CAP1208 datasheet(21/67 Pages) MICROCHIP

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8-Channel Capacitive Touch Sensor

Datasheet

 2014 Microchip Technology Inc.

DS00001570B-page 21

4.3.1

Capacitive Touch Sensing Settings

Controls for managing capacitive touch sensor inputs are determined by the power state.

4.3.1.1

Active State Sensing Settings

The Active state is used for normal operation. Sensor inputs being monitored are determined by the

Sensor Input Enable Register (see Section 5.7, "Sensor Input Enable Register"). Sensitivity is

controlled by the Sensitivity Control Register (see Section 5.5, "Sensitivity Control Register").

Averaging, sample time, and cycle time are controlled by the Averaging and Sampling Configuration

a separate touch detection threshold, as defined in the Sensor Input Threshold registers (see Section

5.19, "Sensor Input Threshold Registers").

4.3.1.2

Standby State Sensing Settings

The Standby state is used for standby operation. In general, fewer sensor inputs are enabled, and they

are programmed to have more sensitivity. Sensor inputs being monitored are determined by the

Standby Channel Register (see Section 5.21, "Standby Channel Register"). Sensitivity is controlled by

the Standby Sensitivity Register (see Section 5.23, "Standby Sensitivity Register"). Averaging, sample

time, and cycle time are controlled by the Averaging and Sampling Configuration Register (see Section

5.22, "Standby Configuration Register"). There is one touch detection threshold, which applies to all

sensors enabled in Standby, as defined in the Standby Threshold Register (see Section 5.24, "Standby

Threshold Register").

4.3.2

Sensing Cycle

Except when in Deep Sleep, the device automatically initiates a sensing cycle and repeats the cycle

every time it finishes. The cycle polls through each enabled sensor input starting with CS1 and

extending through CS8. As each capacitive touch sensor input is polled, its measurement is compared

against a baseline “not touched” measurement. If the delta measurement is large enough to exceed

the applicable threshold, a touch is detected and an interrupt can be generated (see Section 4.8.2,

"Capacitive Sensor Input Interrupt Behavior").

The sensing cycle time is programmable (see Section 5.10, "Averaging and Sampling Configuration

less than the cycle time, the device is placed into a lower power state for the remainder of the sensing

cycle. If the number of active sensor inputs cannot be sampled within the specified cycle time, the

cycle time is extended and the device is not placed in a lower power state.

4.4

Sensor Input Calibration

Calibration sets the Base Count Registers (Section 5.25, "Sensor Input Base Count Registers") which

contain the “not touched” values used for touch detection comparisons. Calibration automatically

occurs after a power-on reset (POR), when sample time is changed, and whenever a sensor input is

newly enabled (for example, when transitioning from a power state in which it was disabled to a power

state in which it is enabled). During calibration, the analog sensing circuits are tuned to the capacitance

of the untouched pad. Then, samples are taken from each sensor input so that a base count can be

established. After calibration, the untouched delta counts are zero.

APPLICATION NOTE: During the calibration routine, the sensor inputs will not detect a press for up to 200ms and

the Sensor Base Count Register values will be invalid. In addition, any press on the

corresponding sensor pads will invalidate the calibration.

The host controller can force a calibration for selected sensor inputs at any time using the Calibration

Activate and Status Register (Section 5.11, "Calibration Activate and Status Register"). When a bit is

set, the corresponding capacitive touch sensor input will be calibrated (both analog and digital). The

bit is automatically cleared once the calibration routine has successfully finished.

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