1 Operation
The QT401 uses a SPI slave mode interface for control
and data communications with a host controller.
Acquisition timings and operating parameters are
under host control; there are no option jumpers and the
device cannot operate in a stand-alone mode.
The positional output data is a 7-bit binary integer
(0...127) indicating position from left (0) to right (127).
Like all QProx™ devices, the QT401 operates using
bursts of charge-transfer pulses; burst mode permits
an unusually high level of control over spectral
modulation, power consumption, and response time.
The QT401 modulates its bursts in a spread-spectrum
fashion in order to heavily suppress the effects of
external noise, and to suppress RF emissions.
VIN
C1
2.2uF
Figure 1-1 QT401 Wiring Diagram
Regulator
VIN
VOUT
GND
C2
2.2uF
R1
22k
R2
100k
13
2
3
4
5
1 QT401
VDD
8
SNS2B
SNS2A 9
DRDY
SDO
/SS
SCLK
SDI
Cs2
100nF
127
Slider Element
60K~150K ohms
total resistance
SNS1A 6
SNS1B 7
SPI BUS
R3
1K
Proximity
Touch Detect
C3 1nF
Cs1
100nF
11 PROX
0
1.1 Synchronized Mode
Refer also to Figure 3-1, page 6.
Sync mode allows the host device to control the rep etition
rate of the acquisition bursts, which in turn govern response
time and power consumption. The maximum spacing from the
end of one burst to the start of the next in this mode is 1
second.
In sync mode, the device will wait for the SPI slave select line
/SS to fall and rise and will then do an acquisition burst;
actual SPI clocks and data are optional. The /SS pin thus
becomes a ‘sync’ input in addition to acting as the SPI
framing control.
Within 35µs of the last rising edge of CLK, the device will
enter a low power sleep mode. The rising edge of /SS must
occur after this time; when /SS rises, the device wakes from
sleep, and shortly thereafter does an acquisition burst. If a
more substantial sleep time is desired, /SS should be made
to rise some delay period later.
By increasing the amount of time spent in sleep mode, the
host can decrease the average current drain at the expense
of response time. Since a burst typically requires 31ms (at
3.3V, reference circuit), and an acceptable response time
might be ~100ms, the power duty cycle will be 31/100 or 31%
of peak current.
If power is not an issue the device can run constantly under
host control, by always raising /SS after 35µs from the last
rising edge of CLK. Constant burst operation can be used by
the host to gather more data to filter the position data further
to suppress noise effects , if required.
12 DETECT
VSS
14
C4 1nF
Mains Sync:
Sync mode can be used to sync to mains
frequency via the host controller, if mains interference is
possible (ie, running as a lamp dimmer control). The host
should issue SPI commands synchronously with the mains
frequency. This form of operation will heavily suppress
interference from low frequency sources (e.g. 50/60Hz),
which are not easily suppressed using spread-spectrum burst
modulation.
Cross-talk suppression:
If two more QT401’s are used in
close proximity, or there are other QTouch™ type device(s)
close by, the devices can interfere strongly with one another
to create position jitter or false triggering. This can be
suppressed by making sure that the devices do not perform
acquisition bursts at overlapping times. The host controller
can make sure that all such devices operate in distinctly
different timeslots, by using a separate /SS line or Sync
signal for each part.
1.2 Free-Run Mode
If /SS stays high, the device will acquire on its own
repetitively approximately every 60ms (Figure 1-2). This
mode can be used to allow the part to function as a prox or
touch detector first, perhaps to wake a host controller. Either
the PROX or DETECT can be used as a wakeup.
In free-run mode, the device does not sleep between acquire
bursts. In this mode the QT401 performs automatic drift
compensation at the maximum rate of one count per 180
acquisition burst cycles, or about one count every 3 seconds
without host intervention. It is not possible to change this
Figure 1-2 Free-Run Timing Diagram ( /SS = high )
~31ms
Acquire Bur
~3.8ms
~30us
~31ms
DRDY from QT
~25ms
lQ
2
QT401 R10.04/0505
QUANTUM [ QUANTUM RESEARCH GROUP ]
