Microchip Technology PIC24FV16KM204 FAMILY, Datasheet

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2013 Microchip Technology Inc.
Advance Information
DS33030A-page 241
PIC24FV16KM204 FAMILY
24.0 CHARGE TIME
MEASUREMENT UNIT (CTMU)
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.
24.1 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 several internal
peripheral modules (OC1, Timer1, any input capture or
comparator module) 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 24-1:
For capacitance measurements, the A/D Converter
samples an external capacitor (C
APP
) on one of its
input channels after the CTMU output’s pulse. A preci-
sion resistor (R
PR
) 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 24-1 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 measur-
ing capacitance and time with the CTMU module is
provided in the “
PIC24F Family Reference Manual” .
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” ,
Section 53. “Charge Time Measurement
Unit (CTMU) with Threshold Detect”
(DS39743).
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