Setting limits using 74190

[asdfjkley]

How can I set the limit of my seconds and minutes thaat it will reset once it reached 59 using 74190 counter?

 

[Reloadron]

Can you post a circuit of what you have?

The 74190 is a SYNCHRONOUS UP/DOWN COUNTERS WITH DOWN/UP MODE CONTROL. You mention 59? You don't mention MM:SS or how you have things configured. For example 59:59 (59 Min and 59 Sec) would require 4 EA of 74190 counters cascaded. You mention 59 but also mention min and sec?

Ron

 

[MrAl]

Hi,

The way this is usually done is to break the 59 down into two separate decade counters, one for the "5" and one for the "9".

The "9" does not need any special work because the counter is a divide by 10 already so it resets to "0" after the "9".

The "5" on the other hand would need a NAND gate to detect the binary code for "6" which is 0110 (MSB on the left, LSB on the right).
Normally you'd need a four input NAND gate for this but because the sequence is also defined we can get away with a two input NAND gate.
The inputs to the NAND gate get connected to bit 1 and bit 2 of the counter output (that is QB and QC) and the output goes to the LOAD' pin.
Also, the data inputs all have to go to ground so that when the LOAD' pin becomes active the counter loads a zero. That makes it go to zero a very very short time after the counter gets the sixth clock pulse.

Any set of digits that need this function can be done the same way, whether it is the hours or the minutes or even the seconds if you have that, but the hours is done slightly different because you may want to show a 12 hour or a 24 hour time format. In that cause you change the inputs to the NAND gate and you can use a flip flop to show AM/PM if
you are doing the 12 hour format.

So to do the format:
12:59:59

you'd need three, two input NAND gates and a flip flop for AM/PM and perhaps another flip flop for the tens of hours digit (it can only be 1 or 0 here). If you do 24 hour time then you need at least one counter or two flip flops for the tens of hours digit.

Note the LOAD' pin is read as "NOT LOAD" or "LOAD NOT" or similar. It is usually shown as the word "LOAD" with a line over the top but i cant show that here.

 

[k7elp60]

I agree with Reloadron & Mr Al. If you were to use 7490's of 74LS90 it be easier to divide by 60 as those two counters have the nand gates in the package to be reset.

 

[asdfjkley]

I've set the limits already thankyou

 

[asdfjkley]

How can we used the flipflop to set limits in hours? That when the tens digit of hours became zero the ones need to be 1-9. And when thye tens digit became 1 the ones place need to be 0-9.

 

[MrAl]

Hi,

Assuming you are doing 12 hour time, you would use a 0 to 9 counter for the one's hour digit and a flip flop for the tens of hours digit.
You would have to detect when the ones digit goes to a "3" and when the tens of hours goes to a "1" with a NAND gate or combination of NAND gates.
When the combination of "1" and "3" is detected, the counter gets set to "1" and the flip flop gets reset (that makes it zero).
So the count goes: 01,02,03,04,05,06,07,08,09,10,11,12, then back to 01 to start over.
Detecting the "1" and the "3" would require a three input NAND gate or combination of two inputs gates.

For 24 hour time format it goes 00,01,02,03,...,21,22,23, then back to 00, so there you need to detect the count "24" in the hours digits.

 

[asdfjkley]

Thaankyouuu ;D

 

[asdfjkley]

The problem now is how to determine if its AM or PM?

 

[MrAl]

Hi,

Ok, so i guess you are most interested in the 12 hour format for now.

AM/PM is just an extension of the hours themselves. It's a binary state either AM or PM, and it changes after each 12 hour roll over, so a single Flip Flop connected to the 'carry' of the 12 hour digits. So when the hours count reaches 13 it sets the hours count to zero and also toggles the Flip Flop for AM/PM.
Really this is the same as if you had 24 hour time except instead of making the tens of hours digit go to 2 we instead toggle the flip flop, and the output of the flip flop tells us the AM/PM. If the flip flop output is zero then it is AM, if the flip flop output is 1 then it is PM.

There is a small catch here though, and that is that when the NAND gate (or combination of NAND gates) detects the "13" hours, it is going to reset the hours digits to "00" and that will cause the output of the NAND gate to cease. This will happen very fast, just after the NAND gate propagation period has ended, and that means that there is a chance that the flip flop will not have enough time to respond to the output of the NAND gate before it changes state. Luckily however after the NAND gate state changes the counters take a small time to reset to "00" so that should be enough time to flip the flip flop to the next AM/PM state. You may want to make sure of this however and add a small delay if needed. If the counter reset (or load depending on device type) is long enough it should work fine. That means the gate changes after one Tp and the counters reset taking Tr time, then the gate changes after another Tp time, so the output pulse will be about Tp+Tr time lenght which is most likely enough for the flip flop toggle input.

 

[ChrisP58]

One thing to remember is that AM and PM don't roll over when 12:59 becomes 1:00, but when 11:59 becomes 12:00.