8 Channel Capacitive Touch Sensor with 2 LED Drivers
Datasheet
4.5.5
4.5.6
Low Frequency Noise Detection
Each sensor input has an EMI noise detector that will sense if low frequency noise is injected onto the
input with sufficient power to corrupt the readings. If this occurs, the device will reject the corrupted
sample and set the corresponding bit in the Noise Status register to a logic ‘1’.
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. If RF noise is detected
on a CS line, that sample is removed and not compared against the threshold.
4.6
ALERT# Pin
The ALERT# pin is an active low (or active high when configured) output that is driven when an
interrupt event is detected.
Whenever an interrupt is generated, the INT bit (see Section 5.1, "Main Control Register") is set. The
ALERT# pin is cleared when the INT bit is cleared by the user. Additionally, when the INT bit is cleared
by the user, status bits are only cleared if no touch is detected.
4.6.1
Sensor Interrupt Behavior
The sensor 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 Section 5.6). See Figure 4.3.
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 4.2).
When the repeat rate is enabled, the device uses an additional control called MPRESS that determines
whether a touch is flagged as a simple “touch” or a “press and hold”. 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 is generated upon
release. If the sensor input detects a touch for longer than this timer value, it is flagged as a “press
and hold” event. So long as the touch is held, interrupts will be generated at the programmed repeat
rate and upon release (if enabled).
APPLICATION NOTE: Figure 4.2 and Figure 4.3 show default operation which is to generate an interrupt upon
sensor pad release and an active-low ALERT# pin.
APPLICATION NOTE: The host may need to poll the device twice to determine that a release has been detected.
SMSC CAP1128
Revision 1.32 (01-05-12)
DATA3S5HEET
SMSC [ SMSC CORPORATION ]
