AN1250 datasheet(3/22 Pages) MICROCHIP

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DS01250A-page 3

AN1250

OVERVIEW OF CAPACITIVE TOUCH

APPLICATIONS

The CTMU is used in capacitive touch applications by

applying the constant current source of the CTMU to

the capacitive touch pad using the following equation:

Where:

• I is the constant current source of the CTMU

• T is a fixed period that the CTMU charges the

capacitive touch circuit

• C is the capacitance of the touch circuit

• V is the voltage read by the A/D converter after

the capacitive touch circuit is finished charging

It is possible to sense a relative shift in capacitance by

observing a change in voltage. The above equation can

be rearranged to:

Since the CTMU current source is constant (I), the

voltage present on the capacitive touch sensor (V)

relies on two variables: the amount of time the touch

circuit is charged (T) and the capacitive size of the

touch circuit (C). If the amount of time the touch circuit

is charged is held constant, then changes in the capac-

itance of the touch circuit will ultimately affect the

voltage that the circuit charges to in the fixed period.

The A/D converter is used to read the voltage that the

touch circuit is charged to with the CTMU.

When the capacitance of a human finger is added to

the touch sensor pad, the capacitance increases and

the result is a lowering of the voltage seen by the A/D

converter (since I and T are held constant).

Setting Up of the CTMU

For a capacitive touch application, each sensor must

be connected directly to a channel of the A/D converter.

See Figure 2 for a block diagram of a Microchip

microcontroller with a CTMU peripheral. This diagram

illustrates that the CTMU is internally connected to the

A/D converter and allows for selection of any of the A/D

channels. With this configuration, a single CTMU unit

can measure many capacitive touch sensors. Note that

the CTMU has been simplified in order to show only the

elements required for capacitive touch, namely the cur-

rent source, the current source control and the current

drain mechanism. These will be explained in detail in

later sections. The current source control, labeled

“Trigger” in Figure 2, is manually controlled in software

by manipulating bits in the CTMU Control register.

FIGURE 2:

CAPACITIVE TOUCH BLOCK DIAGRAM

I x T = C x V

V = (I x T)/C

A/D Converter

Current Source

CTMU

Sensor 0

C A/D

Trigger

A/D MUX

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