Datasheet

 2010 Microchip Technology Inc. Preliminary DS39974A-page 407
PIC18F47J13 FAMILY
26.5 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 26.3.1 “Current Source Calibration”),
C is calculated in the capacitance calibration step
(Section 26.3.2 “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
AD + CEXT, 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 cor-
responding 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 (4-5 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 26-3: TYPICAL CONNECTIONS AND INTERNAL CONFIGURATION FOR TIME
MEASUREMENT
A/D Converter
CTMU
CTED1
CTED2
AN
X
EDG1
EDG2
CAD
CEXT
Current Source
PIC18F47J13
A/D Voltage