Datasheet

 2010 Microchip Technology Inc. Preliminary DS39979A-page 313
PIC18F87J72 FAMILY
25.5 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 25.3.1 “Current Source Calibration”),
C is calculated in the capacitance calibration step
(Section 25.3.2 “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 C
OFFSET provides a valid voltage to
the A/D Converter. For the smallest time measurement,
always set the A/D Channel Select register (AD1CHS)
to an unused A/D channel; the corresponding 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 25-3: TYPICAL CONNECTIONS AND INTERNAL CONFIGURATION FOR TIME
MEASUREMENT
A/D Converter
CTMU
CTEDG1
CTEDG2
AN
X
Output
Pulse
EDG1
EDG2
CAD
RPR
Current Source
PIC18F87J72