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.

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.

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

2

QT401 R10.04/0505

Figure 1-1 QT401 Wiring Diagram

QT401

SCLK

/SS

SDO

VDD

4

3

2

VSS

DRDY

PROX

SDI

SNS1A

SNS1B

SNS2A

8

9

14

Cs2

100nF

SNS2B

7

6

5

11

13

1

VIN

VOUT

GND

Regulator

C1

2.2uF

C2

2.2uF

VIN

Proximity

SPI BUS

R2

100k

Cs1

100nF

R1

22k

127

Slider Element

60K~150K ohms

total resistance

DETECT

12

Touch Detect

C3 1nF

C4 1nF

0

1K

R3

Figure 1-2 Free-Run Timing Diagram ( /SS = high )

~31ms

~31ms

Acquire Bur

~3.8ms

~30us

DRDY from QT

~25ms