SX8635I05AWLTRT SEMTECH [Semtech Corporation], SX8635I05AWLTRT Datasheet - Page 53

• Manufacturer Part Number: SX8635I05AWLTRT
• Description: Low Power, Capacitive Button and Wheel Touch Controller with Enhanced LED
• Manufacturer: SEMTECH [Semtech Corporation]

ADVANCED COMMUNICATIONS & SENSING
As soon as the ticks become larger than the CAP thresholds (see registers of the previous section) plus the
hysteresis (defined in register WhlHysteresis) the debounce counter starts.
In the example of Figure 51 the touch is validated after 2 samples (WhlCfg [1:0] = 01).
The release is detected immediately (WhlCfg [3:2] = 00) at the first sample with a pressure equal to zero.
The position of a finger on a wheel is calculated by the centre of gravity algorithm.
Figure 52 shows an example of a wheel composed of 6 sensors (CAP0, CAP1… CAP5).
The default wheel norm value 320 (WhlNormMsb = 0x01, WhlNormLsb = 0x40), is taken for the example.
A touch on CAP0 gives the wheel position: 0.
A touch on CAP1 gives the wheel position: 10.
A touch on CAP5 gives the wheel position: 50.
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: 5
Assuming the touch is identically distributed on CAP1 and CAP2 then the position will be: 15
Assuming the touch is identically distributed on CAP4 and CAP5 then the position will be: 55
The minimum position of a wheel equals 0.
The maximum position is obtained if the finger is very slightly on CAP5 and heavily on CAP0.
The maximum position (WhlPosMax) is defined by:
Revision 7_8, October 10
- N is the number of sensors,
- A sensor with ticks smaller than the CapThreshold is not taken into account for calculating the position,
- WhlNorm[15:0] is a 16 bit number determined by WhlNormMsb[15:8] and WhlNormLsb[7:0].
- WhlPos is the wheel position (16 bits) which can be read by the host over the I2C registers WhlPosMsb and
WhlPosLsb
with:
N is the number of sensors in the wheel
WhlPos
WhlPosMax
=
WhlNorm
32
(12 sensors) with Enhanced LED Drivers and Proximity Sensing
=
© 2010 Semtech Corp.
Figure 52
Low Power, Capacitive Button and Wheel Touch Controller
WhlNorm
∗
32
∑
N
i
=
−
∑
0
N
Wheel Position
i
1
=
53
−
0
i
1
×
*
(
N
ticks
(
ticks
_
_
diff
diff
) (
i
) (
i
−
−
CapThresh
CapThresh
(
i
(
))
i
))
DATASHEET
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SX8635