LM8400 datasheet(12/20 Pages) NSC

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4.0 Functional Description (Continued)

The default setting is Echo mode disabled.

$A4: Toggle disable/enable pressure measurement

Reply Byte #1: $CA (Acknowledge)

Byte #2: (0 = now disabled, 1 = now enabled)

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

Reply Byte #1: $CA (Acknowledge)

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)

Reply Byte #1: $CA (Acknowledge)

Byte #2: $CB/$CC (Self test OK/Self test fail)

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

calibration)

$CB: Self test OK (done at startup, reset, and after

calibration)

$CA: Acknowledge

$C4: Calibration coordinates OK

This is sent if the coordinates are within the predefined

value.

$C8: Error / TX Buffer overrun

This is added to the last place in transmit buffer to signal that

a buffer overrun has occurred.

$C7: Software Version number

Byte #2: High part of version number (0-127)

Byte #3: Low part of version number (0-127)

$80: Format tablet for Z-axis disabled (see Format Table

section for more info.)

Byte #2: Status, low X and low Y

Byte #3: High X

Byte #4: High Y

$90: Format tablet for Z-axis enabled (see Format Table

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)

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.

TABLE 2. Crystal Oscillator Configuration,

A = 25˚C, VCC =5V

C1 (pF)

C2 (pF)

CKI Freq.

(MHz)

18–36

3.27

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

1 *C2)/(C1 +C2)+

parasitic

2 can be trimmed to obtain the desired frequency. C2

should be less than or equal to C

20037215

FIGURE 2. Crystal Oscillator

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