CAP1088 Datasheet, PDF (38/72 Pages) SMSC Corporation – 8 Channel Capacitive Touch Sensor with 8 LED Drivers

5.5
8 Channel Capacitive Touch Sensor with 8 LED Drivers
Datasheet
The Sensor 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
capacitor associated with a touch on one of the sensors and is referenced to a calibrated base “Not
touched” count value. The delta is an instantaneous change and is updated once per sensor per
sensing cycle (see Section 4.5.1 - sensor 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 5.5).
The value is also capped at a negative value of FFh for negative delta counts which may result upon
a release.
Sensitivity Control Register
ADDR R/W
1Fh
R/W
REGISTER
Sensitivity Control
Table 5.6 Sensitivity Control Register
B7
B6
B5
B4
B3
B2
B1
B0
-
DELTA_SENSE[2:0]
BASE_SHIFT[3:0]
DEFAULT
2Fh
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 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 environment 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).
2
0
0
0
0
1
1
1
1
Revision 1.1 (08-05-09)
Table 5.7 DELTA_SENSE Bit Decode
DELTA_SENSE[2:0]
1
0
0
0
0
1
1
0
1
1
0
0
0
1
1
0
1
1
38
DATASHEET
SENSITIVITY MULTIPLIER
128x (most sensitive)
64x
32x (default)
16x
8x
4x
2x
1x - (least sensitive)
SMSC CAP1088