A single ceramic 0.1uF bypass capacitor, with short traces,
should be placed very close to supply pins 3, 4, 5 and 6 of the
IC. Failure to do so can result in device oscillation, high
current consumption, erratic operation etc. Pins 18, 20, and
21 do not require bypassing.
2.11 Startup / Calibration Times
The devices require initialization times of up to 20ms. A
calibration takes one matrix scan.
Disabled keys are subtracted from the burst sequence and
thus the cal time is shortened. The scan time should be
measured on an oscilloscope.
2.12 Reset Input
The /RST pin can be used to reset the device to simulate a
power-down cycle, in order to bring the device up into a
known state should communications with the device be lost.
The pin is active low, and a low pulse lasting at least 10µs
must be applied to this pin to cause a reset.
The reset pin has an internal 30K
✡
- 60K
✡
resistor. A 2.2µF
capacitor plus a diode to Vdd can be connected to this pin as
a traditional reset circuit, but this is not required.
If an external hardware reset is not used, the reset pin may
be connected to Vdd or left floating.
2.13 Spread Spectrum Acquisitions
QT60xx0 devices use spread-spectrum burst modulation.
This has the effect of drastically reducing the possibility of
EMI effects on the sensor keys, while simultaneously
spreading RF emissions. This feature is hard-wired into the
device and cannot be disabled or modified.
Spread spectrum is configured as a frequency chirp over a
wide range of frequencies for robust operation.
2.14 Detection Integrators
See also Section 6.5, page 18.
The devices feature a detection integration mechanism, which
acts to confirm a detection in a robust fashion. A per-key
counter is incremented each time the key has exceeded its
threshold and stayed there for a number of acquisitions.
When this counter reaches a preset limit the key is finally
declared to be touched.
For example, if the limit value is 10, then the device has to
exceed its threshold and stay there for 10 acquisitions in
succession without going below the threshold level, before
the key is declared to be touched. If on any acquisition the
signal is not seen to exceed the threshold level, the counter is
cleared and the process has to start from the beginning.
The QT60xx0 uses a two-tier confirmation mechanism having
two such counters for each key. These can be thought of as
‘inner loop’ and ‘outer loop’ confirmation counters.
The ‘inner’ counter is referred to as the ‘fast-DI’; this acts to
attempt to confirm a detection via rapid successive
acquisition bursts, at the expense of delaying the sampling of
the next key. Each key has its own fast-DI counter and limit
value; these limits can be changed via the Setups block on a
per-key basis.
The ‘outer’ counter is referred to as the ‘normal-DI’; this DI
counter increments whenever the fast-DI counter has reached
its limit value. If a fast-DI counter failed to reach its terminal
count, the corresponding normal-DI counter is also reset. The
normal-DI counter also has a limit value which is settable on
a per-key basis. If a normal-DI counter reaches its terminal
count, the corresponding key is declared to be touched and
becomes ‘active’. Note that the normal-DI can only be
incremented once per complete keyscan cycle, i .e. more
slowly, whereas the fast-DI is incremented ‘on the spot’
without interruption.
The net effect of this mechanism is a multiplication of the
inner and outer counters and hence a highly noise-resistance
sensing method. If the inner limit is set to 5, and the outer to
3, the net effect is 5x3=15 successive threshold crossings to
declare a key as active.
2.15 Sleep
The device will sleep whenever possible to conserve power.
Periodically, the part will wake automatically, scan the matrix,
and return to sleep unless there is activity which demands
further attention. The part will always return to sleep
automatically once all activity has ceased. The time for which
the part will sleep before automatically awakening can be
configured.
A new communication with the device while it is asleep will
cause it to wake up, service the communication and scan the
matrix. At least one full matrix scan is always performed after
waking up and before returning to sleep.
At the end of each matrix scan, the part will return to sleep
unless recent activity demands further attention. If there has
been recent activity, the part will perform another complete
matrix scan and then attempt to sleep once again. This
process is repeated indefinitely until the activity stops and the
part returns to sleep.
Key touch activity will prevent the part from sleeping. The part
will not sleep if any touch events were detected at any key in
the most recent scan of the key matrix.
lQ
7 QT60240-ISG R8.06/0906
