64GA106 datasheet(241/330 Pages) MICROCHIP

241 / 330 page

 2010 Microchip Technology Inc.

DS39905E-page 241

PIC24FJ256GA110 FAMILY

24.0

CHARGE TIME

MEASUREMENT UNIT (CTMU)

The Charge Time Measurement Unit is a flexible analog

module that provides accurate differential time measure-

ment 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 controls the selection and trim of

the current source.

24.1

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

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 24-1 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 24-1:

TYPICAL CONNECTIONS AND INTERNAL CONFIGURATION FOR

CAPACITANCE MEASUREMENT

Note:

This data sheet summarizes the features

of this group of PIC24F devices. It is not

intended

comprehensive

reference source. For more information,

refer to the “PIC24F Family Reference

Manual

”, Section 11. “Charge Time

Measurement Unit (CTMU)”

(DS39724).

I = C •

PIC24FJ Device

A/D Converter

CTMU

ANx

CAPP

Output Pulse

EDG1

EDG2

RPR

ANY

Timer1

Current Source

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