Rabbit OP7100 User Manual download pdf (Page 37)

User’s Manual 31

3.4.4 Touchscreen

The OP7200 analog touchscreen provides a high-resolution matrix of 4096 × 4096

elements. The touchscreen is mounted to the front of and is the same size as the LCD mod-

ule. A four-conductor flex cable connects the touchscreen to the OP7200 at connector J13.

The inputs from the touchscreen are protected from ESD by ferrite beads, capacitors, and

shunt diodes. The ferrite beads and capacitors also serve to eliminate EMI radiating from

the cable. Ferrite beads rather than resistors are used in series with the inputs to maintain

the most accurate measurement of the touchscreen x,y position.

A reference voltage is applied across the touchscreen. When the touchscreen is touched,

resistances that represent the x,y position are presented at the input circuit. The touchscreen

controller chip U9 converts these resistances into digital form for use by the software.

NOTE: Should you touch two or more different points on the touchscreen simultaneously,

the resistance presented to the input circuit will represent some difference between the

resistances corresponding to the points. This can lead to a different or an unknown

key’s value being processed. To prevent this from happening, exercise care to “touch”

only one point or position on the touchscreen at a time.

3.4.5 Analog Supply Voltage

The analog section is isolated from digital noise generated by other components by way of a

low-pass filter composed of L2, C31, and C32 as shown in the left side of Figure 14. The +V

analog power supply powers the A/D converter chip.

Figure 14. Analog Supply and Voltage Reference Circuits

C91

100 nF

R148

100 W

R150

453 W

JP8

ADC Chip

Internal

Reference

Voltage

U12

A/D Converter

Vcc

C31

100 nF

C32

100 nF

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Rabbit OP7100 User Manual Page 37