PWM Duty Cycle Question

[leels]

Hi,

I'm trying to measure the duty cycle of a PWM signal (ie. from an ADXL202 accelerometer) using a PIC16F873A. I'm using period of 1ms for PWM. I'm trying to avoid using the CCP module, since eventually I will need to take readings from 4 accelerometers. And as far as I can understand it, there are only 2 CCP modules available on the PIC. As you can tell I'm pretty new to using the PIC microcontrollers and will appreciate any help/advice you guys can spare.

Also since I will eventually need to figure it out, if anyone can shed some light on how to connect 4 ADXL to a PIC it would help me out alot.

[newguy]

4 sensors which output a PWM signal, but a PIC with only two CCP inputs.

The other two sensor inputs can be connected to (if your PIC has them) the external interrupt inputs or the change on port B interrupt. They can then be used to measure the PWM duty cycle. It will be a little involved, but it will work.

[MikeValencia]

Furthermore, you can just use one of the CCPs, but you'll need a 4-to-1 multiplexer, thus more i/o lines (to select the channel)

[Kenny]

I would connect Xfilt and Yfilt outputs to the analog inputs of the pic, bypassing the duty cycle modulator on the accelerometer. The modulator doesn't perform well at low accelerations where noise predominates anyway.

However, the internal resistance to the filter capacitor is 32k, and the maximum recommended impedance to the pic analog input to the pic is 10k, so a buffer op amp would be required for each channel to maintain accuracy and sample rate. If this is not a problem the buffer could be left out.

Four ADXL202 accelerometers means eight channels, so an eight channel analog multiplexer chip like the T.I. MPC508A would be needed. In this case only one buffer op amp on the multiplexer output would be needed.
I have used this method with the superlative Sunstrand and Columbia Research accelerometers.