normally not be necessary to ever repeat the sequence
unless an error flag is found or the part is powered down and
back up again.
3.3.4 0x03 - Drift Compensate
7
0
6
0
5
0
4
0
3
0
2
0
1
1
0
1
P is normally in the range from 6 to 10. The prox threshold
has no hysteresis and should only be used for non-critical
applications where occasional detection bounce is not a
problem, like power activation (i.e. to turn on an appliance or
a display).
Both the prox bit in the standard response and the PROX pin
will go high if the signal exceeds this threshold. The PROX
pin can be used to wake an appliance or display as a hand
approaches the slider, however the /SS line must remain high
so that the device acquires continuously, or /SS has to be at
least pulsed regularly (see Section 3.2.1) for this to work.
0x4P power-up default setting: 10
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.
Example:
The host causes a burst to occur by sending a
0x00 Null command every 50ms (20 per second). Every 6th
command the host sends is a 0x03 (drift) command.
The maximum drift compensation slew rate in the reference
level is -
50ms x 6 x 10 = 3.0 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.
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.
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.
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.
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 u nnecessarily
desensitized when a hand lingers near the strip. Most
environmental drift rates are of the order of 10's or 100's of
seconds per count.
3.3.6 0x8T - Set Touch Threshold
7
1
6
0
5
T
5
4
T
4
3
T
3
2
T
2
1
T
1
0
T
0
The lower 6 bits of this command (T5..T0) are used to set the
touch threshold level. Higher numbers are less sensitive (ie
the signal has to travel further to cross the threshold).
Operand ‘T’ can range from 0 to 63. Internally the number is
multiplied by 4 to achieve a wider range. 0 should never be
used.
This number is normally set to 10, more or less depending on
the desired sensitivity to touch and the panel thickness.
Touch detection uses a hysteresis equal to 12.5% of the
threshold setting.
Both the touch bit (bit 7) in the standard response and the
DETECT pin will go high if this threshold is crossed. The
DETECT pin can be used to indicate to the host that the
device has detected a finger, without the need for SPI polling.
However the /SS line must remain high constantly so that the
device continues to acquire continuously, or /SS has to be at
least pulsed regularly (see Section 3.2.1) for this to work.
0x8T power-up default setting: 10
3.4 SPI - What to Send
The host should execute the following commands after
powerup self-cal cycle has completed: (assuming a 50ms SPI
repetition rate):
1. 0x01 - Basic calibration (optional as this is done
automatically on power-up)
2. 0x02 - End calibration (optional)
3. 0x4P - Set prox threshold (optional)
4. 0x8T - Set touch threshold (optional)
5. An endlessly repeating mixture of:
a. 0x00 (Null) - all commands except:
b. 0x03 (Drift compensate) - replace every nth Null
command where typically, n = 6
c. If there is ever an error bit set, send a 0x01 and
optionally, a 0x02.
If the error occurs frequently, then perhaps the ratio of drift
compensation to Nulls should be increased.
Note: the Null can be replaced by an empty /SS pulse if there
is no need for fast updates.
3.3.5 0x4P - Set Proximity Threshold
7
0
6
1
5
P
5
4
P
4
3
P
3
2
P
2
1
P
1
0
P
0
This command is optional, but if it is not given, the proximity
detection function will work at a default setting of 10.
The lower 6 bits of this command (P5..P0) are used to set the
proximity threshold level. Higher numbers are less sensitive
(ie the signal has to travel further to cross the threshold).
Operand ‘P’ can range in value from 0 to 63. Zero (0) should
never be used. Very low settings can cause excessive flicker
in the proximity result due to low level noise and drift.
The host device can require that the Proximity output be
active many times in a row to confirm a detection, to make
prox detection more robust.
lQ
9
QT401 R10.04/0505
QUANTUM [ QUANTUM RESEARCH GROUP ]
