ST AN2843 APPLICATION NOTE

AN2843

Application note

STEVAL-IHI002V1, capacitive touch-sensing

keyboard based on the STMPE1208S

Introduction

The STEVAL-IHI002V1 is a demonstration board designed to modernize, or even fully
replace, a conventional mechanical keyboard.

The demonstration board is based on the STMPE1208S 12-bit GPIO expander with an
additional 12-channel capacitive sensor, which is capable of interfacing a main MCU
through an I²C bus. The demonstration board is designed to work as a daughterboard for
the STEVAL-IHI001V1 demonstration board (washing machine user interface).

Once connected, the STEVAL-IHI002V1 will be automatically detected by the motherboard,
replacing the mechanical keys with a wheel, a slider and 5 buttons.

November 2009 Doc ID 15112 Rev 2 1/9

www.st.com

Contents AN2843

Contents

1 STMPE1208S: 12-channel capacitive touchkey controller . . . . . . . . . . 3

2 Application schematic . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

3 Bill of material . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

4 STEVAL-IHI002V1 demonstration board photos . . . . . . . . . . . . . . . . . . 6

5 Revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

2/9 Doc ID 15112 Rev 2

AN2843 STMPE1208S: 12-channel capacitive touchkey controller

1 STMPE1208S: 12-channel capacitive touchkey

controller

The STMPE1208S is a 12-bit GPIO device with additional 12-channel capacitive sensors,
which is capable of interfacing a main MCU through an I²C bus.

The sensors in the device detect finger contact through the additional capacitance
introduced to the sensor, providing fast and accurate results at very low power consumption.

Figure 1. STMPE1208S block diagram

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When a sensor is touched, the increase in capacitance causes a delay in the clock signal on
the relevant sensing pin. The signal is then compared with a reference clock. The difference
is a direct representation of the additional capacitance introduced by the finger touch.

Environment factors, such as temperature and humidity, can affect the measured
capacitance value. The calibration unit ensures consistent key sensitivity, even in the
presence of significant shifts in temperature, humidity, device lot and PCB characteristics.

The output from the calibration unit provides an instantaneous touch/no-touch status. This
signal is sent to the data filtering stage, which is made up of an integration unit, a filter unit
and a noise filtering unit.

The sensors can be affected by high-frequency noise if working near highly emissive circuits
such as a DC-DC converters or PWM controllers. The noise filtering helps the device
distinguish between a real touch or an emission-related false touch.

The STMPE1208S communicates with a main MCU through an I²C bus interface. It is
configured as a slave device, and supports both standard mode (up to 100 Kbps) and fast
mode (up to 400 Kbps), as well as 7-bit and 10-bit addressing modes.

The device can manage 2 types of interrupt events: a general interrupt related to the GPIO
ports, and a touch interrupt which occurs when touching a sensor.

Doc ID 15112 Rev 2 3/9