© 2009 Microchip Technology, Inc.

DS41393A-Page 11

7.9

ESD Considerations

ESD protection is shown on the 4-wire, 5-wire, and 8-wire interface applications schematics.

7.10 Noise Considerations

Touch sensor filtering capacitors are included in the reference design.

Warning: Changing the value of the capacitors may adversely affect performance of the touch system.

7.11 Touch Reporting Protocol

Touch coordinates are sent from the controller to the host system in a 5-byte data packet, which contains the X-

axis coordinate, Y-axis coordinate, and a “Pen-Up/Down” touch status.

The range for X-axis and Y-axis coordinates is from 0-4095 (12-bit). The realized resolution is 1024, as this

product currently defines, in a non-calibrated state, bits X1:X0 and Y1:Y0 as zeros.

It is recommended that applications be developed to read the 12-bit coordinates from the packet and use them in

a 12-bit format. This enhances the application robustness, as it will work with either 10 or 12 bits of coordinate

information.

The touch coordinate reporting protocol is shown below.

Byte #

Bit 7

Bit 6

Bit 5

Bit 4

Bit 3

Bit 2

Bit 1

Bit 0

1

1

R

R

R

R

R

R

P

2

0

X6

X5

X4

X3

X2

X1

X0

3

0

0

0

X11

X10

X9

X8

X7

4

0

Y6

Y5

Y4

Y3

Y2

Y1

Y0

5

0

0

0

Y11

Y10

Y9

Y8

Y7

Table 7 : Touch Coordinate Reporting Protocol

where:

P

: 0 Pen-Up, 1 Pen-Down

R

: Reserved

X11-X0 : X-axis coordinate

Y11-Y0 : Y-axis coordinate

The capacitance of alternate ESD diodes may adversely affect touch performance. A lower

capacitance is better. The PESD5V0S1BA parts shown in the reference design have a typical

capacitance of 35pF. Test to ensure that selected ESD protection does not degrade touch

performance.

ESD protection is shown in the reference design, but acceptable protection is dependent on your

specific application. Ensure your ESD solution meets your design requirements.