CAP1293 datasheet(17/63 Pages) MICROCHIP

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DS00001566B-page 17

CAP1293

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"). Calibration sensitivity can be adjusted

for each sensor input based on capacitive touch pad capacitance.

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.

4.4.1

AUTOMATIC RECALIBRATION

Each sensor input is regularly recalibrated at a programmable rate(see CAL_CFG[2:0] in Section 5.18, "Recalibration

Configuration Register"). By default, the recalibration routine stores the average 64 previous measurements and peri-

odically updates the base “not touched” setting for the capacitive touch sensor input.

APPLICATION NOTE: Automatic recalibration only works when the delta count is below the active sensor input

threshold. It is disabled when a touch is detected.

4.4.2

NEGATIVE DELTA COUNT RECALIBRATION

It is possible that the device loses sensitivity to a touch. This may happen as a result of a noisy environment, recalibra-

tion when the pad is touched but delta counts do not exceed the threshold, or other environmental changes. When this

occurs, the base untouched sensor input may generate negative delta count values. The NEG_DELTA_CNT[1:0]

bits(see Section 5.18, "Recalibration Configuration Register") can be set to force a recalibration after a specified number

of consecutive negative delta readings. After a delayed recalibration (see Section 4.4.3, "Delayed Recalibration") the

negative delta count recalibration can correct after the touch is released.

APPLICATION NOTE: During this recalibration, the device will not respond to touches.

4.4.3

DELAYED RECALIBRATION

It is possible that a “stuck button” occurs when something is placed on a button which causes a touch to be detected

for a long period. By setting the MAX_DUR_EN bit(see Section 5.6, "Configuration Registers"), a recalibration can be

forced when a touch is held on a button for longer than the duration specified in the MAX_DUR[3:0] bits (see Section

5.8, "Sensor Input Configuration Register").

Note 4-1

Delayed recalibration only works when the delta count is above the active sensor input threshold. If

enabled, it is invoked when a sensor pad touch is held longer than the MAX_DUR bit settings.

Note 4-2

For the power button, which requires that the button be held longer than a regular button, the time

specified by the MAX_DUR[3:0] bits is added to the time required to trigger the qualifying event. This

will prevent the power button from being recalibrated during the time it is supposed to be held.

4.5

Proximity Detection

Each sensor input can be configured to detect changes in capacitance due to proximity of a touch. This circuitry detects

the change of capacitance that is generated as an object approaches, but does not physically touch, the enabled sensor

pad(s). Generally, sensor inputs used to detect proximity have physically larger pads than standard buttons. In addition,

gain should be increased to increase sensitivity. To improve the signal, the signal guard feature may be used.

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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