PIC18F25J50 MICROCHIP [Microchip Technology], PIC18F25J50 Datasheet - Page 404

• Manufacturer Part Number: PIC18F25J50
• Description: 28/44-Pin, Low-Power, High-Performance USB Microcontrollers with nanoWatt XLP Technology
• Manufacturer: MICROCHIP [Microchip Technology]

PIC18F46J50 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 optimal value for R
rent 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
value of 4.2 MΩ. Similarly, if the current source is cho-
sen to be 5.5 A, R
42,000Ω if the current source is set to 55 A.
FIGURE 26-2:
DS39931D-page 404
ANx
R
CAL
CTMU CURRENT SOURCE
CALIBRATION CIRCUIT
CAL
Current Source
MUX
PIC18F46J50 Device
would be 420,000Ω and
CAL
= 2.31V/0.55 A, for a
A/D Converter
CAL
A/D
, the nominal cur-
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 that is to incorporate the trimming bits
from CTMUICON, the resistor value of R
to be adjusted accordingly. R
to allow for available resistor values. R
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.
demonstrates how to initialize the A/D Converter and
the CTMU. This routine is typical for applications using
both modules.
method for the actual calibration routine.
Example 26-2
 2011 Microchip Technology Inc.
CAL
may also be adjusted
demonstrates one
CAL
CAL
Example 26-1
should be of
may need