PIC18F87J90 Datasheet, PDF (319/450 Pages) Microchip Technology – 64/80-Pin, High-Performance Microcontrollers with LCD Driver and nanoWatt Technology

PIC18F87J90 FAMILY
24.5 Measuring Time with the CTMU
Module
Time can be precisely measured after the ratio (C/I) is
measured from the current and capacitance calibration
step by following these steps:
1. Initialize the A/D Converter and the CTMU.
2. Set EDG1STAT.
3. Set EDG2STAT.
4. Perform an A/D conversion.
5. Calculate the time between edges as T = (C/I) * V,
where I is calculated in the current calibration step
(Section 24.3.1 “Current Source Calibration”),
C is calculated in the capacitance calibration step
(Section 24.3.2 “Capacitance Calibration”) and
V is measured by performing the A/D conversion.
It is assumed that the time measured is small enough
that the capacitance, COFFSET, provides a valid voltage
to the A/D Converter. For the smallest time measure-
ment, always set the A/D Channel Select register
(AD1CHS) to an unused A/D channel; the correspond-
ing pin for which is not connected to any circuit board
trace. This minimizes added stray capacitance,
keeping the total circuit capacitance close to that of the
A/D Converter itself (25 pF). To measure longer time
intervals, an external capacitor may be connected to an
A/D channel and this channel selected when making a
time measurement.
FIGURE 24-3:
TYPICAL CONNECTIONS AND INTERNAL CONFIGURATION FOR
TIME MEASUREMENT
CTEDG1
CTEDG2
PIC18F87J90
CTMU
EDG1 _{Current Source}
EDG2
Output Pulse
ANX
RPR
A/D Converter
CAD
 2010 Microchip Technology Inc.
DS39933D-page 319