LM8300 datasheet(13/20 Pages) NSC

13 / 20 page

6.0 Power Save Mode (Low Power

Stand-by)

The power consumption of the controller can be minimized

by enabling the low power stand by mode. The power save

mode can be controlled internally by sending a command of

$A2 to the controller, externally by pulling the SHUTDOWN

pin low, or by issuing a driver shutdown command of $A8.

The self power down mode is enabled/disabled by sending a

command of $A2 to the controller. When enabled, the con-

troller automatically goes into low power stand by if no more

touch activity is detected or if there is no UART communica-

tion. The controller automatically comes out of low power

stand by mode when a touch is detected on the touch panel

or if an incoming communication on the UART is detected. In

the low power stand by mode, all activity is disabled, includ-

ing the oscillator. Also, the LED pin is driven high. To wake

up the controller on the UART, the wake up command byte

must be sent, followed by a minimum time delay of 1ms for

the LM8500 and 3ms for the LM8300/8400 before sending

any command byte. The delay time is needed to allow the

oscillator to restart and stabilize. Table 3 shows the average

startup time for a given operating frequency.

The SHUTDOWN pin will shut down the controller when

pulled low. When the SHUTDOWN pin is pulled low, the

controller will continue to be in the low power stand by mode

until the pin is pulled high or released. While the SHUT-

DOWN pin is pulled low, all activities are stopped and any

touch or communication will be ignored. Immediately follow-

ing the SHUTDOWN pin being released or pulled high, the

controller clears the UART transmit and receive buffers and

resumes normal operation.

The controller can be put in the low power stand by mode by

sending a $A8 command to it. Upon receiving this command,

the controller goes into low power stand by mode. If the self

power down mode is enabled and the controller is put into

low power stand by mode, the controller will wake up if the

wake up command ($A7) is received or if a touch is detected

on the touch panel. If the self power down mode is disabled

and the controller is put into low power stand by mode, the

controller can only be woken up if it receives the wakeup

command. After the controller wakes up, the UART transmit

and receive buffers are cleared and resume normal opera-

tion.

TABLE 3. Startup Times

CKI Frequency

Startup Time

10 MHz

1–10 ms

3.33 MHz

3–10 ms

7.0 Averaging Algorithm

To achieve better accuracy and noise filtering, each X, Y, and

Z coordinate is oversampled by specific amount. The pos-

sible oversampling settings are 1, 2, 4, 8, 16, and 32. The

factory setting is 8. The greater the oversampling, the

greater the accuracy, but the lower the CPPS.

Samples per coordinate vs. CPPS for LM8300/8400

Samples

per coordinate

Coordinate pairs

per second (CPPS)

250

220

190

150

100

Samples per coordinate vs. CPPS for LM8500

Samples

per coordinate

Coordinate pairs

per second (CPPS)

500

430

360

270

190

110

7.1 Delta Algorithm

The delta filter is used to remove large variations in sampled

values due to noise and glitches. The delta filter tries to

predict where the next coordinate could be. This is done by

taking the two previous coordinates and subtracting one

from the other, producing a delta value. This delta value is

then added or subtracted to the last coordinate. The result-

ing value is the predicted coordinate. If the new sampled

coordinate is close to this predicted value, then the new

value is accepted as valid and is passed to the focus algo-

rithm, provided the focus algorithm is enabled. If the new

sampled coordinate is not close to the predicted coordinate,

then the value is discarded and is stored to be used in the

following delta calculations.

The number by which the new sampled coordinate can differ

from the predicted coordinate is controlled by setting the Set

Max Delta value. The Set Max Delta value can be from a

value of 0-63. The factory default is 8.

7.2 Focus Algorithm

The focus algorithm removes some inaccuracy and noise

that could cause the coordinate to differ only by a couple of

pixels. The focus algorithm is used to eliminate the "jittering"

effect of the pointer when the pointer is stationary.

The focus algorithm compares the value of the previous

stored value to the value passed from the delta algorithm

and determines if the difference is greater than or equal to

the Set Focus Value. If the difference is greater or equal to

the value in Set Focus Value then the new coordinate is sent

out through the UART and stored as the previous value. If

the difference is less than the Set Focus Value then the

value is discarded and the stored valued is set through the

UART.

The amount of difference between the new coordinate and

the old coordinate is set in the Set Focus Value. This value

can be from 0-63 with a value of 0 disabling the focus

algorithm. The factory default is 4.

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