PIC24F04KA201 datasheet(161/224 Pages) MICROCHIP

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Preliminary

DS39937B-page 159

PIC24F04KA201 FAMILY

22.0

CHARGE TIME

MEASUREMENT UNIT (CTMU)

The Charge Time Measurement Unit (CTMU) is a

flexible analog module that provides charge measure-

ment, accurate differential time measurement between

pulse sources and 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 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 two registers,

CTMUCON and CTMUICON. CTMUCON enables the

module, and controls edge source selection, edge

source polarity selection, and edge sequencing. The

CTMUICON register selects the current range of

current source and trims the current.

22.1

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 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

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 22-1 displays 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 “PIC24F Family Reference

Manual”

FIGURE 22-1:

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 to be a comprehensive reference

source. For more information on the

Charge Measurement Unit, refer to the

“PIC24F

Family

Reference

Manual”

Section 11. “Charge Time Measurement

Unit (CTMU)”

(DS39724).

-------

⋅

PIC24F Device

A/D Converter

CTMU

ANx

CAPP

Output Pulse

EDG1

EDG2

RPR

ANY

Timer1

Current Source

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