I'm looking at the CRAMPS thermistor calculations in
`hal/user_comps/hal_temp_bbb.py`, and having trouble understanding one
piece.
The comments say:
# The CRAMPS board thermistor input has one side grounded and the other side
# pulled high through a 1.00K resistor to 1.8V. Following this is a 4.7K
# resistor, some protection diodes, and filtering capacitors. The ADC
voltage
# read is the filtered voltage across the thermistor.
Change the pull-up resistor value to 2.00K, and this description matches
variable values in `adc2r_cramps()` as well as the CRAMPS schematic [1]
(p. 5, RHS).
The problem is in function return value. While it seems it should
return the resistance of the thermistor, it instead appears to calculate
the combined resistance of the thermistor and the 4.7k resistor in parallel.
The calculation above in `adc2r_bebopr()` does the calculation I would
expect: the intermediate value of the combined parallel thermistor and
resistor resistance is called `R_TD`, and the calculation goes on to
find the separate thermistor resistance of `R_T`.
Can anyone help clarify?
[1]: http://reprap.org/mediawiki/images/5/5a/CRAMPS.v2.1.sch.pdf
John
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