MAX11800_1010 Datasheet, PDF (35/59 Pages) Maxim Integrated Products – Low-Power, Ultra-Small Resistive Touch-Screen Controllers with I2C/SPI Interface

Low-Power, Ultra-Small Resistive Touch-Screen
Controllers with I²C/SPI Interface
For example:
0000 = 2^-1 LSB = aperture checking disabled
0001 = 2^(1-1) LSB = ±1 LSB
0010 = 2^(2-1) LSB = ±2 LSB
0011 = 2^(3-1) LSB = ±4 LSB
…
1001 = 2^(9-1) LSB = ±256 LSB (1/16 of the touch screen
in each direction)
1010 = 2^(10-1) LSB = ±512 LSB (1/8 of the touch screen
in each direction)
1011 = 2^(11-1) LSB = ±1024 LSB (1/4 of the touch
screen in each direction)
1100 = 2^(12-1) LSB = ±2048 LSB (1/2 of the touch
screen in each direction)
1101 to 1111 = aperture checking disabled
FIFO Aperture Criteria
In autonomous mode with aperture engaged, new data
is written to the FIFO, and an interrupt is issued when
the following conditions occur (aperture mode is not
available in direct conversion mode).
New Panel Touch Initiated
The FIFO updates and issues an interrupt when a new
touch is observed on the panel (data tag = 00). This
event occurs regardless of the current aperture setting
and the previous touch location so that multiple presses
in the same location can be observed and registered.
Continuous Panel Touch Terminated
The FIFO updates and issues an interrupt when a con-
tinuous panel touch is terminated (data tag = 10). This
event occurs regardless of the current aperture setting
and the previous continuous touch location(s) so that
multiple presses in the same location can be observed
and registered.
Continuous Panel Touch
Measurement Meets Aperture Criteria
The MAX11800/MAX11801 log the measurement data
to the FIFO and issue an interrupt when a measurement
during a continuous panel touch (event tag = 01) meets
the aperture criteria (i.e., lies on or outside the aperture
boundary). This event occurs when the point of contact
is dragged across the touch screen. Only the ΔX or ΔY
aperture criteria need to be met and a greater than or
equal to qualification criterion is applied. If the change
in X position or change in Y position exceeds the aper-
ture criteria, then an interrupt is generated.
Applications Information
Using Aperture Mode
Aperture mode is only supported in the MAX11800/
MAX11801. The MAX11800/MAX11801 accommodate
touch-panel applications where limited resolution in
both time and space can be traded off for reduced
microprocessor activity. A simulated keypad is an
example of an application where autonomous conver-
sion mode with aperture checking could yield an effi-
cient solution.
The AP determines the durations of touch-screen
presses. An issuance of TIRQ interrupts accompanies
all FIFO events. The interrupts clear when all existing
data is read back by the AP, allowing the AP to correct-
ly interpret held panel data.
The FIFO updates immediately when a new touch event
is detected. The system assumes that the panel touch is
continuous after the AP receives the interrupt. The
MAX11800/MAX11801 continue to scan the panel at the
user-programmed sample rate. The FIFO updates when
the measurement data shows that the panel touch loca-
tion moves (i.e., a measurement exceeds either of the
selected aperture ranges). The FIFO also updates upon
detection of a panel release. The AP determines the
duration of the press by observing the time between the
leading edge of the touch (tag 00) and the release edge
of the touch (tag 10). All midpress data (tag 01) are inter-
preted as part of a dragged touch event.
All valid touch events log two data points into the FIFO:
an initial data point at the beginning of the touch (tag
00) and a release data point at the termination of the
touch (tag 10). Discard release edge position data as
invalid as the MAX11800/MAX11801 cannot determine
at which point in the ADC conversion cycle the panel is
released during the measurement operation. If the
release occurs while the ADC is actively sampling the
panel, the results are invalid. Only initial and midpress
position data are reliable.
Any touch event too short in duration to log both initial
and release data points is recorded in the FIFO as a
release (tag 10) and discarded as a glitch event.
Measuring durations of panel touches becomes
impractical when the AP services the MAX11800/
MAX11801 at lower than the operating speed of the
devices and the panel combined. The AP cannot time
the duration between panel touches when both the ini-
tial and release data points can be logged before the
initial interrupt is serviced. Do not allow the FIFO to
overflow as touch information can be lost and the FIFO
content becomes invalid.
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