PIC24FJ64GA705 Datasheet, PDF (317/412 Pages) –

PIC24FJ256GA705 FAMILY
27.0 CHARGE TIME
MEASUREMENT UNIT (CTMU)
Note:
This data sheet summarizes the features of
this group of PIC24F devices. It is not
intended to be a comprehensive reference
source. For more information on the
Charge Time Measurement Unit, refer to
the “dsPIC33/PIC24 Family Reference
Manual”, “Charge Time Measurement
Unit (CTMU) and CTMU Operation with
Threshold Detect” (DS30009743), which
is available from the Microchip web site
(www.microchip.com). The information in
this data sheet supersedes the
information in the FRM.
The Charge Time Measurement Unit (CTMU) is a
flexible analog module that provides charge
measurement, accurate differential time measurement
between pulse sources and asynchronous pulse
generation. Its key features include:
• Thirteen External Edge Input Trigger Sources
• Polarity Control for Each Edge Source
• Control of Edge Sequence
• Control of Response to Edge Levels or Edge
Transitions
• Time Measurement Resolution of
One Nanosecond
• Accurate Current Source Suitable for Capacitive
Measurement
Together with other on-chip analog modules, the CTMU
can be used to precisely measure time, measure
capacitance, measure relative changes in capacitance
or generate output pulses that are independent of the
system clock. The CTMU module is ideal for interfacing
with capacitive-based touch sensors.
The CTMU is controlled through three registers:
CTMUCON1L, CTMUCON1H and CTMUCON2L.
CTMUCON1L enables the module, controls the mode
of operation of the CTMU, controls edge sequencing,
selects the current range of the current source and
trims the current. CTMUCON1H controls edge source
selection and edge source polarity selection. The
CTMUCON2L register selects the current discharge
source.
27.1 Measuring Capacitance
The CTMU module measures capacitance by
generating an output pulse, with a width equal to the
time between edge events, on two separate input
channels. The pulse edge events to both input
channels can be selected from four sources: two
internal peripheral modules (OC1 and Timer1) and up
to 13 external pins (CTED1 through CTED13). This
pulse is used with the module’s precision current
source to calculate capacitance according to the
relationship:
EQUATION 27-1:
I=C•
dV
dT
For capacitance measurements, the A/D Converter
samples an external Capacitor (CAPP) on one of its
input channels, after the CTMU output’s pulse. A
Precision Resistor (RPR) provides current source
calibration on a second A/D channel. After the pulse
ends, the converter determines the voltage on the
capacitor. The actual calculation of capacitance is
performed in software by the application.
Figure 27-1 illustrates the external connections used
for capacitance measurements, and how the CTMU
and A/D modules are related in this application. This
example also shows the edge events coming from
Timer1, but other configurations using external edge
sources are possible. A detailed discussion on
measuring capacitance and time with the CTMU
module is provided in the “dsPIC33/PIC24 Family
Reference Manual”, “Charge Time Measurement
Unit (CTMU) and CTMU Operation with Threshold
Detect” (DS30009743).
 2016 Microchip Technology Inc.
DS30010118B-page 317