LM8400 National Semiconductor, LM8400 Datasheet - Page 8

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

www.national.com
Resolution
DNL
DNL
INL
INL
Offset Error
Offset Error
Gain Error
Gain Error
Input Voltage Range
Analog Input Leakage Current
Analog Input Resistance (Note 8)
Analog Input Capacitance
Operating Current on AV
Note 4: Maximum rate of voltage change must be
Note 5: Supply and IDLE currents are measured with CKI driven with a
square wave Oscillator, CKO driven 180˚ out of phase with CKI, inputs
connected to V
Note 8: Resistance between the device input and the internal sample and hold capacitance.
4.0 Functional Description
4.1 General
The LM8300/8400/8500 is a 4-wire resistive touch screen
controller. The primary communication is through the built in
UART operating at a baud rate of 38400. The LM8300/8400/
8500 has the ability to measure pressure on the Z-axis, in
addition to the X-Y coordinates.
The device has the capability to do a 2, 5, and 13 point
calibration. All calibration data is stored in the internal non-
volatile storage element. In addition, all settings pertaining to
the controller are stored internally. This feature negates the
need for external EEPROM.
The device has three built in averaging algorithms: oversam-
pling, delta, and focus. These algorithms help to minimize
noise and A/D variation due to noise. Refer to the averaging
algorithm for a more detail explanation. These algorithms are
implemented on-chip, freeing the main processor from these
tasks. To further minimize noise in extremely noisy environ-
ments, the device has the ability to route the signal from the
touch panel to an external filtering stage before A/D conver-
sions are performed.
To minimize power consumption, the device can be put into
power save mode. The device can be set to go into power
save mode automatically or manually by pulling the external
shutdown pin low.
4.2 Advanced Pin Descriptions
CLK_SET — This pin is the selection pin used to determine
the operating frequency of the controller. On power up, the
controller polls this pin to determine if the operating fre-
quency is set to 10 MHz or 3.3 MHz. If the pin is left floating
or tied high, then the operating frequency is 10 MHz. If the
pin is tied low, then the operating frequency is set to 3.3
MHz.
Note: This is available on the LM8500 only.
Datasheet min/max specification limits are guaranteed by design, test, or statistical analysis.
CC
and outputs driven low but not connected to a load.
Parameter
A/D Converter Electrical Characteristics (0˚C ≤ T
CC
<
0.5 V/ms.
V
V
V
V
V
V
V
V
2.7V ≤ V
AV
CC
CC
CC
CC
CC
CC
CC
CC
CC
= 5V
= 3V
= 5V
= 3V
= 5V
= 3V
= 5V
= 3V
Conditions
= 5.5V
CC
<
5.5V
8
Note 6: The HALT mode will stop CKI from oscillating. Measurement of I
HALT is done with device neither sourcing nor sinking current; all inputs tied
to V
Note 7: Absolute Maximum Ratings should not be exceeded.
SHUTDOWN — This is the external shut down pin. When
pulled low, the controller goes into power saving mode. This
selection pin state has higher priority than the internal power
save mode settings.
WAKE_UP — This pin is used to wake the controller from
power save mode. When the device is in power save mode,
the pin must be tied to one of X-Y lines coming from the
touch screen panel.
X+ — Connect to X+ terminal of the resistive screen.
X- — Connect to X- terminal of the resistive screen.
Y+ — Connect to Y+ terminal of the resistive screen.
Y- — Connect to Y- terminal of the resistive screen.
Filter_out — Analog output to external filter. The use of the
external filter is controlled by sending a command byte of
$A0 on the UART to the controller.
Filter_in — Analog input from external filter. The use of the
external filter is controlled by sending a command byte of
$A0 on the UART to the controller.
LED — Optional LED output. When the controller is oper-
ating in normal (non power save) mode, a low is output to the
pin. When the controller is in power save mode, a high is
output to the pin.
RESET — Reset pin. When pulled low, a manual reset is
executed. For normal operation, this pin must be pulled high.
Under no circumstances should the pin be left floating.
Note: For the LM8400, an external reset must be used as outlined in the
TTL_TX — UART transmit pin. The signal is inverted for use
with standard RS-232 drivers.
TTL_RX — UART receive pin. The signal is inverted for use
with standard RS-232 drivers.
DTR — Data Terminal Ready signal for the UART. If a low
level is detected on the pin, this signals that the UART is not
ready. If a high level is detected on the pin, this signals that
the UART is ready.
A
CC
≤ +70˚C) (Single-ended mode only)
Brownout Reset section.
; A/D converter and clock monitor and BOR disabled.
Min
0
Typ
0.2
Max
±
±
±
±
V
0.5
0.6
10
±
±
±
±
6k
1.5
2.5
1.5
2.5
7
CC
1
1
2
4
Units
LSB
LSB
LSB
LSB
LSB
LSB
LSB
LSB
Bits
mA
µA
pF
V
Ω
DD