PIC24FJ256GB108-I Datasheet, PDF (283/352 Pages) Microchip Technology – 64/80/100-Pin, 16-Bit Flash Microcontrollers with USB On-The-Go (OTG)

PIC24FJ256GB110 FAMILY
25.0 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, refer to the
associated “PIC24F Family Reference
Manual” chapter.
The Charge Time Measurement Unit is a flexible
analog module that provides accurate differential time
measurement between pulse sources, as well as
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 1 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 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 has controls the selection and trim
of the current source.
25.1 Measuring Capacitance
The CTMU module measures capacitance by generat-
ing 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:
I = C  ^d-_d---^VT--
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 preci-
sion 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 25-1 shows 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”.
FIGURE 25-1:
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.
DS39897C-page 283