CAP1133 datasheet(37/63 Pages) MICROCHIP

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DS00001625B-page 37

CAP1133

Bits 2 - 0 - STBY_SENSE[2:0] - Controls the sensitivity for sensor inputs that are active in Standby. The sensitivity set-

tings act to scale the relative delta count value higher or lower based on the system parameters. A setting of 000b is the

most sensitive while a setting of 111b is the least sensitive. At the more sensitive settings, touches are detected for a

smaller delta C corresponding to a “lighter” touch. These settings are more sensitive to noise however and a noisy envi-

ronment may flag more false touches than higher sensitivity levels.

APPLICATION NOTE: A value of 128x is the most sensitive setting available. At the most sensitivity settings, the

MSB of the Delta Count register represents 64 out of ~25,000 which corresponds to a touch

of approximately 0.25% of the base capacitance (or a

ΔC of 25fF from a 10pF base

capacitance). Conversely a value of 1x is the least sensitive setting available. At these

settings, the MSB of the Delta Count register corresponds to a delta count of 8192 counts

out of ~25,000 which corresponds to a touch of approximately 33% of the base capacitance

(or a

ΔC of 3.33pF from a 10pF base capacitance).

6.23

Standby Threshold Register

The Standby Threshold register stores the delta threshold that is used to determine if a touch has been detected. When

a touch occurs, the input signal of the corresponding sensor pad changes due to the capacitance associated with a

touch. If the sensor input change exceeds the threshold settings, a touch is detected.

6.24

Sensor Input Base Count Registers

The Sensor Input Base Count registers store the calibrated “Not Touched” input value from the capacitive touch sensor

inputs. These registers are periodically updated by the re-calibration routine.

The routine uses an internal adder to add the current count value for each reading to the sum of the previous readings

until sample size has been reached. At this point, the upper 16 bits are taken and used as the Sensor Input Base Count.

The internal adder is then reset and the re-calibration routine continues.

TABLE 6-41:

STBY_SENSE BIT DECODE

STBY_SENSE[2:0]

Sensitivity Multiplier

210

128x (most sensitive)

001

64x

32x (default)

011

16x

100

101

110

1x - (least sensitive)

TABLE 6-42:

STANDBY THRESHOLD REGISTER

ADDR

R/W

B5B4

B3B2

B1B0

Default

43h

R/W

Standby Thresh-

old

-64

40h

TABLE 6-43:

SENSOR INPUT BASE COUNT REGISTERS

ADDR

R/W

B5B4

B3B2

B1B0

Default

50h

Sensor Input 1

Base Count

128

C8h

51h

Sensor Input 2

Base Count

128

C8h

52h

Sensor Input 3

Base Count

128

C8h

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