SX8661 datasheet(57/88 Pages) SEMTECH

57 / 88 page

ADVANCED COMMUNICATIONS & SENSING

DATASHEET

Revision v3.2, January 2011

www.semtech.com

SX8661

Low Power, Capacitive Button Touch and Proximity Controller

(8 sensors) with LED Drivers and Analog Output

CapThreshold + CapThreshold * hysteresis.

A release is detected if the ticks are getting smaller as the value defined by:

CapThreshold - CapThreshold * hysteresis.

BtnStuckAtTimeout

The stuckat timer can avoid sticky buttons.

If the stuckat timer is set to one second then the touch of a finger will last only for one second and then a

compensation will be performed and button hence considered released, even if the finger remains on the

button for a longer time. After the actual finger release the button can be touched again and will be reported as

usual.

In case the stuckat timer is not required it can be set to zero.

BtnStrongestHysteresis

This parameter defines the hysteresis value for the adjacent button filtering engine. This parameter is only

valid when BtnCfg has been configured to report the strongest touch.

When the SX8661 device has been configured to report the strongest touch, a situation may arise where the

CAP signals of two sensors are of approximately equal value. Environmental noise can cause the signals of

these two sensors to fluctuate as shown in Figure 55 (b).

As a result of that, the output of the SX8661 device would also change very quickly as each of the two sensors

becomes the sensor with the strongest touch value. To eliminate this jitter, the SX8661 device adds a

hysteresis element to the calculation of the strongest touch sensor. In that respect, the strongest CAP sensor

is calculated as the sensor whose value is greater that the second detected strongest CAP sensor by the

Strongest hysteresis amount.

For example, as shown in Figure 55, the strongest CAP sensor is initially CAP0 (Figure 55 (b)). CAP1

becomes the strongest detected touch only if at some point in time the following holds true:

CAP1 signal = CAP0 signal + StrongestHysteresis

Similarly, if CAP2 is now also touched, it will only become the strongest detected touch if:

CAP2 signal = CAP1 signal + StrongestHysteresis.

BtnLongPressTimer

Figure 55

Strongest touch and Hysteresis

(a)

(b)

Fluctuation

Strongest

Hysteresis

CAP1

CAP0

SX8661 Datasheet(PDF) 57 Page - Semtech Corporation

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