Datasheet

PIC18F66K80 FAMILY
DS39977F-page 248  2010-2012 Microchip Technology Inc.
18.6 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 18.4.1 “Current Source
Calibration”
)
•
C is calculated in the capacitance calibra-
tion step (
Section 18.4.2 “Capacitance
Calibration”
)
•
V is measured by performing the A/D conversion
It is assumed that the time measured is small enough
that the capacitance,
CAD + CEXT, provides a valid
voltage to the A/D Converter. For the smallest time
measurement, always set the A/D Channel Select bits
CHS<4:0> (ADCON0<6:2>) to an unused A/D channel,
the corresponding pin for which is not connected to any
circuit board trace. This minimizes added stray capaci-
tance, 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 18-3: TYPICAL CONNECTIONS AND INTERNAL CONFIGURATION FOR TIME
MEASUREMENT
PIC18F66K80
A/D Converter
CTMU
CTED1
CTED2
AN
X
A/D Voltage
EDG1
EDG2
CAD
CEXT
Current Source