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.
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 LVD set to 2.7 volts.
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.
The reset pin has an internal 30K ~ 80K 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.
A Force Reset command, 0x04 is also provided which
generates an equivalent hardware reset.
QT60xx8 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.
If an external hardware reset is not used, the reset pin may be
connected to Vdd or left floating.
The FMEA testing is done on all enabled keys in the matrix, and
results are reported via the serial interface through a dedicated
status command (page 13). Disabled keys are not tested. The
existence of an error is also reported in normal key reporting
commands such as Report 1st Key, page 13.
2.14 Spread Spectrum Acquisitions
QT60xx8 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.
All FMEA tests are repeated every second or faster during
normal run operation. Sometimes, FMEA errors can occur
intermittently, for example due to momentary power
fluctuations. It is advisable to confirm a true FMEA fault
condition by making sure the error flags persist for a several
seconds.
Spread spectrum is configured as a frequency chirp over a
wide range of frequencies for robust operation.
2.15 Detection Integrators
Since the devices only communicate in slave mode, the host
can determine immediately if the QT has suffered a
catastrophic failure.
See also Section 5.4, page 19.
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.
The FMEA tests performed are:
X drive line shorts to Vdd and Vss
X drive line shorts to other pins
X drive signal deviation
Y line shorts to Vdd and Vss
Y line shorts to other pins
X to Y line shorts
The QT60xx8 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.
Cs capacitor checks including shorts and opens
Vref test
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.
Key gain test
Other tests incorporated into the devices include:
A test for signal levels against a preset min value (LSL
setup, see page 21). If any signal level falls below this
level, an error flag is generated.
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, ie more slowly, whereas the
fast-DI is incremented ‘on the spot’ without interruption.
CRC communications checks on all critical command and
data transmissions.
‘Last-command’ command to verify that an instruction was
properly received.
Some very small key designs have very low X-Y coupling. In
these cases, the amount of signal will be very small, and the
key gain will be low. As a result, small keys can fail the LSL
test (page 21) or the FMEA key gain test (above). In such
cases, the burst length of the key should be increased so that
the key gain increases. Failing that, a small ceramic capacitor,
for example 3pF, can be added between the X and Y lines
serving the key to artificially boost signal strength.
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.
For those applications requiring it, Quantum can supply sample
FMEA test data on special request.
2.16 FMEA Tests
FMEA (Failure Modes and Effects Analysis) is a tool used to
determine critical failure problems in control systems. FMEA
lQ
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