PIC18F26K80-I Datasheet, PDF (248/622 Pages) Microchip Technology – 28/40/44/64-Pin, Enhanced Flash Microcontrollers with ECAN™ and nanoWatt XLP Technology

PIC18F66K80 FAMILY
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
CTED1
CTED2
ANX
CEXT
PIC18F66K80
CTMU
EDG1 _{Current Source}
EDG2
A/D Voltage
A/D Converter
CAD
DS39977F-page 248
 2010-2012 Microchip Technology Inc.