PIC18F25K80 MICROCHIP [Microchip Technology], PIC18F25K80 Datasheet - Page 247

• Manufacturer Part Number: PIC18F25K80
• Manufacturer: MICROCHIP [Microchip Technology]

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 R
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
R
if the current source is chosen to be 5.5  A, R
be 420,000 Ω, and 42,000 Ω if the current source is set
to 55  A.
FIGURE 18-2:
 2011 Microchip Technology Inc.
CAL
= 2.31V/0.55 A , for a value of 4.2 M Ω . Similarly,
ANx
R
CAL
CTMU CURRENT SOURCE
CALIBRATION CIRCUIT
Current Source
MUX
PIC18F66K80
A/D Converter
A/D
CAL
A/D
Trigger
CTMU
, the nominal
CAL
would
Preliminary
PIC18F66K80 FAMILY
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 R
ingly. R
resistor values. R
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
A/D Converter and the CTMU.
This routine is typical for applications using both
modules.
Example 18-2
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>).
CAL
also may be adjusted to allow for available
demonstrates how to initialize the
demonstrates one method for the
CAL
CAL
should be of the highest precision
may need to be adjusted accord-
DS39977C-page 247