LM8400 National Semiconductor, LM8400 Datasheet - Page 9

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

4.0 Functional Description
WD_Out — Watch dog output. For correct operation, this
pin should be connected to the RESET pin.
OSC_OUT — Clock oscillator output pin.
OSC_IN — Clock oscillator input pin
GND — Digital Ground
AGND — Analog ground
DV
AV
pin should be connected to a filter.
Users of the LLP package are cautioned to be aware that the
central metal area and the pin 1 index mark on the bottom of
the package may be connected to GND. See figure below:
4.3 USART Framing Format
The device communicates with the touch screen driver using
the UART set at a baud rate of 38400, 8, n, 1.
All communication from the host software and the controller
consists of a command byte and occasionally data byte(s). If
data byte(s) follow a command byte, it must be sent directly
after the command byte. If the device is in the power save
mode, a delay must be inserted between the wake up com-
mand and the command byte. Refer to the Power Save
mode section for more details. For every command byte sent
to the controller, an acknowledge byte is sent back and
sometimes data byte(s).
A command byte is distinguished by having the 7th bit of the
command byte set (1). If any data byte(s) is to follow, the 7th
bit of the data byte is reset (0).
Data packets sent to the host software can be either four or
five bytes in length. If the pressure measurement is enabled,
five bytes are sent. If the pressure measurement is not
enabled, four bytes are sent.
The first byte of each data packet is a header byte. This
header byte is used to synchronize and differentiate between
four or five byte packages. If a four byte data package is
sent, the 4th bit of the header byte is reset (0). If a five byte
data package is sent, the 4th bit of the header byte is set (1).
The 6th bit of the second byte of the data packet is used to
indicate the touch state. If the bit is set (1), then a touchdown
or continuous touch is detected. If the bit is reset (0), then a
liftoff is detected.
CC
CC
— Digital power supply.
— Analog power supply. For best performance, this
FIGURE 1. LLP Package Bottom View
20037216
(Continued)
9
4.3.1. Data packet format table
L - Package length. (0 = 4 bytes, 1 = 5 bytes)
S - Touch state (0 = liftoff, 1 = touchdown or continuous
touch)
X
Y
Z
pressure measurement is enabled
4.3.2 Command Bytes
Read
clock-speed
(3,33MHz,
10MHz)
Read
parameters
Read
software
version
number
Read # of
calibration
points
Read stored
calibration
points
Set focus
value (# of
pixels on
touch panel)
Set # of
samples per
coordinate (1,
2, 4, 8, 16,
32)
Set
communication
mode
(stream,
touchdown,
liftoff)
Byte 1
Byte 2
Byte 3
Byte 4
Byte 5
Z
X
Y
Command
- Z positon (7 bit value), this byte is only transmitted if the
- X position (10 bit value)
- Y position (10 bit value)
PC
Bit 7 Bit 6 Bit 5 Bit 4 Bit 3 Bit 2 Bit 1 Bit 0
1
0
0
0
0
$B0
$B1
$B2
$B3
$B4
$B8
$BA
$BB
X
Y
Z
Byte 1
S
0
6
9
9
PC
X
X
Y
Z
0
5
2
8
8
$0-$3F
$01, $02,
$04, $08,
$10, $20
$01, $02,
$04
Byte 2
X
X
Y
PC
Z
L
4
1
7
7
X
X
Y
Z
0
0
6
6
3
$CA
$CA
$C7
$CA
$CA
$CA
$CA
$CA
Byte 1
TSC
Y
X
Y
Z
0
2
5
5
2
$00, $01
4.3.3
Advanced
Command
Bytes
Descriptions
4.3.3
Advanced
Command
Bytes
Descriptions
$00, $02,
$05, $0D
4.3.3
Advanced
Command
Bytes
Descriptions
$
$01, $02,
$04, $08,
$10, $20
$01, $02,
$04
00-$3F
www.national.com
Y
X
Y
Z
0
Byte 2
1
4
4
1
TSC
Y
X
Y
Z
0
0
3
3
0