Measuring Temperature with a dsPIC® Digital Signal Controller (DSC) CTMU

When measuring temperature with the CTMU consider the following:

  1. The diode current needs to between 5.5 µA or 55 µA. Low current will have an error due to leakage paths in the mux, while higher current will make the curves nonlinear.
  2. The slope and intercept of the VF vs. temperature line shift as the diode current level changes.
  3. The diode VF can vary lot-to-lot due to process variations. (Calibration offsets may be needed.)
  4. The current is steered to the diode only when EDG1 and EDG2 of the CTMU are equal (00) or (11). Since the POR state of the EDG1/2 bits is 00, there is no need to assert these signals to steer the current to the diode.
  5. The datasheet gives the diode voltage at 25°C.
  6. Use the 10-bit ADC configuration. Not all parts have the diode connected in 12-bit mode.

This is example is based on information provided in the datasheet:

  • ADCVal - ADC value of the forward voltage VDD used as reference (3.3 V)
  • ADC_STEPS - 1023 (for 10-bit ADC) VF - Diode forward voltage reading
  • SLOPE: Rate of change (See section 30-56: Electrical Specification)
  • DIODE_25C: Internal diode forward voltage at 25°C
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