sx8644 Semtech Corporation, sx8644 Datasheet - Page 49

• 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
Small environmental and system noise cause the ticks to vary slowly around the zero idle mode value.
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
BtnCompNegThresh and by the number of ticks below the negative thresholds defined in register
BtnCompNegCntMax. An example is shown in Figure 50.
BtnCompNegThresh
BtnCompNegCntMax
BtnHysteresis
BtnStuckAtTimeout
Revision 7_3, January 10
BtnAvgThresh. 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 BtnAvgThresh value simultaneously then the SX8644 will start an
offset compensation procedure.
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.
The hysteresis percentage is identical for all buttons.
A touch is detected if the ticks are getting larger as the value defined by:
A release is detected if the ticks are getting smaller as the value defined by:
The stuckat timer can avoid sticky buttons.
CapThreshold + CapThreshold * hysteresis.
CapThreshold - CapThreshold * hysteresis.
Figure 50
Negative Ticks Offset Compensation Trigger
Touch Controller (12 sensors) with Enhanced LED Drivers
© 2009 Semtech Corp.
49
Ultra Low Power, Capacitive Button and Slider
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SX8644