© Quantum Research Group Ltd.
o accomplish. Only a full recalibration via a reset or a
2.8 Reference Guardbanding
See also commands ^N, ^O, page 27; L, page 28
recalibration command will perform a complete recalibration
involving both the R2R Offset and Cz capacitors (Section
2.10).
QT60xx5 devices provide for a method of self-checking that
allows the host device to ascertain whether one or more key
reference levels are 'out of spec'. This feature can be used to
determine if an X or Y line has broken, the matrix panel has
delaminated from the control panel, or there is a circuit fault.
After a fast recalibration has taken place, the affected key will
once again function normally even if it is still being contacted
by the foreign object. This feature is set on a per-key basis
using setup ^L. It can be disabled if desired by setting this
parameter to zero, so that it will not recalibrate automatically.
Guardbanding alerts the host controller when the reference
level of a key falls outside of acceptable absolute levels. The
guardband is expressed in percent of absolute reference from
the reference level of each individual key. The normal
reference levels can be locked into internal eeprom via the
Lock command 'L' during production; deviations in references
that fall outside the guardbands centered on these reference
levels are then reported as errors.
2.6 Detect Integrator (‘DI’)
See also command ^J, page 26
To suppress false detections caused by spurious events like
electrical noise, the QT60xx5 incorporates a 'detection
integrator' or DI counter that increments with each sample
where the signal passes below the negative threshold, until a
user-defined DI limit is reached, at which point the detection
is confirmed and the corresponding detect bit is set.
The calculations required for guardbanding are performed
after the device has recalibrated or been reset after the ‘L’
command.
If before the DI limit is reached, the signal rises to a point
between the hysteresis and threshold levels, the DI counter is
decremented with each such sample to a limit of zero.
Positive excursion guarding is treated separately from
negative excursion guarding. The possible negative settings
are from 1% to 99% of absolute signal reference in steps of
1% as set by command ^O. Positive excursions can run from
10% to 1,000% in steps of 10% as set by command ^N. A
setting of 0 disables the corresponding guardband direction.
If before the DI limit is reached, the signal rises above the
hysteresis level, the DI counter is immediately cleared.
When an active key is released, the DI must count down to
zero before the key state is cleared. Clearing a key’s DI limit
disables that key although the bursts for that key continue.
Since the circuit uses a segmented ADC approach with a
'coarse' (based on Cz states) and 'fine' (based on R2R ladder
drive) offsets, the determination of percentage reference
deviation from 'normal' presents a problem. The contributions
of the Cz caps and the R2R ladder must be factored into the
determination in order to make an accurate assessment of
the error band. There are three commands which set
coefficients used to convert the Cz and DAC offset values to
'absolute signal' values, according to the following equation,
for each key:
The DI is extremely effective at reducing false detections at
the expense of slower reaction times. In some applications a
slow reaction time is desirable; the DI can be used to
intentionally slow down touch response in order to require the
user to touch longer to operate the key.
There are 16 possible values for the DI limit.
2.7 Positive Recalibration Delay
See also command ^K, page 26
TotalRef(k) = (C1 x nCz) + (C2 x Offset) + SigRef
Where -
TotalRef(k) is the equivalent absolute reference for key ‘k’;
C1 is a global constant set by commands ^T and ^U;
C2 is a global constant set by command ^V;
nCz is the number of Cz caps switched in for key ‘k’;
Offset is the noted value of the R2R DAC for key ‘k’;
SigRef is the noted current 'window reference' for key ‘k’.
A recalibration can occur automatically if the signal swings
more positive than the positive threshold level. This condition
can occur if there is positive drift but insufficient positive drift
compensation, or if the reference moved negative due to a
recalibration, and thereafter the signal returned to normal.
As an example of the latter, if a foreign object or a finger
contacts a key for period longer than the Negative Recal
Delay, the key is by recalibrated to a new lower reference
level. Then, when the condition causing the negative swing
ceases to exist (e.g. the object is removed) the signal can
suddenly swing back positive to near its normal reference.
The percent deviations are computed in relation to
TotalRef(k) on a per-key basis at the time the 'L' command is
executed. Once the L command has recorded all values of
relating to TotalRef into eeprom, the part will compare the
actual running reference level of each key to its
corresponding computed TotalRef value to see if it falls
outside the guardbands specified by global parameters ^N
and ^O.
It is almost always desirable in these cases to cause the key
to recalibrate to the new signal level so as to restore normal
touch operation. The device accomplishes this by simply
setting Reference = Signal.
Values which correspond to the reference circuit of Figure 3-1
are:
The time required to detect this condition before recalibrating
is governed by the Positive Recalibration Delay command. In
order for this feature to operate, the signal must rise through
the positive threshold level (Section 2.2) for the proscribed
user-set interval determined by ^K.
C1 = 1513; ^T value = 0x05, ^U value = 0xE9
C2 = 8; ^V value = 0x08
Guardbanding tests should not be confused with Reference
Boundary errors (Section 2.11). Guardbanding can report
errors that occur even if the signal is properly centered in the
ADC window, while Reference Boundary error reporting
cannot. Guardband tests do however require that the key
After the Positive Recal Delay interval has expired and the
fast-recalibration has taken place, the affected key will once
again function normally. This interval can be set on a per-key
basis; it can also be disabled by setting ^K to zero.
lQ
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