© Quantum Research Group Ltd.
The rated resistance of an R2R ladder is also its Thevenin frequency source can also be connected to XTI; XTO should
be left unconnected.
equivalent resistance which affects the scaling of the offset
injected into the amplifier, in terms of mV/bit. The scaling of
offset injection also affects the crossover points for the
switching of each Cz capacitor. If during the calibration cycle
the R2R network is found to not provide enough offset to
bring the signal to the midpoint of the ADC's range, a Cz
capacitor is switched in to create an additional offset.
The frequency of oscillation should be 6MHz +/-2%.
3.16 Startup / Calibration Times
The QT60xx5B requires initialization times as follows:
1. From very first powerup to ability to communicate:
2,000ms (One time event to initialize all of eeprom)
If the R2R drive value and Cz values are not properly
matched, the circuit may not be able to converge on all
calibration points, i.e. there will be acquisition ‘holes’. This will
happen if the Cz cancellation voltage step is too large with
respect to the amount of full-scale influence of the R2R
ladder on the analog offset. It is recommended that the
reference circuits shown in Figures 3-1 and 3-2 should not be
altered to avoid problems.
2. Normal cold start to ability to communicate:
70ms (Normal initialization from any reset)
3. Calibration time per key vs. burst spacings:
spacing = 250µs: 425ms
spacing = 300µs: 510ms
spacing = 400µs: 680ms
spacing = 500µs: 850ms
spacing = 1ms: 1,700ms
spacing = 2ms: 3,400ms
3.13 Water Film Suppression
Water films on the user surface can cause problems with
false detection under certain conditions. Water films on their
own will not normally cause false detections. The most
common problem occurs when surface water bridges over 2
or more keys, and a user touches one of the keys and the
water film causing an adjacent key to also trigger. Essentially,
the water film transports the touch contact to adjacent keys.
To the above, add 2,000ms or 70ms from (1) or (2) for
the total elapsed time from reset to ability to report key
detections.
Keys that cannot calibrate for some reason require 5 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.
The recommended circuit suppresses water coupling by
means of a short sample dwell time: a short dwell time
reduces the signal from resistive films by limiting the amount
of time during which charge is collected. Charge from distant
regions of the film take longer to return, and so a short dwell
time will prevent such charge from being sensed. This effect
has nothing to do with the frequency of the burst itself, it is
purely a time-domain phenomenon; changing the burst or
pulse spacings (i.e. sample frequency) will have no effect on
water film suppression.
3.17 Sleep_Wake / Noise Sync
The Sleep_wake and Noise Sync features depend on the use
of pin X2WS as an input. To prevent interference with scan
line X2 during acquisitions, a resistor equal to the rating of
the R2R ladder (i.e. 100K) must be used in series. The Sleep
and Sync features can be used simultaneously; the part can
be put into Sleep mode, but awakened by a noise sync signal
which is gated in at the time desired.
Sleep mode: See also command ‘Z’, page 29.
To create short dwell times, a CMOS PLD is configured with a
simple timing circuit to control the ‘Y’ gate (Section 3.9).
The device can be put into an ultra low-power sleep mode
using the ‘Z’ command. When this command is received, the
Sleep line must be placed immediately thereafter into a
logic-high state. The part will complete an ongoing burst
before entering Sleep. The part can be awakened by a low
transition on the X2WS pin lasting at least 5µs. One
convenient way to wake the part is to connect pin X2WS to
MOSI via the 100K resistor, and have the host send a null
command to the device. The part will wake and the null
command will not be processed. The MOSI line in turn
requires a pullup resistor to prevent the line from floating low
and causing an unintentional wake from sleep.
Mechanical means can also be used to suppress cross-
coupling due to moisture films, for example raised plastic
barriers between keys, or placing keys in shallow wells or on
raised areas to lengthen the electrical path from key to key.
AKS - Adjacent Key Suppression - is included in these
devices to enhance moisture performance (Section 2.9).
3.14 Reset Input
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.
During Sleep the oscillator is shut down, and the part
hibernates with microamp levels of current drain. When the
part wakes, the part resumes normal functionality from the
point where it left off. It will not recalibrate keys or engage in
other unwarranted behavior.
To provide for proper operation during power transitions the
devices have an internal brown-out detector set to 4 volts.
Before going to sleep the part will respond with a 'Z'. In
slave-only SPI mode (see Section 4.3), the SS line must be
floated high by the host as soon as it receives this response;
if SS does not float high, sleep will fail and the device will
instead completely reset after about 2 seconds. Upon waking
the part will issue another 'Z' byte back to the host.
A reset command, ‘r’, is also provided which generates an
equivalent hardware reset (page 28).
3.15 Oscillator
The oscillator can use either a quartz crystal or a ceramic
resonator. In either case, the XTI and XTO must both be
loaded with 22pF capacitors to ground. 3-terminal resonators
having onboard ceramic capacitors are commonly available
and are recommended. An external TTL-compatible
Noise sync: See also command ^W, page 30.
lQ
14
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