Re: Continuous Pulse-Width Measurement

jawereq@xxxxxxxxxxx wrote: 
Hi,

I'm wondering if anyone knows of a microcontroller or some cheap hardware in combination with a micro that can be used to continuously measure and buffer all of the high and low pulse-widths of a binary signal. I'd like to be able to buffer about 1000 16-bit width measurements in one shot.

The catch is that the smallest pulse width measurable needs to be 700ns or better (500ns would be nice). I've looked at using an input capture pin to a 20MHz or faster auto-cleared timer in combination with a DMA controller to transfer the captured pulse-widths to RAM, but all the transfer cycles generally add up to well over 1 micro-second. The 20MHz timer gives sufficient resolution, but the time to buffer the data is too long.

Right now, the company I work for accomplishes this exact feat using an ASIC. Together, the current ASIC and micro cost about $7. Can the same be done for less money, and possibly with a single chip?

I appreciate anyone's suggestions on this problem.

How much resolution do you need and how long of a period do you need to capture over? How many bits? If 500ns of resolution is sufficient and you have the RAM space, you might just do a CPU copy into RAM space and then do the measurements as a postprocessing step looking through RAM for transitions. For single bit monitoring, an 8 bit shift register that is clocked from an accurate source would give you either 8x the resolution or 1/8 the storage space. Read the parallel outputs of the shift register and copy them to RAM. Do the measurements as a software postprocessing step. Obvously this approach will only work if you have enough free RAM and don't need continuous operation.

Bob
.

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