2.7 AKS Function Pins

The QT1080 features an adjacent key suppression ( AKS)

function with two modes. Option resistors act to set this

feature according to Tables 1.2 and 1.6. AKS can also be

disabled, allowing any combination of keys to become active

at the same time. When operating, the modes are:

Global: AKS functions operates across all eight keys. This

means that only one key can be active at any one time.

Groups: AKS functions among two groups of four keys:

0-1-4-5 and 2-3-6-7. This means that up to two keys can

be active at any one time.

In Group mode, keys in one group have no AKS interaction

with keys in the other group.

Note that in Fast Detect mode, AKS can only be off.

2.8 MOD_0, MOD_1 Inputs

In full option mode, MOD_0 and MOD_1 resistors are used

to set the ‘Max On-Duration’ recalibration timeouts. If a key

becomes stuck on for a lengthy duration of time, this feature

will cause an automatic recalibration event of that specific

key only once the specified on-time has been exceeded.

Settings of 10s, 60s, and infinite are available.

The Max On-Duration feature operates on a key-by-key

basis; when one key is stuck on, its recalibration has no

effect on other keys.

The logic combination on the MOD option pins sets the

timeout delay (see Table 1.3).

Simplified mode MOD timing: In simplified mode, the max

on-duration is fixed at 60 seconds.

2.9 Fast Detect Mode

In many applications, it is desirable to sense touch at high

speed. Examples include scrolling ‘slider’ strips or ‘Off’

buttons. It is possible to place the device into a ‘Fast Detect’

mode that usually requires under 1 0ms to respond. This is

accomplished internally by setting the Detect Integrator to

only two counts, i.e. only two successive detections are

required to detect touch.

In LP mode, ‘Fast’ detection will not speed up the initial

delay (which could be up to 360ms nominal depending on

the option setting). However, once a key is detected the

device is forced back into normal speed mode . It will remain

in this faster mode until another LP pulse is received.

When used in a ‘slider’ application, it is normally desirable to

run the keys without AKS.

In both normal and ‘Fast’ modes, the time required to

process a key release is the same . It takes six sequential

confirmations of non-detection to turn a key off.

Fast Detect mode can be enabled as shown in Tables 1.2

and 1.6.

2.10 Simplified Mode

A simplified operating mode which does not require the

majority of option resistors is available. This mode is set by

connecting a resistor labelled SMR between pins SNS6K

and SNS7 (see Figure 1.2).

In this mode there is only one option possible - AKS enable

or disable. When AKS is disabled, Fast Detect mode is

enabled; when AKS is enabled, Fast Detect mode is off.

AKS in this mode is Global only (i.e. operates across all

functioning keys).

The other option features are fixed as follows:

OUT_n, DETECT Pins: Push-pull, active high, direct

outputs

SYNC/LP Function: LP mode, ~200ms response time

Max On-Duration: 60 seconds

See Tables 1.6 and 1.7.

2.11 Unused Keys

Unused keys should be disabled by removing the

corresponding Cs, Rs, and Rsns components and

connecting SNS pins as shown in the ‘Unused’ column of

Table 1.1. Unused keys are ignored and do not factor into

the AKS function (Section 2.7).

3 Design Notes

3.1 Oscillator Frequency

The QT1080’s internal oscillator runs from an external

resistor network connected to the OSC and SS pins as

shown in Figures 1.1 and 1.2 to achieve spread spectrum

operation. If spread spectrum mode is not required, the OSC

pin should be connected to Vdd with an 18K

Ω 1% resistor.

Under different Vdd voltage conditions the resistor network

(or the solitary 18K

Ω resistor) might require minor

adjustment to obtain the specified burst center frequency.

The network should be adjusted slightly so that the positive

pulses on any key are approximately 2µs wide in the ‘solitary

18K

Ω resistor’ mode, or 2.15µs wide at the beginning of a

burst with the recommended spread -spectrum circuit (see

next section).

In practice, the pulse width has little effect on circuit

performance if it varies in the range from 1.5µs to 2.5µs. The

only effects will be seen in non-LP mode, as proportional

variations in Max On-Duration times and response times.

3.2 Spread-spectrum Circuit

The QT1080 offers the ability to spectrally spread its

frequency of operation to heavily reduce susceptibility to

external noise sources and to limit RF emissions. The SS pin

is used to modulate an external passive RC netw ork that

modulates the OSC pin. OSC is the main oscillator current

input. The circuit is shown in both Figures 1.1 and 1.2.

The resistors Rb1 and Rb2 should be changed depending

on Vdd. As shown in Figures 1.1 and 1.2, two sets of values

are recommended for these resistors depending on Vdd.

The power curves in Section 4.6 also show the effect of

these resistors.

The circuit can be eliminated , if it is not desired, by using an

18K

Ω resistor from OSC to Vdd to drive the oscillator, and

connecting SS to Vss with a 100K

Ω resistor. This mode

consumes significantly less current than spread spectrum

mode.

The spread-spectrum RC network might need to be modified

slightly if the burst lengths are particularly long. Vdd

variations can shift the center frequency and spread slightly.

8

QT1080 R11.06/0806