SX8635I05AWLTRT SEMTECH [Semtech Corporation], SX8635I05AWLTRT Datasheet - Page 54

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

ADVANCED COMMUNICATIONS & SENSING
WhlOffset
Slow varying wheel ticks due to environmental changes are handled as buttons in the previous section.
If the ticks pass below the wheel negative threshold for more than the compensation negative max counter then
an offset compensation phase will be triggered.
If the ticks pass above the wheel average positive threshold then the averaging filters will be held.
A finger that moves very slowly over the wheel is not considered as a rotation. The status rotate clockwise and
rotate counter clockwise will not be set.
A finger that moves faster on the wheel will change the rotation status.
A rotation is detected if the difference of the position for two succeeding samples at the scanning rate goes
beyond the rotation threshold (WhlRotateThresh). A large rotation threshold requires very rapid finger rotations,
while a small rotation threshold detects more easily rotations but gets sensitive to noise variations as well.
WhlCfg
WhlAvgThresh
Small environmental and system noise cause the ticks to vary slowly around the zero idle mode value.
Revision 7_8, October 10
The wheel offset adds an offset to the wheel position.
Therefore the wheel is divided in 256 segments. Examples are shown in Figure 53.
If the offset equals zero then the calculated position remains unchanged.
If the offset is set to 64, that means an angle offset of 64/256 * 360 degree, the position zero will be shifted
90°.
If the offset is set to 128, that means an angle offset of 128/256 * 360 degree, the position zero will be shifted
180°.
If the offset is set to 192, that means an angle offset of 192/256 * 360 degree, the position zero will be shifted
270°.
In noisy environments it may be required to debounce the touch and release detection decision.
In case the debounce is enabled the SX8647 will count up to the number of debounce samples WhlCfg [1:0],
WhlCfg [3:2] before taking a touch or release decision. The sample period is identical to the scan period.
Small environmental and system noise cause the ticks to vary slowly around the zero idle mode value.
In case the ticks get slightly positive this is considered as normal operation. Very large positive tick values
indicate a valid touch. The averaging filter is disabled as soon as the average reaches the value defined by
WhlAvgThresh. This mechanism avoids that a valid touch will be averaged and finally the tick difference
becomes zero.
In case three or more sensors reach the WhlAvgThresh value simultaneously then the SX8647 will start an
offset compensation procedure.
Figure 53
(12 sensors) with Enhanced LED Drivers and Proximity Sensing
Wheel Position zero with different offsets
© 2010 Semtech Corp.
Low Power, Capacitive Button and Wheel Touch Controller
54
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SX8635