PIC18F44J50-I/SPSQTP 查看數據表（PDF） - Microchip Technology

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PIC18F44J50-I/SPSQTP

28/44-Pin, Low-Power, High-Performance USB Microcontrollers with nanoWatt XLP Technology

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 RCAL, the nominal cur-

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 RCAL = 2.31V/0.55 A, for a

value of 4.2 MΩ. Similarly, if the current source is cho-

sen to be 5.5 A, RCAL would be 420,000Ω and

42,000Ω if the current source is set to 55 A.

FIGURE 26-2:

CTMU CURRENT SOURCE

CALIBRATION CIRCUIT

PIC18F46J50 Device

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 that is to incorporate the trimming bits

from CTMUICON, the resistor value of RCAL may need

to be adjusted accordingly. RCAL may also be 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 26-1

demonstrates how to initialize the A/D Converter and

the CTMU. This routine is typical for applications using

both modules. Example 26-2 demonstrates one

method for the actual calibration routine.

ANx

RCAL

A/D Converter

A/D

MUX

DS39931D-page 404

 2011 Microchip Technology Inc.

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