How to determine logic to drive hours of a TTL digital clocks.

[LarryManalo]

Hello,

I know there are many threads about building TTL digital clocks, but I am not here to ask for a schematic or a complete design. I have actually been working on a design for my electronics class using 74LSxx series IC, such as the 74LS61 for the counter chips for example. I am sorry I do not have a circuit diagram of what I have so far to present, but the clock will use a crystal oscillator and 7HC4060 to lower the frequency to what is desired and the seconds and minutes just use logic to create 5- and 9- detectors to reset and rollover to the next counter to count up. I have corresponding decoder chips for the displays (which happen to be IN-18 nixie tubes) and I even created a 12V power supply to supply my voltage regulator to output 5V and 170V. The only issue I am having, which is probably the hardest part about the clock, is figuring out how to drive the hours section of the clock correctly to work with a 12-hr format. So far, I know that the 10s hr counter chip will drive the 1s hr counter chip (since when the 10s hr chip is outputting 0, the 1s hr chip will need to display 1-9, and when the 10s hr chip is outputting 1, the 1s hr chip will need to display 0-2). I also know that it is possible to create the roll-over logic without using flip-flops, but I do not know how to set up a Karnaugh map for this situation or how the wiring would go. I do not want a complete answer, but just some hints to help me start solving this issue on my own. Thank you for your time and do have a nice day.

 

[chuckey]

Draw up a truth table, columns:-output of counters /display. Select which states are required to reset 10s counter, build gate circuit to accomplish this.
Frank

 

[betwixt]

For a 12 hour clock you actually need to detect '3' in the hours digit and use that to reset the counters. You can do it as suggested by inverting bits that are zero then ANDing them with the other bits so when '3' is reached all the bits are at the same logic level.

An alternative, and slightly cleverer way is to use a TTL 8-bit comparator (or two 4-bit comparators) and feed it with the tens of hours and unit hours bits on one side and a pre-set logic number 13 on the other. When the numbers match the 'equal' output of the comparator will go active and you can use that to reset the counters. It is more versatile to do this because if you change the comparison from 13 to 25 you get a 24 hour clock instead.

Brian.