9

AT42QT1060 [DATASHEET]

9505H–AT42–04/2015

2.11

Guard Channel

The device has a guard channel option, which allows any key, or combination of keys, to be configured as a guard

channel to help prevent false detection. Guard channel keys should be more sensitive than the other keys (physically

bigger or larger Cs), subject to burst length limitations (see Section 2.12.3).

With guard channel enabled, the designated key(s) is connected to a sensor pad which detects the presence of

touch and overrides any output from the other keys using the chip AKS feature. The guard channel option is enabled

by an I2C command.

To enable a guard channel the relevant key should be removed from the key mask (see Table 5-1 on page 16). In

addition, the guard channel needs to be included within the AKS mask with the other keys for the guard function to

operate. Note that a detection on the guard channel does not cause a change request.

With the guard channel not enabled, all the keys work normally.

Figure 2-1.

Guard Channel Example

2.12

Signal Processing

2.12.1 Detect Threshold

The device detects a touch when the signal has crossed a threshold level and remained there for a specified number

of counts (see Section 5.11 on page 19). This can be altered on a key-by-key basis using the key threshold I2C

commands.

2.12.2 Detect Integrator

The device features a fast detection integrator counter (DI filter), which acts to filter out noise at the small expense of

slower response time. The DI filter requires a programmable number of consecutive samples confirmed in detection

before the key is declared to be touched. There is also a fast DI on the end of the detection (see Section 5.20 on

page 23). The fast DI will not be applied at the start of a detection if a detection on any other channel has already

been declared.

2.12.3 Burst Length Limitations

In a balanced system common signals are regarded as thermal shifts and are removed by the relative referencing

drifting, if enabled. This means that the burst lengths must be similar. This can be checked by reading the reference

values (Address 52 – 63) and making sure that they are similar. The absolute maximum difference is that the

maximum value of reference is less than three times the minimum value amongst all the channels. It is

recommended having the burst lengths (references) as close together as possible, through better routing and layout.

For example, if the keys have references of 250, 230, 220, 240, 200 and 210, this is acceptable. If the keys have

references of 250, 230, 220, 240, 200 and 710, the efficiency of the relative referencing drifting will be affected. The

last key’s (710) layout should be changed or relative referencing be disabled. The closer the references are in value,

the better the relative referencing drifting performs.

Guard channel