Datasheet

 2010 Microchip Technology Inc. Preliminary DS39979A-page 303
PIC18F87J72 FAMILY
25.0 CHARGE TIME
MEASUREMENT UNIT (CTMU)
The Charge Time Measurement Unit (CTMU) is a
flexible analog module that provides accurate differen-
tial time measurement between pulse sources, as well
as asynchronous pulse generation. By working 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 with a specific time delay. The CTMU is
ideal for interfacing with capacitive-based sensors.
The module includes the following key features:
• Up to 13 channels available for capacitive or time
measurement input
• On-chip precision current source
• 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
• High-precision time measurement
• Time delay of external or internal signal
asynchronous to system clock
• Accurate current source suitable for capacitive
measurement
The CTMU works in conjunction with the A/D Converter
to provide up to 13 channels for time or charge
measurement, depending on the specific device and
the number of A/D channels available. When config-
ured for time delay, the CTMU is connected to one of
the analog comparators. The level-sensitive input edge
sources can be selected from four sources: two
external inputs or CCP1/CCP2 Special Event Triggers.
Figure 25-1 provides a block diagram of the CTMU.
FIGURE 25-1: CTMU BLOCK DIAGRAM
CTEDG1
CTEDG2
Current Source
Edge
Control
Logic
CTMUCON
Pulse
Generator
A/D Converter Comparator 2
Input
CCP2
CCP1
Current
Control
ITRIM<5:0>
IRNG<1:0>
CTMUICON
CTMU
Control
Logic
EDGEN
EDGSEQEN
EDG1SELx
EDG1POL
EDG2SELx
EDG2POL
EDG1STAT
EDG2STAT
TGEN
IDISSEN
CTTRIG
A/D Trigger
CTPLS
Comparator 2 Output