SX8661 datasheet(56/88 Pages) SEMTECH

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ADVANCED COMMUNICATIONS & SENSING

DATASHEET

Revision v3.2, January 2011

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SX8661

Low Power, Capacitive Button Touch and Proximity Controller

(8 sensors) with LED Drivers and Analog Output

In case the debounce is enabled the SX8661 will count up to the number of debounce samples BtnCfg [1:0],

BtnCfg [3:2] before taking a touch or release decision. The sample period is identical to the scan period.

BtnAvgPosThresh

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

BtnAvgPosThresh. 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 BtnAvgPosThresh value simultaneously then the SX8661 will start an

offset compensation procedure.

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 54.

Figure 54

Negative Ticks Offset Compensation Trigger

BtnCompNegThresh

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.

BtnCompNegCntMax

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.

BtnHysteresis

The hysteresis percentage is identical for all buttons.

A touch is detected if the ticks are getting larger as the value defined by:

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