QT60xx6 spread spectrum operates by using a frequency chirp
within each burst, in four different frequency bands.
Table 2-1 Calibration Timings
This feature is hardwired into the device and cannot be
disabled or modified.
Burst Spacing, Cal Time, ms, Cal Time, ms,
ms
32 keys
48 keys
*Auto
0.50
0.75
1.00
1.25
1.50
1.75
2.00
2.25
2.50
2.75
3.00
see text
see text
280
384
2.15 Detection Integrators
387
543
See also Section 5.4, page 21.
495
702
602
862
The devices feature a detection integration mechanism, which
acts to confirm a detection in a robust fashion. The basic idea is
to increment a per-key counter each time the key has crossed
its threshold. When this counter reaches a preset limit the key
is finally declared to be touched. Example: If the limit value is
10, then the device has to detect a threshold crossing 10 times
in succession without interruption, before the key is declared to
be touched. If on any sample the signal is not seen to cross the
threshold level, the counter is cleared and the process has to
start over from the beginning.
709
1,021
1,180
1,339
1,500
1,658
1,817
1,976
816
923
1031
1138
1245
1352
To the above, add 2,083ms, 36 ms, or 3ms from (1), (3),
or (4) for the total elapsed time from reset to ability to
report key detections.
The QT60xx6 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.
*Auto mode determination time: In Auto mode, burst
spacings are assigned just after a reset event in a process that
requires 30ms worst case per enabled key. Thus, if there are
32 keys enabled, the Auto mode calculation process requires
32 x 30ms = 960ms. Subsequent to the auto mode calculation
time, the keys enter calibration mode. Thus, the startup time of
the part is almost 1s longer than normal due to this
‘determination time’, and should be factored into the startup
delay time.
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 (at the
same burst spacing timing).
Calibration time: The calibration time is shown in Table 2-1.
Disabled keys are subtracted from the burst sequence and thus
the cal time will be proportionately shorter than the numbers
shown for lower key counts. In auto burst spacing mode, the
burst spacing time should be measured on an oscilloscope and
used to look up the calibration time value in the table.
Keys that cannot calibrate for some reason require 5 full cal
cycles before they report as errors. However, the device can
report back during this interval that the key(s) affected are still
in calibration via status function bits. Keys in calibration also
report back as being in error (Section 4.11) since keys in
calibration are ‘blind’ to touch.
The net effect of this mechanism is a multiplication of the inner
and outer counters and hence a highly noise-resistant 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.
2.13 Reset Input
2.16 FMEA Tests
The /RST pin can be used to reset the device to simulate a
power down cycle, in order to bring the part up into a known
state should communications with the part 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.
FMEA (Failure Modes and Effects Analysis) is a tool used to
determine critical failure problems in control systems. FMEA
analysis is being applied increasingly to a wide variety of
applications including domestic appliances. To survive FMEA
testing the control board must survive any single problem in a
way that the overall product can either continue to operate in a
safe way, or shut down.
To provide for proper operation during power transitions the
devices have an internal brownout detector set to 4 volts.
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 overkill.
The most common FMEA requirements regard opens and
shorts analysis of adjacent pins on components and
connectors. However other criteria must usually be taken into
account, for example complete device failure, and the use of
redundant signaling paths.
A Force Reset command, 0x04 is also provided which
generates an equivalent hardware reset.
If an external hardware reset is not used, this pin may be
connected to Vdd or left floating.
QT60xx6 devices incorporate special self-test features which
allow products to pass such FMEA tests easily. These tests are
performed during a dummy timeslot after the last enabled key.
The sequence of tests are performed repeatedly during normal
running once all initialization, include the burst spacing
optimization in auto mode, is complete. During initialization, all
2.14 Spread Spectrum Acquisitions
QT60xx6 devices use spread-spectrum acquisition modulation.
This has the effect of drastically reducing EMI effects on the
signals, while reducing the level of detectable RF emissions.
lQ
7
QT60486-AS R8.01/0105
QUANTUM [ QUANTUM RESEARCH GROUP ]
