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Signal problem from ADC to prop pin — Parallax Forums

Signal problem from ADC to prop pin

frank freedmanfrank freedman Posts: 1,983
edited 2012-11-11 10:58 in Propeller 1
Odd problem recently with the output from an MCP3201. The output from the ADC is a nice clean wave form, 0-5v as it is running from 5v supply to get max sps. The output is fed to a prop pin through a 3.9k resistor. The wave is about 3v or so, but the lows are not flat, rather a kind of downward spike loking, almost looks like a shark tooth shape. Just can't seem to pull it down cleanly. Resistor to low?

Thanks,

FF

Comments

  • TubularTubular Posts: 4,703
    edited 2012-11-11 02:40
    If you're running flat out (100ksps?) it could be the input circuit capacitance combined with your 3k9 giving you a lowpass filter effect. You could try reducing the resistor to see if it improves things (5.0-3.6)/(500uA) ~ 2k7. You could try lower than that for a a short while just to verify.

    And yes resistor to ground (effectively a resistive divider) might also work if you can use relatively low value resistors. Other than that you could use some kind of buffer or level translator
  • frank freedmanfrank freedman Posts: 1,983
    edited 2012-11-11 10:58
    Well further playing around shows the problem to be the output of the adc and not the prop. Also the initial versions of the circuit and object were done using the usb GG board which turns out to have a resistor network on one of the pins. I have put one of the 40dip props on the breadboard to consolidate the circuits and reduce noise etc. That is when the problem started. Now to figure out why. Also need to find the value of the resistor network. Fun....

    FF

    Later edit:

    Seems to be resolved (sorta) need to research and understand the cause of this issue. The resolution so far seems to be to run the output of the device into the input pin on the prop even though the supply to the device is 5v and the serial output is running about 4v. I noticed (unless I miss-read the print) that the C3 is taking the output from the MCP3202 directly into the prop pin in this design as well. Now I have (almost) the same drivers running clock timing and ADC capture as I had under pure PASM but now they are called and running under C using the LMM. May have to go back to the GG USB if I start playing with other memory models.
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