LM8400 datasheet(17/20 Pages) NSC

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9.0 Calibration (Continued)

9.1 General Calibration Procedures

Calibration is invoked by sending a command byte of $BD

followed by a command byte to the device. The command

byte is broken into two parts: the high nibble states the

number of calibration points to performs and the lower nibble

states the active calibration point. For example, to do the first

calibration point for the two point calibration, the command

bytes of $BD and $11 are sent to the device. The device then

echos $BD and $11 back to the TS driver and waits for a

touch on the panel. When a touch is detected, the device

checks to see if the point is within the specified parameters

if the calibration point checking is enabled. If the calibration

point checking is disabled, the device will send a $C8 for OK

byte to the TS driver regardless of where the touch was

detected. This calibration point checking can be enabled or

disabled by sending a command byte of $A5 to the device.

When all the calibration points are done, the device will do a

self-test and send a command of $CB for OK or a command

of $CC for failed. If a $CC is received, the TS driver should

issue a warning stating the calibration was not done properly

and redo the calibration procedures. The flowchart for the

calibration procedures is shown in Figure 8.

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FIGURE 8. Calibration Procedures with Coordinates Checking Enabled

LM8400 Datasheet(PDF) 17 Page - National Semiconductor (TI)