Datasheet

 2012 Microchip Technology Inc. DS30575A-page 521
PIC18F97J94 FAMILY
26.6 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 26.4.1 “Current Source
Calibration”)
• C is calculated in the capacitance calibra-
tion step (Section 26.4.2 “Capacitance
Calibration”)
• V is measured by performing the A/D conversion
It is assumed that the time measured is small enough
that the capacitance, C
AD + CEXT, provides a valid
voltage to the A/D Converter. For the smallest time
measurement, always set the A/D Channel Select bits
via ADCON1L/H to an unused A/D channel; the corre-
sponding 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 26-3: CTMU TYPICAL CONNECTIONS AND INTERNAL CONFIGURATION FOR TIME
MEASUREMENT
PIC18F97J94
A/D Converter
CTMU
CTED1
CTED2
AN
X
A/D Voltage
EDG1
EDG2
CAD
CEXT
Current Source