PIC18F45K80-I Datasheet, PDF (241/622 Pages) Microchip Technology – 28/40/44/64-Pin, Enhanced Flash Microcontrollers with ECAN and nanoWatt XLP Technology

PIC18F66K80 FAMILY
The CTMU current source may be trimmed with the
trim bits in CTMUICON, using an iterative process to
get the exact current desired. Alternatively, the nominal
value without adjustment may be used. That value may
be stored by software, for use in all subsequent
capacitive or time measurements.
To calculate the optimal value for RCAL, the nominal
current must be chosen.
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 calculated 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 18-2:
CTMU CURRENT SOURCE
CALIBRATION CIRCUIT
PIC18F66K80
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. If an exact current is chosen to
incorporate the trimming bits from CTMUICON, the
resistor value of RCAL may need to be adjusted accord-
ingly. RCAL also may be adjusted to allow for available
resistor values. RCAL should be of the highest precision
available, in light of the precision needed for the circuit
that the CTMU will be measuring. A recommended
minimum would be 0.1% tolerance.
The following examples show a typical method for
performing a CTMU current calibration.
• Example 18-1 demonstrates how to initialize the
A/D Converter and the CTMU.
This routine is typical for applications using both
modules.
• Example 18-2 demonstrates one method for the
actual calibration routine.
This method manually triggers the A/D Converter to
demonstrate the entire step-wise 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
 2010-2012 Microchip Technology Inc.
DS39977F-page 241