PIC24F04KA201 Datasheet, PDF (161/224 Pages) Microchip Technology – 14/20-Pin General Purpose, 16-Bit Flash Microcontrollers with nanoWatt XLP™ Technology

PIC24F04KA201 FAMILY
22.0 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”,
Section 11. “Charge Time Measurement
Unit (CTMU)” (DS39724).
The Charge Time Measurement Unit (CTMU) is a
flexible analog module that provides charge measure-
ment, accurate differential time measurement between
pulse sources and asynchronous pulse generation. Its
key features include:
• Four edge input trigger sources
• Polarity control for each edge source
• Control of edge sequence
• Control of response to edges
• 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 two registers,
CTMUCON and CTMUICON. CTMUCON enables the
module, and controls edge source selection, edge
source polarity selection, and edge sequencing. The
CTMUICON register selects the current range of
current source and trims the current.
22.1 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 two
external pins (CTEDG1 and CTEDG2). This pulse is
used with the module’s precision current source to
calculate capacitance according to the relationship:
C = I ⋅ ^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 22-1 displays 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”.
FIGURE 22-1:
TYPICAL CONNECTIONS AND INTERNAL CONFIGURATION FOR
CAPACITANCE MEASUREMENT
PIC24F Device
Timer1
CTMU
EDG1 _{Current Source}
EDG2
Output Pulse
A/D Converter
ANx
ANY
CAPP
RPR
© 2009 Microchip Technology Inc.
Preliminary
DS39937B-page 159