QST108 datasheet(13/51 Pages) STMICROELECTRONICS

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QST108

Device operating modes

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Device operating modes

4.1

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.

4.2

Burst operation

The device operates in “Burst” mode. Each key touch is acquired using a burst of charge-

transfer sensing pulses whose count varies depending on the value of the sense capacitor

CS and the load capacitance CX. Key touches are acquired using two successive bursts of

pulses:

●

Burst A: Keys 1, 2, 3, and 4

●

Burst B: Keys 5, 6, 7, and 8

Bursts always operate in an A-B sequence. If Keys 5 to 8 are not implemented, the QST

device will not perform the Burst B to improve the response time and reduce the power

consumption when in Low Power (LP) mode.

In Low Power mode, the device sleeps in an ultra-low current state between bursts to

conserve power.

4.3

Low power mode

In order to reduce the device power consumption, the QST family include scalable low

power modes.

●

Standard low power mode

When the device is in standard low power mode, a window with very low power

consumption is inserted between the acquisition of the last active key and the following

acquisition of the first active key.

This window duration is programmable as the 'sleep duration time'.

Note that the sleep window insertion is cancelled in the following conditions:

–

If a change is detected on a key, in order to speed up the DI process, the sleep

window insertion is skipped until the end of the DI process.

–In I2C mode, when a key change is actually detected and reported with a negative

pulse on the IRQ pin. In this case, the low power mode is disabled until a

command is received from the host.

–

Inside an I2C command, between the Write and the Read I2C frames, the sleep

period is skipped.

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