Datasheet

PIC18F87K90 FAMILY
DS39957D-page 420  2009-2011 Microchip Technology Inc.
27.6 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. To do that:
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 27.4.1 “Current Source Calibration”)
•
C is calculated in the capacitance calibration step
(
Section 27.4.2 “Capacitance Calibration”)
•
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 that channel
selected whenever making a time measurement.
FIGURE 27-3: TYPICAL CONNECTIONS AND INTERNAL CONFIGURATION FOR TIME
MEASUREMENT
A/D Converter
CTMU
CTED1
CTED2
AN
X
Output Pulse
EDG1
EDG2
CAD
RPR
Current Source
PIC18F87K90