LM8400 National Semiconductor, LM8400 Datasheet - Page 12

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

www.national.com
4.0 Functional Description
The default setting is Echo mode disabled.
$A4: Toggle disable/enable pressure measurement
The pressure measurement sets the touch screen to also
sample the Z-axis when reading the X and Y coordinates.
The default setting is pressure measurement enabled.
$A7: Wakeup
There is no reply byte to this command.
When the self-power down mode of the device is enabled,
the touch screen driver must send a wakeup command prior
to any command byte(s). If the self-power down mode of the
TSC is enabled. The wakeup command must also to be sent
if the driver puts the TSC in power-down mode via the
shutdown command.
$A8: Shutdown
When the TSC driver wants the controller to go into power
save mode it sends a shutdown command to the controller.
The driver needs to send a wakeup to the controller before
starting up the communication again.
With the TSC has the self-power-down mode enabled, then
a touchdown on the touch screen will wake-up the TSC from
shutdown mode in addition to sending the wake up com-
mand. If the self-power-down mode is disabled, then only the
wakeup command can wake-up the controller from shut-
down mode (i.e. wake-up on touchdown is disabled).
$AF: Soft reset (restart the controller)
When the PC driver sends the soft reset command, the
touch screen controller executes a soft reset, which clears
and re-initializes all internal RAM configuration registers
from on-chip FLASH and performs a self-check of internal
RAM and program memory.
CONTROLLER REPLIES
$CF: Timeout.
Communication timeout has occurred, and current command
has been aborted.
$CE: Re-send
Request the TSC driver to resend the last command. This
command is used if the controller does not understand the
received command or a buffer overrun condition occurs.
$CC: Self test fail (done at startup, reset, and after
$CB: Self test OK (done at startup, reset, and after
$CA: Acknowledge
$C4: Calibration coordinates OK
This is sent if the coordinates are within the predefined
value.
$C8: Error / TX Buffer overrun
Reply Byte #1: $CA (Acknowledge)
Byte #2: (0 = now disabled, 1 = now enabled)
Reply Byte #1: $CA (Acknowledge)
Reply Byte #1: $CA (Acknowledge)
Byte #2: $CB/$CC (Self test OK/Self test fail)
calibration)
calibration)
(Continued)
12
This is added to the last place in transmit buffer to signal that
a buffer overrun has occurred.
$C7: Software Version number
$80: Format tablet for Z-axis disabled (see Format Table
$90: Format tablet for Z-axis enabled (see Format Table
5.0 Oscillator
OSC_IN is the clock input while OSC_OUT is the clock
generator output to the crystal. Table 2 shows the compo-
nent values required for various standard crystal values.
Figure 2 shows the crystal oscillator connection diagram.
The crystal and other oscillator components should be
placed in close proximity to the OSC_IN and OSC_OUT pins
to minimize printed circuit trace length.
The values for the external capacitors should be chosen to
obtain the manufacturer’s specified load capacitance for the
crystal when combined with the parasitic capacitance of the
trace, socket, and package (which can vary from 0 to 8 pF).
The guideline in choosing these capacitors is:
Manufacturer’s specified load cap = (C
C
C
should be less than or equal to C
parasitic
2
can be trimmed to obtain the desired frequency. C
section for more info.)
Byte #2: Status, low X and low Y
Byte #3: High X
Byte #4: High Y
section for more info.)
Byte #2: Status, low X and low Y
Byte #3: High X
Byte #4: High Y
Byte #5: Z-axis (0-127)
Byte #2: High part of version number (0-127)
Byte #3: Low part of version number (0-127)
TABLE 2. Crystal Oscillator Configuration,
C1 (pF)
FIGURE 2. Crystal Oscillator
18–36
18
T
A
= 25˚C, V
C2 (pF)
18–36
18
CC
20037215
1
.
= 5V
CKI Freq.
(MHz)
1
3.27
10
* C
2
) / (C
1
+ C
2
) +
2