Designing a stopwatch( 3 digit counter)

Thread Starter

mpn10

Joined Oct 5, 2020
13
I need to make a 3 digit synchronous up decade counter that can count up to a certain number. I can get it to count from 000 to 999 but I cant get it to stop.
 

Thread Starter

mpn10

Joined Oct 5, 2020
13
I forgot to attach it. I also meant that the counter needs to reach a number and then reset.
Im using a 74LS192 sync counter with a 7447 BCD.
 

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dl324

Joined Mar 30, 2015
11,914
I forgot to attach it.
It's unreadable without a lot of effort by the reader. A good schematic would use blocks for functions, not IC's with connections to unlabeled pins (few of us use 74LS enough to remember pin functions).

This counter symbol is more typical:
clipimage.jpg

The preferred flow in a schematic is left to right and top to bottom. For counters, I'd probably use right to left and bottom to top; but that's an exceptional case to make the schematic more readable:
clipimage.jpg
 
Last edited:

dl324

Joined Mar 30, 2015
11,914
OK I tried to draw the schematic excluding the connections to the 7 segments.
That's much easier to understand than first attempt. Is that first picture from a simulator?
  1. You don't show any attempt at stopping the counter.
  2. You show the down and load inputs as being floating.
  3. What have you been taught to do with unused inputs?
  4. How does the unused clock input need to be connected for the other clock to be active?
 

Thread Starter

mpn10

Joined Oct 5, 2020
13
Yeah the Simulator is called Atanua.
Down and clock I left floating and it was able to count 000-999. I'm not sure if I made a mistake there??

OK here's what I tried. I tried to stop the count at 777 using the NAND gate below for each counter and connecting it to the load. I did it for the 100th counter first, stopping the count at 700 and goes back to 000 successfully. But I fail when I try to add a NAND gate to the 10th counter and make it stop at 770. It counts 000 to 070 and then goes to 100-101 instead of 071-072
 

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dl324

Joined Mar 30, 2015
11,914
I tried to stop the count at 777 using the NAND gate below for each counter and connecting it to the load.
Your definition of stop is different than mine. Stop to me means stop; as in stop the clock. Load isn't stopping the counters.
I did it for the 100th counter first, stopping the count at 700 and goes back to 000 successfully.
You can call that counter 100's, but not 100th. That would mean counter number 100, implying there are 99 before it.
But I fail when I try to add a NAND gate to the 10th counter and make it stop at 770.
Why do you think it did something at 70 instead of 770?

Your simulator must be doing something weird with floating inputs. The counter shouldn't increment or decrement unless the other clock input is HIGH.
1603325340732.png
 

Thread Starter

mpn10

Joined Oct 5, 2020
13
Your definition of stop is different than mine. Stop to me means stop; as in stop the clock.
I meant to say the count should reset at 777 instead of 999. I'm not quite sure how I can make the 10's continue to count up to 99 instead of 70 before incrementing the 100's. And the 1's also increment the 10's until 7, not 9.

There is only one clock input( 10 Hz) and And for unused inputs, they're connected to either connected to VCC or ground.
 

dl324

Joined Mar 30, 2015
11,914
I meant to say the count should reset at 777 instead of 999. I'm not quite sure how I can make the 10's continue to count up to 99 instead of 70 before incrementing the 100's. And the 1's also increment the 10's until 7, not 9.
Post your schematic. It would also help if you posted the entire text of the problem because what you're describing would make a poor stopwatch.
 

Thread Starter

mpn10

Joined Oct 5, 2020
13
OK I drew the schematic with what I did in order to try and get the count to reset at 777. But like I said, the count only increments until 007 for 1s and 070 for 10s

The homework requires us to count from 000 to a specific number abc. So yeah it's not really a stopwatch.
 

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dl324

Joined Mar 30, 2015
11,914
OK I drew the schematic with what I did in order to try and get the count to reset at 777. But like I said, the count only increments until 007 for 1s and 070 for 10s
Try describing in words the conditions required to reset counter to zero, and compare that with what you've implemented.

The way you're labeling the inputs to the NAND gates is ambiguous. You have 3 QA's, 3 QB's, etc. Why don't you just connect the inputs to their corresponding outputs? That's the way it would normally be done.
 

Thread Starter

mpn10

Joined Oct 5, 2020
13
Ok I used the 3 NAND gates representing the number 7 to try and get the whole count to count from 000-777, having an increment of 00.1 seconds until 00.9, where it will then go to 01.0 seconds. And for the 10's it should go from 01.0 until 09.9, where it will then go to 10.0 seconds. The 100's should also go from 10.0 until 77.7 seconds. But what Ive implemented instead has the 1's incrementing until 00.7 and then going to 01.0, the 10's incrementing until 07.7 and then going to 10.0.
 

dl324

Joined Mar 30, 2015
11,914
Have you analyzed what your circuit is doing?

You're decoding 7 from the first counter and using that condition to load something (you said those inputs were HIGH, but now you're implying they're LOW) into the first counter. Why do you think the other counters don't advance?
 

djsfantasi

Joined Apr 11, 2010
6,970
Ok I used the 3 NAND gates representing the number 7 to try and get the whole count to count from 000-777, having an increment of 00.1 seconds until 00.9, where it will then go to 01.0 seconds. And for the 10's it should go from 01.0 until 09.9, where it will then go to 10.0 seconds. The 100's should also go from 10.0 until 77.7 seconds. But what Ive implemented instead has the 1's incrementing until 00.7 and then going to 01.0, the 10's incrementing until 07.7 and then going to 10.0.
Think carefully about what you said and compare it to what you want.

When each digit reaches its max, do you want to reset/load/stop the counter?

Or do you want to reset the count when all digits reach their max?
 

Thread Starter

mpn10

Joined Oct 5, 2020
13
When each digit reaches its max, do you want to reset/load/stop the counter?

Or do you want to reset the count when all digits reach their max?
I want the count to reset once it has reached the number 777 like how a normal 3 digit decade counter would reset at 999 and begin the count again from 000
 
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