sx8644 Semtech Corporation, sx8644 Datasheet - Page 53

• Manufacturer Part Number: sx8644
• Description: Ultra Low Power, Capacitive Button And Slider Touch Controller 12 Sensors With Enhanced Led Drivers
• Manufacturer: Semtech Corporation

ADVANCED COMMUNICATIONS & SENSING
The pressure represents the finger touch on the sensors of the slider and it used to determine if a slider is
touched or released.
In case the pressure equals zero the slider status is released.
In case the pressure is larger as the Slider
The position of a finger on a slider is calculated by the centre of gravity algorithm.
Figure 51 shows an example of a slider composed of 6 sensors (CAP0, CAP1… CAP5).
The default slider norm value 12 (SldNormMsb = 0x01, SldNormLsb = 0x80), is taken for the example.
A touch on CAP0 gives the slider position: 0.
A touch on CAP1 gives the slider position: 12.
A touch on CAP5 gives the slider position: 60.
If a touch occurs on CAP0 and CAP1 the centre of gravity algorithm will interpolate.
Assuming the touch is identically distributed on CAP0 and CAP1 then the position will be: 6
Assuming the touch is identically distributed on CAP1 and CAP2 then the position will be: 18
Assuming the touch is identically distributed on CAP4 and CAP5 then the position will be: 54
The maximum slider position (for this example) that can be obtained is 60.
The minimum position of a slider equals 0.
Revision 7_3, January 10
- N is the number of sensors,
- A sensor with ticks smaller than the CapThreshold is not taken into account for calculating the pressure
- N is the number of sensors,
- A sensor with ticks smaller than the CapThreshold is not taken into account for calculating the position,
- SldNorm[15:0] is a 16 bit number determined by SldNormMsb[15:8] and SldNormLsb[7:0].
- SldPos is the slider position (16 bits) which can be read by the host over the I2C registers SldPosMsb and
SldPosLsb
SldPressur
SldPos
=
SldNorm
e
32
=
Touch Controller (12 sensors) with Enhanced LED Drivers
Hysteresis
© 2009 Semtech Corp.
Figure 51
∑
N
i
∗
=
−
0
1
(
N
∑
i
=
ticks
−
the slider status is touched.
∑
0
N
i
1
=
Slider Position
−
53
0
1
i
*
Ultra Low Power, Capacitive Button and Slider
(
ticks
(
_
ticks
diff
_
_
diff
) (
diff
i
) (
i
−
) (
i
−
CapThresh
−
CapThresh
CapThresh
(
i
(
(
))
i
i
))
))
DATASHEET
www.semtech.com
SX8644