STMPE321 ST Microelectronics, STMPE321 Datasheet - Page 8

• Manufacturer Part Number: STMPE321
• Description: 3-channel capacitive touch key controller
• Manufacturer: ST Microelectronics

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Capacitance compensation
2.1.1
2.1.2
8/40
Noise filtering
When the STMPE321 is operating in the vicinity of highly emissive circuits (DC-DC
converters, PWM controllers/drives etc.), the sensor inputs can be affected by high-
frequency noise. In this situation, the time-integrating function can be used to distinguish
between a real touch, or an emission-related false touch.
The INTEGRATION TIME and STRENGTH THRES registers are used to configure the time-
integrating function of the STMPE321.
Data filtering
The output from the calibration unit provides an instantaneous TOUCH or NO TOUCH
status. This output is directed to the filtering stage where the TOUCH is integrated across a
programmable period of time. The output of the integration stage is a “STRENGTH" (in the
STRENGTH register) that indicates the number of times a TOUCH is detected across the
integration period.
The STRENGTH is then compared to the value in STRENGTH THRESHOLD register. If
STRENGTH exceeds the STRENGTH THRESHOLD, it is considered a final, filtered
TOUCH status.
In the data filtering stage, 3 modes of operation are supported:
Mode 1: Only the touch channel with highest STRENGTH is taken
Mode 2: All touch channels with STRENGTH > STRENGTH THRESHOLD are taken
Mode 3: The 2 touch channels with the highest STRENGTH are taken.
These modes are selected using the FEATURE SELECTOR register. The final, filtered data
is accessible through the Touch Byte register.
Doc ID 15791 Rev 1
STMPE321