Datasheet

PIC18F46J11 FAMILY
DS39932D-page 390  2011 Microchip Technology Inc.
25.6 Creating a Delay with the CTMU
Module
A unique feature on board the CTMU module is its
ability to generate system clock independent output
pulses based on an external capacitor value. This is
accomplished using the internal comparator voltage
reference module, Comparator 2 input pin and an
external capacitor. The pulse is output onto the CTPLS
pin. To enable this mode, set the TGEN bit.
See Figure 25-4 for an example circuit. C
PULSE is
chosen by the user to determine the output pulse width
on CTPLS. The pulse width is calculated by
T =(C
PULSE/I)*V, where I is known from the current
source measurement step (Section 25.3.1 “Current
Source Calibration”) and V is the internal reference
voltage (CV
REF).
An example use of this feature is for interfacing with
variable capacitive-based sensors, such as a humidity
sensor. As the humidity varies, the pulse width output
on CTPLS will vary. The CTPLS output pin can be con-
nected to an input capture pin and the varying pulse
width is measured to determine the humidity in the
application.
Follow these steps to use this feature:
1. Initialize Comparator 2.
2. Set CPOL = 1.
3. Initialize the comparator voltage reference.
4. Initialize the CTMU and enable time delay
generation by setting the TGEN bit.
5. Set EDG1STAT.
6. When C
PULSE charges to the value of the voltage
reference trip point, an output pulse is generated
on CTPLS.
FIGURE 25-4: TYPICAL CONNECTIONS AND INTERNAL CONFIGURATION FOR PULSE
DELAY GENERATION
25.7 Operation During Sleep/Idle
Modes
25.7.1 SLEEP MODE AND DEEP SLEEP
MODES
When the device enters any Sleep mode, the CTMU
module current source is always disabled. If the CTMU
is performing an operation that depends on the current
source when Sleep mode is invoked, the operation may
not terminate correctly. Capacitance and time
measurements may return erroneous values.
25.7.2 IDLE MODE
The behavior of the CTMU in Idle mode is determined
by the CTMUSIDL bit (CTMUCONH<5>). If CTMUSIDL
is cleared, the module will continue to operate in Idle
mode. If CTMUSIDL is set, the module’s current source
is disabled when the device enters Idle mode. If the
module is performing an operation when Idle mode is
invoked, in this case, the results will be similar to those
with Sleep mode.
25.8 Effects of a Reset on CTMU
Upon Reset, all registers of the CTMU are cleared. This
leaves the CTMU module disabled, its current source is
turned off and all configuration options return to their
default settings. The module needs to be re-initialized
following any Reset.
If the CTMU is in the process of taking a measurement at
the time of Reset, the measurement will be lost. A partial
charge may exist on the circuit that was being measured,
and should be properly discharged before the CTMU
makes subsequent attempts to make a measurement.
The circuit is discharged by setting and then clearing the
IDISSEN bit (CTMUCONH<1>) while the A/D Converter
is connected to the appropriate channel.
C2
CVREF
CTPLS
PIC18F46J11 Device
Current Source
Comparator
CTMU
CTED1
C2INB
CPULSE
EDG1