CAP1203 datasheet(15/59 Pages) MICROCHIP

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DS00001572B-page 15

4.3.2

SENSING CYCLE

Except when in Deep Sleep, the device automatically initiates a sensing cycle and repeats the cycle every time it fin-

ishes. The cycle polls through each enabled sensor input starting with CS1 and extending through CS3. As each capac-

itive 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 gen-

erated (see Section 4.8.2, "Capacitive Sensor Input Interrupt Behavior").

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

tion 5.21, "Standby Configuration Register"). If all enabled inputs can be sampled in 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 5.24, "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 cir-

cuits 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

RegisterSection 5.10.1, "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.

If analog calibration fails for a sensor input, the corresponding bit is not cleared in the Calibration Activate and Status

be generated. Analog calibration will fail if a noise bit is set or if the calibration value is at the maximum or minimum

value. If digital calibration fails to generate base counts for a sensor input in the operating range, which is +12.5% from

the ideal base count (see TABLE 4-1:), indicating the base capacitance is out of range, the corresponding BC_OUTx bit

is set in the Base Count Out of Limit Register(Section 5.16, "Base Count Out of Limit Register"), and the BC_OUT bit

is set in the General Status Register (Section 5.2, "Status Registers"). An interrupt can be generated. By default, when

a base count is out of limit, analog calibration is repeated for the sensor input; alternatively, the sensor input can be

sampled using the out of limit base count(Section 5.6, "Configuration Registers").

During normal operation there are various options for recalibrating the capacitive touch sensor inputs. Recalibration is

a digital adjustment of the base counts so that the untouched delta count is zero. After a recalibration, if a sensor input’s

base count has shifted +12.5% from the ideal base count, a full calibration will be performed on the sensor input.

TABLE 4-1:

IDEAL BASE COUNTS

Ideal Base Count

Sample Time

3,200

320us

6,400

640us

12,800

1.28ms

25,600

2.56ms

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