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DS30010118D-page 317

PIC24FJ256GA705 FAMILY

27.0 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:

CTMUCON1L, CTMUCON1H and CTMUCON2L.

CTMUCON1L enables the module, controls the mode

of operation of the CTMU, controls edge sequencing,

selects the current range of the current source and

trims the current. CTMUCON1H controls edge source

selection and edge source polarity selection. The

CTMUCON2L register selects the current discharge

source.

27.1

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 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 27-1:

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 27-1 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

measuring capacitance and time with the CTMU

module is provided in the “dsPIC33/PIC24 Family

Reference Manual”, “Charge Time Measurement

Unit (CTMU) and CTMU Operation with Threshold

Detect” (DS30009743).

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 Time Measurement Unit, refer to

the “dsPIC33/PIC24 Family Reference

Manual”, “Charge Time Measurement

Unit (CTMU) and CTMU Operation with

Threshold Detect” (DS30009743), which

is available from the Microchip website

(www.microchip.com). The information in

this

data

sheet

supersedes

the

information in the FRM.

I = C • dV

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