24 hour digital clock logic circuit

ericgibbs

Joined Jan 29, 2010
18,766
hi a,
You don't reset the counters at 23:59:59, but when it gets clocked to 24:00:00

So that’s when the Hours Units is switching to '4' when the Hours Tens is at '2'.

Use a AND gate on the Hours Tens & Units at 24:xx.xx
Remember the Bit weighting.
E
BYW: why for a 24 hour clock do you want a divide by 12 7492.??
 

Thread Starter

arian78

Joined Nov 22, 2021
37
hi a,
You don't reset the counters at 23:59:59, but when it gets clocked to 24:00:00

So that’s when the Hours Units is switching to '4' when the Hours Tens is at '2'.

Use a AND gate on the Hours Tens & Units at 24:xx.xx
Remember the Bit weighting.
E
BYW: why for a 24 hour clock do you want a divide by 12 7492.??
Sounds good, i'll try my best.
Is it wrong to use 7492 then? Cause professor said i must do that... so that's why i'm doing it.
 

dl324

Joined Mar 30, 2015
16,845
Is it wrong to use 7492 then? Cause professor said i must do that... so that's why i'm doing it.
If you were told that you need to use 7492, then you need to use 7492. Read the datasheet to determine the count sequence (or read the table I posted in #27. That will indicate which digits can use that counter.
 

MrChips

Joined Oct 2, 2009
30,708
Sorry for joining the party late.

We were taught Digital Electronics in the final year of a 4-year Engineering Physics program.
We were taught boolean algebra, logic gates, four types of flip-flops, registers, counters, ripple and synchronous counters, etc.

We were never given part numbers.
We were not told the difference between 7490 and 7492, or 7447 and 7448. We had to figure out these on our own by reading manufacturers' datasheets.

Edit: In other words, we were taught the theory in the lectures. We had to explore and discover practical circuits in the laboratory on our own. We were not given any practical assignments. We had to be creative and design our own circuits. Of course, in those days there were no CAD simulators and no internet.
 

MrChips

Joined Oct 2, 2009
30,708
I must be missing something by not reading through the entire thread carefully.
I think 7490 and 7492 circuits were designed particularly for building 24-hour clock circuits.
You can cascade 7490 and 7492 directly for counting from 00 to 59. No additional gates are required.

Have I missed out on something?
 

MrChips

Joined Oct 2, 2009
30,708
I don't see why you need a gate.

Connect QC, QB, QA to the 7447. Make sure the D-input of the 7447 is tied to GND.
Take QC output to the next 7490 stage.

Edit: In other words, the circuit shown in post #6 is correct except for the floating inputs.
 

dl324

Joined Mar 30, 2015
16,845
I don't see why you need a gate.

Connect QC, QB, QA to the 7447. Make sure the D-input of the 7447 is tied to GND.
Take QC output to the next 7490 stage.
You need the 7492 to reset itself and the 7490 when the count gets to 60. That requires a HIGH on reset. If you use Qc, it would reset the counters at the count of 40. If you use Qd, which goes HIGH at 60, for reset, that signal needs to be inverted to clock the next stage because they clock on the falling edge.

EDIT: You're right. It can work with no gates, but you'd need to use multiple outputs.
 

MrChips

Joined Oct 2, 2009
30,708
QC, QB, QA will count from 0-5. There is no need to reset.

Qc goes high on 4 and goes low on 0 (i.e. 8 = count 6) . The falling edge will clock the next 7490 correctly.
 

Thread Starter

arian78

Joined Nov 22, 2021
37
You need the 7492 to reset itself and the 7490 when the count gets to 60. That requires a HIGH on reset. If you use Qc, it would reset the counters at the count of 40. If you use Qd, which goes HIGH at 60, for reset, that signal needs to be inverted to clock the next stage because they clock on the falling edge.

EDIT: You're right. It can work with no gates, but you'd need to use multiple outputs.
Yes it's working without gates right now, and i did let proteus work for one hour just to be sure it will reset at 59:59 which it did... now i can carry on and connect it to the hours.

Just to be clear... i do need AND gates to connect tens of minutes to units of hours and again AND gate to connect units of hours to the tens of hours, right? Here's a picture of the whole thing right now.
I made it from buttom to top, just to not get confused. but for the final design i will make it from left to right on the paper and on the proteus.
Frop top to buttom there's tens of hours, units of hours, tens of minutes, units of minutes, tens of seconds, units of seconds.
 

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dl324

Joined Mar 30, 2015
16,845
I made it from buttom to top, just to not get confused. but for the final design i will make it from left to right on the paper and on the proteus.
Left to right makes no more sense than bottom to top.

A note on style. Since the functionality for the seconds digits is the same as the minutes digits, there's no reason for the schematic layout to be different either.

EDIT: The symbol Proteus uses doesn't make much sense to me. It's very common for the Qa output to be used to provide the clock to the other 3 flip flops. This makes more sense to me:
1640713196931.png
 
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Thread Starter

arian78

Joined Nov 22, 2021
37
Left to right makes no more sense than bottom to top.

A note on style. Since the functionality for the seconds digits is the same as the minutes digits, there's no reason for the schematic layout to be different either.
You're right, they are exactly the same. Thanks for all the help, and how can i work with the hour part now? I believe the units of hour can go up to 9 no problem, for like hour 19 and 20 and 21... but with tens of hour it shouldn't go up to 23 ... so... Can you give me some advices about it please?
 

ericgibbs

Joined Jan 29, 2010
18,766
Use a AND gate on the Hours Tens & Units at 24:xx.xx
Remember the Bit weighting.
hi a,
It is the same as I said earlier.
AND gate , Input '2' from Tens and the other input '4' on the other from Units counter
E
 

MrChips

Joined Oct 2, 2009
30,708
Just to be clear... i do need AND gates to connect tens of minutes to units of hours and again AND gate to connect units of hours to the tens of hours, right? Here's a picture of the whole thing right now.
No. You do not need to reset the seconds and minutes. They are self-resetting.
You do not need gates to cascade from minutes to the hours.

You only need to reset hours when 24 is reached.
 

MrChips

Joined Oct 2, 2009
30,708
We have shown you how to approach this problem.
Now it is your turn to put some effort into it. This is your homework, not our homework.
 
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