LM8400 National Semiconductor, LM8400 Datasheet - Page 11

• Manufacturer Part Number: LM8400
• Description: Four Wire Resistive Touchscreen Controller with Brownout
• Manufacturer: National Semiconductor

4.0 Functional Description
$B2: Read software version number
$B3: Read # of calibration points
$B4: Read stored calibration points
Continue until all coordinates have been sent. A zero is send
back if calibration has not been performed and there are no
data bytes.
$B8: Set focus value
The set focus command allows the setting of different values
to improve touch screen focusing. Focusing is defined as the
ability of the touch screen controller to detect exactly identi-
cal coordinate values from measurement to measurement if
the pointer on the touch screen has not moved. The focus
values are equivalent to pixels of touch screen resolution. If
for example a value of 2 is selected, this means that every
coordinate value that is within two pixels of the previously
measured coordinate value is considered to be identical to
that previous value and that in this case the touch screen
controller transmits the previous coordinate information. This
keeps the mouse pointer steady at the point being touched,
rather than "jumping around" the point. A Focus value of zero
disables the focusing algorithm. The default setting is 4.
$BA: Set number of samples per coordinate
This command allows the selection of different sample num-
bers per X, Y, and Z coordinates. The higher the number of
samples per X, Y, and Z coordinates, the better the accuracy,
but the lower the coordinates per second data rate. The
default setting is 8.
$BB: Set communication mode
Byte #2: Communication mode (
touchdown,
Reply Byte #1: $C7 (Software version number)
Byte #2: First byte in version number, year 20 (00-99)
Byte #3: Second byte in version number, month (1-12)
Byte #4: Third byte in version number, day (1-31)
Reply Byte #1: $CA (Acknowledge)
Byte #2: (
$
Reply Byte #1: $CA (Acknowledge)
Byte #2: (
$
Byte #3: X-max (2 MSB for coordinate 1)
Byte #4: X-min (8 LSB for coordinate 1)
Byte #5: Y-max (2 MSB for coordinate 1)
Byte #6: Y-min (8 LSB for coordinate 1)
Byte #2: Focus value (0-63)
Reply Byte #1: $CA (Acknowledge)
Byte #2: Focus value (0-63)
Byte #2: Number of samples per coordinate ($01 - 1
samples/coordinate, $02 - 2 samples/coordinate, $04
- 4 samples/coordinate, $08 - 8 samples/coordinate,
$10
samples/coordinate)
Reply Byte #1: $CA (Acknowledge)
Byte #2: Number of samples per coordinate ($01 - 1
samples/coordinate, $02 - 2 samples/coordinate, $04
- 4 samples/coordinate, $08 - 8 samples/coordinate,
$10
samples/coordinate)
05 — 5 points or
05 — 5 points or
-
-
$
$
00 — no calibration done,
00 — no calibration done,
16
16
$
04 = liftoff)
samples/coordinate,
samples/coordinate,
$
$
0D — 13 points)
0D — 13 points)
$
01 = stream,
$
$
02 — 2 points,
02 — 2 points,
$20
$20
(Continued)
-
-
$
02 =
32
32
11
See the communication modes section for a description of
the stream, touchdown and liftoff modes. This command
selects the communication mode. The default setting is
stream mode.
$BC: Set max delta
See the averaging algorithms section for a detailed descrip-
tion of this setting. Simply put, this command sets how much
the "coordinate velocity" can change from one coordinate to
the next. The default setting is 8.
$BD: Set calibration points
Refer to the calibration section for details.
$BE: Set minimum pressure
This setting controls how high the pressure (Z-axis) must be
in order for samples to be accepted. Setting this value too
low may result in having faulty coordinates accepted. This
value is internally multiplied by two in the controller (due to
the 7-bit limitation in the communication format, which can
not send 8-bit values larger than 127 in one byte). The
default setting is 40.
$A0: Toggle disable/enable external filter path
This command enable/disable external filter path. The exter-
nal filter path enabled option will require the addition of a
single external low pass filter (either R/C or active OpAmp
based), which is then applied to the touch screen signal
lines. This option can be used in high noise environments to
significantly improve performance and accuracy of the touch
screen controller.
The default setting is filter path enabled.
$A2: Toggle disable/enable self-power down
This command can switch between self-power down mode
enable or disabled. Refer to the Power Save Mode section
for details. The default setting is Self-Power Down mode
enabled. If the echo mode is enabled, any command byte
send to the device will be echo back and executed.
$A3: Toggle disable/enable echo mode
The echo mode is available for debugging purposes. If en-
abled, the touch screen controller will echo back any data
that is received via the UART interface.
Byte #2: Max delta value (0-63)
Reply Byte #1: $CA (Acknowledge)
Byte #2: Max delta value (0-63)
Reply Byte #1: $CA (Acknowledge)
Byte #2: (0 = now disabled, 1 = now enabled)
Reply Byte #1: $CA (Acknowledge)
Byte #2: (0 = now disabled, 1 = now enabled)
Reply Byte #1: $CA (Acknowledge)
Byte #2: (0 = now disabled, 1 = now enabled)
Byte #2: Minimum pressure value (0-127)
Reply Byte #1: $CA (Acknowledge)
Byte #2: Minimum pressure value (0-127)
Reply Byte #1: $CA (Acknowledge)
Byte #2: Communication mode (
touchdown,
Byte #2: High nibble: Number of calibration points
(
Low nibble: Active calibration cross (1-13 = cross #)
Reply Byte #1: $CA (Acknowledge)
Byte #2: High nibble: Number of calibration points
(
Low nibble: Active calibration cross # (1-13)
$
$
01 = two,
01=two,
$
02=five,
$
$
02 = five,
04 = liftoff)
$
04=thirtheen)
$
04 = thirteen)
$
01 = stream,
www.national.com
$
02 =