CAP1203 Datasheet, PDF (17/59 Pages) Microchip Technology – 3-Channel Capacitive Touch Sensor

4.7 Noise Controls
4.7.1 LOW FREQUENCY NOISE DETECTION
Each sensor input has a low frequency noise detector that will sense if low frequency noise is injected onto the input
with sufficient power to corrupt the readings. By default, if this occurs, the device will reject the corrupted samplesee
DIS_ANA_NOISE bit in Section 5.6.1, "Configuration - 20h") and the corresponding bit is set to a logic ‘1’ in the Noise
Flag Status register (see SHOW_RF_NOISE bit in Section 5.6.2, "Configuration 2 - 44h").
4.7.2 RF NOISE DETECTION
Each sensor input contains an integrated RF noise detector. This block will detect injected RF noise on the CS pin. The
detector threshold is dependent upon the noise frequency. By default, if RF noise is detected on a CS line, that sample
is removed and not compared against the threshold (see DIS_RF_NOISE bit in Section 5.6.2, "Configuration 2 - 44h").
4.7.3 NOISE STATUS AND CONFIGURATION
The Noise Flag Status (see Section 5.3, "Noise Flag Status Registers") bits can be used to indicate RF and/or other
noise. If the SHOW_RF_NOISE bit in the Configuration Register (see Section 5.6, "Configuration Registers") is set to
0, the Noise Flag Status bit for the capacitive sensor input is set if any analog noise is detected. If the
SHOW_RF_NOISE bit is set to 1, the Noise Flag Status bits will only be set if RF noise is detected.
The CAP1208 offers optional noise filtering controls for both analog and digital noise.
For analog noise, there are options for whether the data should be considered invalid. By default, the DIS_ANA_NOISE
bit (see Section 5.6.1, "Configuration - 20h") will block a touch on a sensor input if low frequency analog noise is
detected; the sample is discarded. By default, the DIS_RF_NOISE bit (see Section 5.6.2, "Configuration 2 - 44h") will
block a touch on a sensor input if RF noise is detected; the sample is discarded.
For digital noise, sensor input noise thresholds can be set (see Section 5.19, "Sensor Input Noise Threshold Register").
If a capacitive touch sensor input exceeds the Sensor Noise Threshold but does not exceed the touch threshold (Sensor
Threshold (see Section 5.18, "Sensor Input Threshold Registers") in the Active state or Sensor Standby Threshold in
the Standby state (Section 5.23, "Standby Threshold Register")), it is determined to be caused by a noise spike. The
DIS_DIG_NOISE bit (see Section 5.6.1, "Configuration - 20h") can be set to discard samples that indicate a noise spike
so they are not used in the automatic recalibration routine (see Section 4.4.1, "Automatic Recalibration").
4.8 Interrupts
Interrupts are indicated by the setting of the INT bit in the Main Control Register(see Section 5.1, "Main Control Regis-
ter") and by assertion of the ALERT# pin. The ALERT# pin is cleared when the INT bit is cleared by the user. When the
INT bit is cleared by the user, status bits may be cleared (see Section 5.2, "Status Registers").
4.8.1 ALERT# PIN
The ALERT# pin is an active low output that is driven when an interrupt event is detected.
4.8.2 CAPACITIVE SENSOR INPUT INTERRUPT BEHAVIOR
Each sensor input can be programmed to enable / disable interrupts(see Section 5.11, "Interrupt Enable Register").
When enabled for a sensor input and the sensor input is not the designated power button, interrupts are generated in
one of two ways:
1. An interrupt is generated when a touch is detected and, as a user selectable option, when a release is detected
(by default - see INT_REL_n in Section 5.6.2, "Configuration 2 - 44h"). See FIGURE 4-4:.
2. If the repeat rate is enabled then, so long as the touch is held, another interrupt will be generated based on the
programmed repeat rate (see FIGURE 4-3:).
When the repeat rate is enabled for a sensor input (see Section 5.12, "Repeat Rate Enable Register"), the device uses
an additional control called MPRESS that determines whether a touch is flagged as a simple “touch” or a “press and
hold” (see Section 5.9, "Sensor Input Configuration 2 Register"). The MPRESS[3:0] bits set a minimum press timer.
When the button is touched, the timer begins. If the sensor pad is released before the minimum press timer expires, it
is flagged as a touch and an interrupt (if enabled) is generated upon release. If the sensor input detects a touch for lon-
ger than this timer value, it is flagged as a “press and hold” event. So long as the touch is held, interrupts will be gener-
ated at the programmed repeat rate (see Section 5.8, "Sensor Input Configuration Register") and upon release (if
enabled).
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