CAP1128 datasheet(34/75 Pages) MICROCHIP

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CAP1128

DS00001622B-page 34

 2015 Microchip Technology Inc.

6.4

Sensor Input Delta Count Registers

The Sensor Input Delta Count registers store the delta count that is compared against the threshold used to determine

if a touch has been detected. The count value represents a change in input due to the capacitance associated with a

touch on one of the sensor inputs and is referenced to a calibrated base “Not Touched” count value. The delta is an

instantaneous change and is updated once per sensor input per sensing cycle (see Section 5.5.1, "Sensing Cycle").

The value presented is a standard 2’s complement number. In addition, the value is capped at a value of 7Fh. A reading

of 7Fh indicates that the sensitivity settings are too high and should be adjusted accordingly (see Section 6.5).

The value is also capped at a negative value of 80h for negative delta counts which may result upon a release.

6.5

Sensitivity Control Register

The Sensitivity Control register controls the sensitivity of a touch detection.

Bits 6-4 DELTA_SENSE[2:0] - Controls the sensitivity of a touch detection. The sensitivity settings act to scale the rel-

ative 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 capacitance

corresponding to a “lighter” touch. These settings are more sensitive to noise, however, and a noisy environment may

flag more false touches with 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).

TABLE 6-6:

SENSOR INPUT DELTA COUNT REGISTERS

ADDR

R/W

B5B4

B3B2

B1B0

Default

10h

Sensor Input 1

Delta Count

Sign

00h

11h

Sensor Input 2

Delta Count

Sign

00h

12h

Sensor Input 3

Delta Count

Sign

00h

13h

Sensor Input 4

Delta Count

Sign

00h

14h

Sensor Input 5

Delta Count

Sign

00h

15h

Sensor Input 6

Delta Count

Sign

00h

16h

Sensor Input 7

Delta Count

Sign

00h

17h

Sensor Input 8

Delta Count

Sign

00h

TABLE 6-7:

SENSITIVITY CONTROL REGISTER

ADDR

R/W

Default

1Fh

R/W

Sensitivity Control

DELTA_SENSE[2:0]

BASE_SHIFT[3:0]

2Fh

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