Table 1-1 Pin Descriptions
PIN
1
2
3
4
5
6
7
8
9
10
11
12
13
14
NAME
VDD
SDO
/SS
SCLK
SDI
SNS1A
SNS1B
SNS2B
SNS2A
N/A
PROX
DETECT
DRDY
VSS
TYPE
Power
O
I
I
I
I/O
I/O
I/O
I/O
O
O
O
O
Ground
DESCRIPTION
Positive power pin (+2.5 .. +5V)
Serial data output
Slave Select pin. Active low input to enable serial clocking. 1K ohms in series recommended.
Serial clock input. Clock idles high
Serial data input
Sense pin (to Cs1)
Sense pin (to Cs1, Rs1); connects to ‘0’ end of slider element
Sense pin (to Cs2, Rs2); connects to ‘127’ end of slider element
Sense pin (to Cs2)
Leave open
Active high when a hand is near the slider. May be left unconnected.
Note (1)
Active high when slider is touched. May be left unconnected.
Note (1)
Data ready output. Goes high to indicate it is possible to communicate with the QT401.
Note (1)
Negative power pin
Note (1):
Pin floats briefly after wake from Sleep mode.
setting of drift compensation in Free-Run mode. See also
Section 3.3.4.
1.5 Position Data
The position value is internally calculated and can be
accessed only when the slider is touched (Detect pin is high).
The position data is a 7-bit number (0..127) that is computed
in real time; the end numbers (0, left; 127, right) map to the
physical ends by one of two possible calibration methods
(see Section 1.6). The position data will update either with a
single rapid touch or will track if the finger is moved
lengthwise along the surface of the slider element. The
position data ceases to be reported when touch detection is
no longer sensed.
1.3 Sleep Mode
After an SPI transmission, the device will enter a low power
sleep state; see Figure 3-1, page 6, and Section 3.2.4, page
7 for details. This sleep state can be extended in order to
lower average power, by simply delaying the rise of /SS.
Coming out of sleep state when /SS rises, the PROX,
DETECT, and DRDY pins will float for ~400µs; it is
recommended that these pins be pulled low to Vss to avoid
false signalling if they being monitored during this time .
Note: Pin /SS clamps to Vss for 250ns after coming out of
sleep state as a diagnostic pulse. To prevent a possible pin
drive conflict, /SS should either be driven by the host as an
open-drain pull-high drive (e.g. with a 100K pullup resistor), or
there should be a ~1K resistor placed in series with the /SS
pin. See Figure 1-1, R3.
1.6 Calibration
Calibration is possible via two methods:
1) Power up or power cycling (there is no reset input).
2) On command from host via SPI (Command 0x01: see
Section 3.3.2).
The calibration period requires 10 burst cycles, which are
executed automatically without the need for additional SPI
commands from the host. The spacing between each Cal
burst is 2ms, and the bursts average about 23ms each when
Cs1, Cs2 are 100nF, ie the Cal command requires ~220ms to
execute. Lower values of Cs will result in shorter bursts and
hence shorter cal times.
In addition to the basic calibration, it is also possible to
request that the QT401 adjust its reported data to achieve
physically calibrated end points (0, 127) via a serial command
(command 0x02: Section 3.3.3). This requires an immediately
preceding reference calibration command (command 0x01:
see Section 3.3.2) in order to work correctly.
Calibration should be performed when there is no hand
proximity to the element, or the results may be in error.
Should this happen, the error flag (bit 1 of the standard
response, see Section 3.3) will activate when the hand is
withdrawn again. In most cases this condition will self-correct
if drift compensation is used, and it can thus be ignored. See
also Section 1.8 below.
1.4 PROX, DETECT Outputs
There are two active-high output pins for detection of hand
proximity and slider position:
PROX output:
This pin goes high when a hand is detected
in free space near the slider. This condition is also found
as bit 0 in the standard response when there is no touch
detection (Section 3.3).
DETECT output:
This pin goes high when the signal is
large enough to allow computation of finger position. This
condition is also found as bit 7 in the standard response
(Section 3.3).
The sensitivities of these functions can be set using serial
commands (Sections 3.3.5 and 3.3.6).
These outputs will float for ~400µs after wake from Sleep
mode (see Section 1.3). If Sleep mode is used, it is
recommended that PROX and DETECT (if used) be shunted
to ground with 1nF capacitors to hold their states during the
400µs float interval when emerging from Sleep.
lQ
3
QT401 R10.04/0505
QUANTUM [ QUANTUM RESEARCH GROUP ]
