Microchip Technology ARD00385 Data Sheet
PIC18F87K90 FAMILY
DS39957D-page 420
2009-2011 Microchip Technology Inc.
27.6
Measuring Time with the CTMU
Module
Module
Time can be precisely measured after the ratio (C/I) is
measured from the current and capacitance calibration
step. To do that:
1.
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
where:
• I is calculated in the current calibration step
• C is calculated in the capacitance calibration step
• V is measured by performing the A/D conversion
It is assumed that the time measured is small enough
that the capacitance, C
that the capacitance, C
OFFSET
, 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.
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
MEASUREMENT
A/D Converter
CTMU
CTED1
CTED2
AN
X
Output Pulse
EDG1
EDG2
C
AD
R
PR
Current Source
PIC18F87K90