counter clock

[111]

I am trying to build counters using 7476 ICs. But the problem is with the clock I have made a manual pulse generator and it works well. But when i apply the clock to the clock input the counter is not counting the sequence properly. I clear the outputs and then also tried but again it is counting randomnly or sometimes it is not counting. Each time it is counting in different manner. i am sure with the circuit.
I then tried by connecting the clock to the ground temporarily for each clock pulse. Now the counter is counting as expected. What went wrong with the clock?

 

[fsahmed]

I am trying to build counters using 7476 ICs. But the problem is with the clock I have made a manual pulse generator and it works well.

Er... what exactly do u mean by a manual pulse generator? For the expression gives the notion that u r switching manually between levels.

I then tried by connecting the clock to the ground temporarily for each clock pulse. Now the counter is counting as expected. What went wrong with the clock?

I dont understand what u mean by connecting the clock to the ground? The clock is supposed to oscillate between Vcc & Ground.

 

[ljcox]

I have made a manual pulse generator and it works well. But when i apply the clock to the clock input the counter is not counting the sequence properly. I clear the outputs and then also tried but again it is counting randomnly or sometimes it is not counting. Each time it is counting in different manner.

i am sure with the circuit.
I then tried by connecting the clock to the ground temporarily for each clock pulse. Now the counter is counting as expected. What went wrong with the clock?

I assume you mean that the "manual pulse generator" is a push button switch. If so then you need a bounce suppressor.

Mechanical switches "bounce" on opening and closing. Bounce means that there is not a single close/open but multiple ones. So the counter counts each one. Hence the random results you are experiencing.

If you look at a switch contact when it is pressed or released with a storage oscilloscope, it looks like a picket fence.

A bounce suppressor is essentially a low pass filter followed by a Schmitt Trigger. Several have been discussed and circuits posted on this forum. So I suggest you do a search fro bounce suppressor.

I don't understand what you mean in the second part of the quote.

Len

 

[Dean Huster]

A debounced manual clock is not only mandatory so you get one pulse per switch actuation, but the clock edge has a maximum risetime limit. In other words, unless you have a Schmitt-trigger clock input (e.g., the 74121), a slow transition of the clock input will be ignored by the chip. At one time, this specification used to be given on data sheets, but I've not seen it in a long time and don't know why.

Dean

 

[mstechca]

Depending on the IC, you either attach a resistor (between 100 ohms and 1K) between ground and the clock pin or attach it between the clock pin and VCC. This will force a default setting on the clock rather than leaving it floating high.

One of those two connections should work.

 

[111]

Manual pulse generator is for generating clock pulses manually for each press of the push button. I have provided bounce eliminator for the pulser yet it failed to provide me the desired result.

By temporarily connecting the clock to gnd i connected the clock input to the ground as for each pulse i.e instead for logic1 and logic0 I performed logic0 and open circuit. Then the counter worked!

Anyway i traced the problem and it is with the pulser but what may be its trouble ?. I now connected astable multivibrator as the clock input and it is counting as i expected.

Again a problem, by using 7490 decade counter and astable multi as clk input it is not counting anything. What may went wrong again?

 

[Dean Huster]

The 7490, although it is probably the decade counter I've used the most, is a royal pain in the tooshie to make work, primarily because it has four reset pins (two for reset to 0, two for reset to 9) and because it's split into a divide-by-2 and a divide-by-5 counter and they must be connected together.

So, be sure that the Qa output is connected to the clock B input and use the clock A input as the input from your source to be counted. You MUST use this configuration if the counter is to drive a BCD-to-7 segment decoder driver. If there is no readout, you can use the clock B as your main input from your source, and connect the Qd output to the clock A input. Your final divide-by-10 output will then be on Qa. The only reason for the second method is that you'll get a perfect square wave from your decade output which may be necessary in some applications. But you can't use a decoder driver in any fashion whatsoever in this configuration, for you'll never get a proper count-up sequence.

As for the resets, these are active HIGH resets, so to count, all four pins must be tied to ground or a logical LOW. If left unconnected, they will float high and strangle the counter.

Dean

 

[mstechca]

Anyway i traced the problem and it is with the pulser but what may be its trouble ?. I now connected astable multivibrator as the clock input and it is counting as i expected.

Again a problem, by using 7490 decade counter and astable multi as clk input it is not counting anything. What may went wrong again?

Throw the "Pulser" in the garbage, and use an astable multivibrator.