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I need to make a circuit that generates a PWM signal for a motor. The motor has 2 inputs - P and Q. They are the inverse of each other. Now, both signals aren't allowed to be high at the same time. That's why there needs to be a programmable Dead Time - the time between the signal that is high to go low and the low signal to go high.

Now I know how to easily make a PWM signal. It's allowed to have a fixed signal width and fixed period - because it's for educational purposes. To generate this, I just have a comparator with on one input a 8-bit-counter continuously counting, and a constant on the other.

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Now, I need to somehow check whether the signal toggles upwards or downwards. If it goes upwards, it can go straight through, but else it has to load a loadable 8-bit counter with certain value and wait for it to overflow before the new high output gets outputted.

For the loadable counter part I should have enough knowledge, but I don't know how to detect the edges (preferably asynchronous stuff should be avoided)

jdepypere
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1 Answers1

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You can produce two outputs from one PWM signal using an RC filter and a couple of logic gates. Try this: -

enter image description here

Because you don't want either to be high at the same time, invert the OR gate's output. Use schmitt trigger gates for the best result.

Andy aka
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  • Thanks Andy, for the answer, can you please explain how the RC Filter work here? I am not able to understand it. – 0xF1 Mar 02 '14 at 20:08
  • Ok!! Let me tell you first what I think how it works: At the rising edge of square pulse the CAP provides a delay since it takes a while to get charged to HIGH level. So AND gate outputs a '1' late, then again it holds the charge for a while at the falling edge; now since original PWM pulse goes to '0', AND outputs a LOW. In both the cases the OR Gate always has at least one input HIGH hence it produces '1' for long. Is that right? – 0xF1 Mar 02 '14 at 20:14
  • @MadHatter spot on!!! – Andy aka Mar 02 '14 at 21:01