sx8649 Semtech Corporation, sx8649 Datasheet - Page 55

• Manufacturer Part Number: sx8649
• Description: Ultra Low Power, Capacitive Button And Wheel Touch Controller 8 Sensors With Enhanced Led Drivers
• Manufacturer: Semtech Corporation

ADVANCED COMMUNICATIONS & SENSING
In case the ticks get slightly negative this is considered as normal operation. However large negative values will
trigger an offset compensation phase and a new set of DCVs will be obtained.
The decision to trigger a compensation phase based on negative ticks is determined by the value in the register
WhlCompNegThresh and by the number of ticks below the negative thresholds defined in register
WhlCompNegCntMax. An example is shown in Figure 50.
WhlCompNegThresh
WhlCompNegCntMax
WhlHysteresis
WhlStuckAtTimeout
Revision 7_6, February 10
Small negative ticks are considered as normal operation and will occur very often.
Larger negative ticks however need to be avoided and a convenient method is to trigger an offset
compensation phase. The new set of DCV will assure the idle ticks will be close to zero again.
A trade-off has to be found for the value of this register. A negative threshold too close to zero will trigger a
compensation phase very often. A very negative threshold will never trigger.
As soon as the ticks get smaller than the Negative Threshold the Negative Counter starts to count.
If the counter goes beyond the Negative Counter Max then the offset compensation phase is triggered.
The recommended value for this register is ‘1’ which means that the offset compensation starts on the first tick
below the negative threshold.
In case the pressure is larger as the wheel
The stuckat timer can avoid sticky sensors.
If the stuckat timer is set to one second then the touch of a finger will last only for one second and considered
released, even if the finger remains on the wheel for a longer time. After the actual finger release the wheel
can be touched again and will be reported as usual.
In case the stuckat timer is not required it can be set to zero.
Figure 54
Negative Ticks Offset Compensation Trigger
© 2010 Semtech Corp.
Touch Controller (8 sensors) with Enhanced LED Drivers
hysteresis
55
Ultra Low Power, Capacitive Button and Wheel
the wheel status is touched.
DATASHEET
www.semtech.com
SX8649