STM8T143 datasheet(23/65 Pages) STMICROELECTRONICS

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STM8T143

STM8T143 processing

5.5.3

Reference freeze and reference freeze timeout

To prevent an object under detection from influencing the reference value, the ECS is halted

as soon as a proximity detection happens: PTh <

Δ(signal) < TTh. Consequently, the

reference is frozen.

In order to recover from a sudden environment change, the reference freeze ends after a

maximum programmable delay called the “reference freeze timeout” (tRFT).

When a detection lasts longer than the tRFT, a recalibration process occurs. The recalibration

consists of setting the reference to the current burst count value minus 8. Then, ECS is re-

enabled and the reference moves along the signal. After a period of time that depends on

the signal variation speed, the difference between the signal and the reference becomes

smaller than the detection threshold and the device reports no detection. The process delay

after the timeout, to get the reference aligned with the current signal, is called the

recalibration time (tRECAL).

Figure 14 describes the situation where an unwanted detection is solved by the reference

freeze timeout. The left-hand side of the image (Reference freeze timeout enabled) shows

the reference freeze timeout configured to launch a calibration if water droplets are poured

onto the electrode. The droplet capacitance is not sufficient to make the device enter into

touch detection but it is enough to trigger a proximity detection. If the proximity detection

caused by the droplets lasts longer than the reference freeze timeout delay, the device

recalibrates to the new “wet” environment. Consequently, the detection output is cleared.

The right-hand side of the image (Infinite reference freeze timeout) shows the device

behavior when the reference freeze timeout is disabled. The device leaves proximity

detection only after the droplets are removed.

Figure 14. Unwanted detection managed by the reference freeze timeout

STM8T143 Datasheet(PDF) 23 Page - STMicroelectronics