PIC18F87J90 Datasheet, PDF (312/450 Pages) Microchip Technology – 64/80-Pin, High-Performance Microcontrollers with LCD Driver and nanoWatt Technology

PIC18F87J90 FAMILY
The CTMU current source may be trimmed with the
trim bits in CTMUICON using an iterative process to get
an exact desired current. Alternatively, the nominal
value without adjustment may be used; it may be
stored by the software for use in all subsequent
capacitive or time measurements.
To calculate the value for RCAL, the nominal current
must be chosen and then the resistance can be
calculated. For example, if the A/D Converter reference
voltage is 3.3V, use 70% of full scale or 2.31V as the
desired approximate voltage to be read by the A/D
Converter. If the range of the CTMU current source is
selected to be 0.55 A, the resistor value needed is cal-
culated as RCAL = 2.31V/0.55 A for a value of 4.2 MΩ.
Similarly, if the current source is chosen to be 5.5 A,
RCAL would be 420,000Ω, and 42,000Ω if the current
source is set to 55 A.
FIGURE 24-2:
CTMU CURRENT SOURCE
CALIBRATION CIRCUIT
PIC18F87J90
Current Source ^CTMU
A value of 70% of full-scale voltage is chosen to make
sure that the A/D Converter is in a range that is well
above the noise floor. Keep in mind that if an exact cur-
rent is chosen to incorporate the trimming bits from
CTMUICON, the resistor value of RCAL may need to be
adjusted accordingly. RCAL may be also adjusted to
allow for available resistor values. RCAL should be of
the highest precision available, keeping in mind the
amount of precision needed for the circuit that the
CTMU will be used to measure. A recommended
minimum would be 0.1% tolerance.
The following examples show one typical method for
performing a CTMU current calibration. Example 24-1
demonstrates how to initialize the A/D Converter and the
CTMU; this routine is typical for applications using both
modules. Example 24-2 demonstrates one method for
the actual calibration routine. Note that this method
manually triggers the A/D Converter. This is done to
demonstrate the entire stepwise process. It is also
possible to automatically trigger the conversion by
setting the CTMU’s CTTRIG bit (CTMUCONH<0>).
ANx
RCAL
A/D
Trigger
A/D Converter
A/D
MUX
DS39933D-page 312
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