PIC24FJ128GC010 Datasheet, PDF (389/472 Pages) Microchip Technology – 16-Bit Flash Microcontrollers with 12-Bit Pipeline A/D, Sigma-Delta A/D, USB On-The-Go and XLP Technology

PIC24FJ128GC010 FAMILY
32.0 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 Measurement Unit, refer to the
“PIC24F Family Reference Manual”,
“Charge Time Measurement Unit
(CTMU) with Threshold Detect”
(DS39743).
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:
CTMUCON1, CTMUCON2 and CTMUICON.
CTMUCON1 enables the module and controls the mode
of operation of the CTMU, as well as controlling edge
sequencing. CTMUCON2 controls edge source selec-
tion and edge source polarity selection. The CTMUICON
register selects the current range of current source and
trims the current.
32.1 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 (CTEDG1 through CTEDG13). This
pulse is used with the module’s precision current
source to calculate capacitance according to the
relationship:
EQUATION 32-1:
I = C  ^d----^V--
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 32-1 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 “PIC24F Family Reference
Manual”, “Charge Time Measurement Unit (CTMU)”.
 2012-2013 Microchip Technology Inc.
DS30009312B-page 389