6.2 Spread-spectrum Circuit
5 Reset
The QT1106 offers the ability to spectrally spread its
frequency of operation to heavily reduce susceptibility to
external noise sources and to limit RF emissions. The
SPREAD pin is used to modulate an external passive RC
network that modulates the OSC pin. OSC is the main
oscillator current input. The circuit and recommended values
are shown in Figure 2.1.
5.1 Introduction
When starting from power-up or /RST reset there are a few
additional factors to be aware of. In most applications the
host will not need to take special action.
During hardware reset all outputs are disabled. To define the
levels of the CHANGE and DRDY during reset these signals
should pulled down by resistors to 0V. Otherwise, they may
drift high causing the host to detect a false logic 1.
The resistors Rb1 and Rb2 should be changed, depending
on Vdd. As shown in Figure 2.1, three sets of values are
recommended for these resistors, depending on Vdd. The
power curves in Section 7.6 also show the effect of these
resistors.
When the initial reset phase ends, CHANGE and DRDY
outputs are enabled. DRDY will drive low and CHANGE will
drive high.
The spread-spectrum circuit can be eliminated if it is not
desired; see Section 6.1. Non spread-spectrum mode
consumes less current in the low power modes.
5.2 Delay to SPI Functionality
The QT1106 SPI interface is not operational while the device
is being reset. However, SPI is made operational early in the
start-up procedure.
The spread-spectrum RC network should be adjusted to suit
the acquire burst lengths. The sawtooth waveform observed
on SPREAD should reach a crest height as follows:
After any reset (either via the /RST pin or via power-up), SPI
typically becomes operational within 100ms of /RST going
high or power-up. This is indicated to the host by DRDY
being pulsed high for at least 450µs, as occurs between
groups of acquire bursts when in Free Run mode. The
maximum delay is:
Vdd >= 3.6V: 17% of Vdd
Vdd < 3.6V:
20% of Vdd
The Css capacitor connected to SPREAD (see Figure 2.1)
should be adjusted so that the waveform approximates the
above amplitude, ±10%, during normal operation in the
target circuit. If this is done, the circuit will give a spectral
modulation of 12 to 15%.
Vdd >= 4.5V: 150ms
Vdd < 4.5V:
200ms
In cases where the three acquire bursts 1, 2, 3 are of
different lengths, the Css capacitor should be adjusted for
the longest acquire burst.
5.3 Reset Delay to Touch Detection
After power up or reset, the QT1106 calibrates all electrodes .
During this time, touch detection cannot be reported. Four
dummy bursts are performed in 80ms after exiting from the
reset start-up delay. Calibration completes after 14 burst
cycles, which normally requires an additional 280ms.
6.3 Cs Sample Capacitors - Sensitivity
The Cssample capacitors accumulate the charge from the
key electrodes and determine sensitivity. Higher values of C s
make the corresponding sensing channel more sensitive.
The values of Cs can differ for each channel, permitting
differences in sensitivity from key to key or to balance
unequal sensitivities.
In total, 460ms are required from reset or power-up for the
device to be fully functional.
Disabled Keys: Keys with missing Cs capacitors, or that
otherwise have an out-of-range signal during calibration , are
considered to be unused or faulty and are disabled. Disabled
keys are re-examined for operation after each reset or
recalibration event.
Unequal sensitivities can occur due to key size and
placement differences and stray wiring capacitances. More
stray capacitance on a sense trace will desensitize the
corresponding key; increasing the Cs for that key will
compensate for the loss of sensitivity.
5.4 Mode Setting After Reset
After a reset the device will enter Free Run mode, with AKS
disabled.
The Cs capacitors can be virtually any plastic film or low to
medium-K ceramic capacitor. The ‘normal’ Cs range is 1nF
to 100nF for the keys and 4.7nF to 220nF for the
wheel/slider, depending on the sensitivity required; the larger
values of Cs require better quality to ensure reliable sensing.
Acceptable capacitor types for most uses include PPS film,
polypropylene film, and NP0 and X7R ceramics. Lower
grade ceramics than X7R are not advised; the X5R grade
should be avoided because it is less stable than X7R .
6 Design Notes
6.1 Oscillator Frequency
The oscillator uses an external network connected to the
OSC and SPREAD pins as shown in Figure 2.1. The charts
in this figure show the recommended values to use
depending on nominal operating voltage and
spread-spectrum mode.
6.4 Thermal Stability
The QT1106 can operate with or without the wheel/slider and
supports up to seven keys. Channels not fitted with a sense
capacitor will automatically be switched off during calibration.
If spread-spectrum mode is not used, only resistor RB1
should be used, the Css capacitor eliminated, and the
SPREAD pin pulled to Vss with a 100K resistor .
For better thermal stability while operating with only one key
it is best to fit a sense capacitor of the same type and value
for another spare key channel. Additionally a small value Cx
(5pF COG) should be fitted to simulate electrode
capacitance. This provides a stable reference for increased
thermal stability.
An out-of-specification oscillator can induce timing problems
such as large variations in response times as well as on the
SPI port.
Lq
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QT1106-ISG R8I.05/0906
