TABLE 3-1 - Command Summary
Hex
0x00
Command
What it does
Shift out data; cause acquire burst (if /SS rises again)
Null
Force recalibration of reference; causes 10 sequential bursts
0x01
Calibrate
Power up default value = calibrated
0x03
0x04
Drift Comp
Error Status
Drift compensation request; causes acquire burst. Max drift rate is 1 count per ten 0x03 commands.
On the following SPI shift, returns the error status of the part; causes acquire burst. See Section 3.3.4.
Set touch threshold; causes acquire burst. Bottom 6 bits (‘T’) are the touch threshold value. (10TT TTTT)
0x8T
Threshold
Power up default value = 10
This command is predominant once the device has been
calibrated and is running normally.
If the Drift command is issued while the device is in touch
detection (ie bit 7 of the Standard Response byte =1), the drift
function is ignored.
3.3.2 0x01 - Calibrate
Drift compensation during Free-Run mode is fixed at 6, which
results in a maximum rate of drift compensation rate of about
3secs / count; see Section 1.2.
7
0
6
0
5
0
4
0
3
0
2
0
1
0
0
1
This command takes ~325ms @ 3.3V to complete.
The drift compensation rate should be made slow, so that it
does not interfere with finger detection. A drift compensation
rate of 3s ~ 5s is suitable for almost all applications. If the
setting is too fast, the device can become unnecessarily
desensitized when a hand lingers near the element. Most
environmental drift rates are of the order of 10's or 100's of
seconds per count.
0x01 causes the device to do a basic recalibration. After the
command is given the device will execute 10 acquisition
bursts in a row in order to perform the recalibration, without
the need for /SS to trigger each of the bursts. The host should
wait for DRDY to rise again after the calibration has
completed before shifting commands again.
The response to this command is the Standard Response
byte, returned on the next SPI shift.
This command should be given if there is an error
reported via the 0x04 command.
On power-up the device calibrates itself automatically and so
a 0x01 command is not required on startup.
3.3.4 0x04 - Error Status
7
0
6
0
5
0
4
0
3
0
2
1
1
0
0
0
The response to this command is the Standard Response
byte, returned on the next SPI shift. During calibration,
device communications are suspended.
This command is used to read the current status of the
device. In particular it is used to detect if there is a sensing
error caused by a calibration or power-on at a bad time, ie
when a finger is on the sensing strip and thereafter removed.
3.3.3 0x03 - Drift Compensate
7
0
6
0
5
0
4
0
3
0
2
0
1
1
0
1
The reported bits are as follows:
Bit 7 = 1 indicates touch;
0x03 causes the sensor to perform incremental drift
compensation. This command must be given periodically in
order to allow the sensor to compensate for drift. The more
0x03 commands issued as a percentage of all commands,
the faster the drift compensation will be.
The 0x03 command must be given 10 times in order for the
device to do one count of drift compensation in either
direction. The 0x03 command should be used in substitution
of the Null command periodically.
= 0 indicates no touch
Bit 6 = 0 indicates Linear type (QT401 or QT411)
= 1 indicates Wheel type (QT510 or QT511)
Bits 5, 4, 3, 2: unused (0)
Bit 1 = 1 if calibration error
Bit 0 reserved (reports 0 or 1)
All bits except Bit 1 can be safely ignored.
The status byte should be read the first time there is a
detection just after a power-on reset or after a 0x01
calibration. If Bit 1 = 1, there was a calibration error and the
device should be immediately calibrated again using the 0x01
command. After the second calibration it should be checked
yet again (and so on) until there is no error.
Example: The host causes a burst to occur by sending a
0x00 Null command every 50ms (20 per second). Every 10th
command the host sends is a 0x03 (drift) command.
The maximum drift compensation slew rate in the reference
level is -
If there is no error according to the sequence of the above
paragraph, it is not required to read this byte again.
50ms x 10 x 10 = 5.0 seconds
The actual rate of change of the reference level depends on
whether there is an offset in the signal with respect to the
reference level, and whether this offset is continuous or not.
The error byte is returned on the following SPI shift.
It is possible to modulate the drift compensation rate
dynamically depending on circumstances, for example a
significant rate of change in temperature, by varying the mix
of Drift and Null commands.
lQ
8
QT411-ISSG R6.01/1005
QUANTUM [ QUANTUM RESEARCH GROUP ]
