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Solution to interview question about clock circuits

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wanchope

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Hi, all,
I just been to a company for a hardware eng position interview.
There is one digital design question as illustrated in the attached picture.
Can anyone think of how to design a circuit that can transform from 'A' to 'B'?

regards
 

Re: interview question

This looks like a monostable multivibrator? Here's a quote from wiki on multivibrators:

"When triggered by an input pulse, a monostable multivibrator will switch to its unstable position for a period of time, and then return to its stable state. If repeated application of the input pulse maintains the circuit in the unstable state, it is called a retriggerable monostable. If further trigger pulses do not affect the period, the circuit is a non-retriggerable multivibrator."

Sounds like you have a retriggerable monostable multivibrator.
 

interview question

As I see in the figure a divide by 16 can generate B from A.
 

Re: interview question

Looks like a divided by 32 with a delay retrigger.

Added after 4 minutes:

Easiest circuit would be a PIC12C508A set up to count input pulses. Incorporate a watch dog timer and your done. Single chip solution under $5.00. Probably 1 hr of engineering time. Total cost $140.00. Not bad.

Red
 

Re: interview question

You could use simple RC filter. That would do the trick... It is not digital solution, but it works
 

Re: interview question

Thanks all for your reply.
There are 2 more conditions
1. it may not be necessarily 16 clock cycles. What if 15 or 14?
2. The pulse width of clock in Diagram A is 50ms
3. The duration of B is around 15-20 seconds. Therefore RC may not be a good idea.
 

Re: interview question

hi.............
i think this is a counter circuit....it counts the give pulses..when all pulses are counted it change its state..from high to low or vice versa...
i think so....its counting pulses may very depending upon given pulses...may be.........but i m not sure....

hope itz true...

best luck...
bye
 

Re: interview question

I also thought it should be counter circuit. Any good example?
 

Re: interview question

Counter, or maybe timer, since you dont know how many pulses there is...
But if you know the duration of pulse and pause, you can use timer... Just measure time from last pulse to 50 or 60ms...
 

Re: interview question

Can a '555' do this?
How should I connect the trigger pin?
 

Re: interview question

the LM555 can do that. check its datasheet and formulate/calculate the necessary Resistances (Ra and Rb). The pulses above correspond to the turn-on and turn-off of the device. the question is... what is the period of the First Pulse? the Second Pulse? after you identify the t (time in period) you can calculate the resistances and the capacitor for oscillation.

best regards,
randell_xtian :D:D:D
 

Re: interview question

Again, use a PIC12CXXXX. It only has 8 pins and can be reconfigured easily.
It is faster to develop and change than a 555.

Hope this helps,

Red
 

Re: interview question

Hi
I understand that the author needs a discrete circuit that does this job, so I can't see why people are suggesting to use powerful ucontrollers for such a simple task.

If you know the pulse duration of pulse B, I think all you need is a monostable multivinrator , which is normally off, but has an ON time of 15or 20 sec, whichever you need, when triggered.

I dont think you need a retriggarable multivibrator. A monostable using 555 timer should be all you need. You can get the circuit anywhere on the net, or in any good book on electronics.

Hope this helps you.
:)
 

interview question

its definitely a counter circuit . as it gives one pulse for 16 pulses it is a divide by 16 counter. the A signal acts a triggering pulse (negative pulse) for the clock signal of that counter.
 

Re: interview question

Look at the 555 as a missing pulse detector.

BTW, I todays age, as an EE for a defense subcontractor, I would like to see this implemented in different ways. I would like to see someone suggest some type of RC circuit, but if they suggested a uC, I would take it. An RC solution is going to be a uni-tasker. But the uC will be expandable.

For the same pin count as a 555 and not having to use RC components, I would take a 8-pin uC in any circuit to replace a 555.

Anybody looking for a job in Dallas?

Hope this helps,
Red
 

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