QST108 datasheet(11/38 Pages) STMICROELECTRONICS

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QST108

Device operating modes

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Device operating modes

4.1

Mode selection

The device options are configured by connecting pull-up or pull-down resistors on OPTn

pins. The device operating mode is selected using option pin 1 (OPT1) while the device

settings are configured using option pins OPT2 to OPT6 (Table 3). Option pins are sampled

at power-up and after a reset.

To fit most applications, the QST108 device offers two different operating modes:

●

Stand-alone mode

This mode allows the user to simply replace existing mechanical switches with a

capacitive sensing solution. It is designed for maximum flexibility and can

accommodate most popular sensing requirements via option resistors (AKS, Low

power, Max On-Duration and output modes).

In this mode, the 8 output pins reflect the status of the 8 sensing channels.

●

I2C mode

In this mode, which is the most open one, the device is driven using the I2C interface.

To avoid polling, the QST device features an output interrupt pin (IRQ). The IRQ line

reports all key changes to the Master device. The QST (Slave) device can drive up to

five general-purpose outputs.

4.2

Reset and power-up

At power-up, the device configures itself according to the pull-up or pull-down option

resistors present on pins OPT1 to OPT6. The device start-up and configuration may take up

to tSetup.

When the power is established, it is possible to force a new device configuration by applying

a negative pulse on the RESET pin.

The RESET pin is a bidirectional pin with an internal pull-up. The line is forced low when the

device resets itself (through an I²C command, for example).

A 10nF capacitor is recommended on the RESET pin to ensure reliable start-up and noise

immunity.

Table 3.

Operating modes

OPT1: Mode selection

Option resistor function

OPT2

OPT3

OPT4

OPT5

OPT6

Pin OPT1 is high at start-up Stand-alone mode

AKS

MOD_0 MOD_1

Pin OPT1 is low at start-up

I2C mode

ADD0

ADD1

ADD2

Unused Unused

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