8

AT42QT1060 [DATASHEET]

9505H–AT42–04/2015

2.7

Change Line

The Change line (see CHG in Figure 1-1 on page 4) signals when there is a change in state in the Detection or Input

status bytes and is active low. It is cleared (allowed to float high) when the host reads the status bytes.

If the status bytes change back to their original state before the host has read the status bytes (for example, a touch

followed by a release), the CHG line will be held low. In this case, a read to any memory location will clear the CHG

line.

The CHG line is open-drain and should be connected via a 100k

 resistor to Vdd. It is necessary for minimum power

operation as it ensures that the QT1060 can sleep for as long as possible. Communications wake up the QT1060

from sleep causing a higher power consumption if the part is randomly polled.

The keys enabled by the key bit mask or a change in the Input port status cause a key change interrupt (see Table 5-

1 on page 16). Create a guard channel by removing that key from the key mask and including it in the AKS mask.

Touching the guard channel does not cause an interrupt. The key and AKS masks are set by using the mask

commands (see Table 5-1).

2.8

Types of Reset

2.8.1

External Reset

An external reset logic line can be used if desired, fed into the RST pin. However, under most conditions it is

acceptable to tie RST to Vdd.

2.8.2

Soft Reset

The host can cause a device reset by writing a nonzero value to the reset byte. This soft reset triggers the internal

watchdog timer on a ~16 ms interval.



After ~16 ms the device resets and wakes again.



After a further 30 ms initialization period the device begins responding to its I2C slave address.



After another ~80 ms the device asserts the CHG line to indicate it is ready for touch sensing.

The device NACKs any attempts to communicate with it during the first 30 ms of its initialization period.

After CHG goes low, the device calibrates the sensing channels. When complete, the CHG pin is set low once again.

2.9

Moisture Tolerance

The presence of water (condensation, sweat, spilt water, and so on) on a sensor can alter the signal values

measured and thereby affect the performance of any capacitive device. The moisture tolerance of QTouch devices

can be improved by designing the hardware and fine-tuning the firmware following the recommendations in the

application note Atmel AVR3002: Moisture Tolerant QTouch Design (www.atmel.com/Images/doc42017.pdf).

2.10

Calibration

The command byte can force a recalibration at any time by writing a nonzero value to the calibration byte. This can

be useful to clear out a stuck key condition after a prolonged period of uninterrupted detection.

When the device recalibrates, it also autosenses which keys are enabled by examining the burst length of each

electrode. If the burst length is either too short (if there is a missing or open Cs capacitor) or too long (a Cs capacitor

is shorted), the key is ignored until the next calibration.

The count of the number of currently enabled keys is found in the status response byte. This number can change

after a CAL command; for example, if a Cs capacitor is intermittent.