based on discrete logic ICs and a stable clock source.
For the display LEDs, I am using common anode 7 segment displays. This variant is 0.56” tall and has a red illumination color. There are 2 sets of LEDs as use the “colons” in a typical clock display.
The circuit keep track of time, in 12 hour format, using an external 1 Hz source. The 1 Hz source comes from a 4 pin header, and allows to be changed out in the future.
The second counter comprises of 4 ICs: 74LS47 (2x),74HC393N (1x), and 74HC08N (1x)
The 74LS47 is an active low 7 segment LCD driver. The 74HC393N is a dual 4 bit binary counter that used to count both the ONES and TENS section of the seconds part of the clock. One half of the IC counts the ONES and the other half of the TENS.
When the ONES counter reaches “1010” in binary, the AND gate resets the counter. The reason it’s using binary “1010” is because it is an active high reset. If “1001” was used, it would reset as soon as it hits 9, never actually seeing it. When it resets, it also increments the TENS counter.
When the TENS counter reaches “0110” in binary, the AND gate resets the counter. The reason it’s using binary “0110” is because it is an active high reset. If “0101” was used, it would reset as soon as it hits 5, never actually seeing it.
Since seconds and minutes both count from 0 to 59, this circuitry can be repeated for counting minutes as well.”