AN2927 datasheet(8/12 Pages) STMICROELECTRONICS

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Firmware implementation

AN2927

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3.3

Touched effect

The electrode shape and size, the layout from the touch controller device to the electrode

(and more specifically the ground coupling) as well as the dielectric panel material and

thickness are the main parameters that define the value of the electrode capacitance (CX).

As a consequence, the RC charge and discharge values are directly dependant on this CX

value. Figure 8 illustrates this “touched effect”.

The time <t1’> (where the VIH level is reached) is greater than the time <t1>. This is the

same for the VIL level with times t2’ and t2.

Figure 8.

“Touched effect” example

Table 2.

Capacitance charge/discharge measurement steps

Step

Description

Load I/O pin is set in Output mode at VDD.

Acq I/O pin is set in Output mode at VDD.

Timer counter VIH start value is saved (vih_start).

Acq I/O pin is set in Input mode Hi-Z.

--> The electrode capacitance CX is charging.

After the voltage on the Acq I/O pin reaches VIH:

Timer counter VIH stop value is saved (vih_stop) and the VIH measurement is

calculated and saved (vih_stop – vih_start).

Acq I/O pin is set in Output mode at VDD.

Load I/O pin is set in Output mode at GND.

Timer counter VIL start value is saved (vil_start).

Acq I/O pin is set in Input mode Hi-Z.

--> The electrode capacitance CX is discharging.

After the voltage on the Acq I/O pin reaches VIL:

Timer counter VIL stop value is saved (vil_stop) and the VIL measurement is

calculated and saved (vil_stop – vil_start).

The two measurements “vih_meas” and “vil_meas” are added and saved.

The process continues with Step 1.

VIL

VIH

VDD

“Acq I/O” pin

“touched effect”

t2’

t1’

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