TSC2008

The control byte begins with a start bit followed by seven control bits. For the command to be valid, the start bit

must be '1'. Do not use '0' for the start bit; it is reserved for factory use.

Initiate Start—The first bit is the start bit (S), and must always be high to initiate the start of a user-controllable

control byte. When the start bit = '0', it is reserved for factory use.

Addressing and Command Decoding—The next three bits in the control byte following the start bit are three

addressing bits A[2:0] used to select the active input channel(s) of the input multiplexer (see Table 4 and

Figure 24), enable the touch screen drivers, select the reference inputs, or decode other commands.

Bit[6:4] = '010' is the setup command that is used to configure the TSC.

Bit[3:0] followed by the setup command are the configuration bits and are used to select the pull-up resistor

value, bypass the noise filter (in the preprocessing unit), select the SDO output timing, and perform the software

reset. Bit[3:1] are static—that is, they do not change once programmed unless either the device is powered off,

one of the reset conditions occur (power-on reset, software reset, or SureSet), or unless changed with the setup

command. Note that if any reset occurs, bit[3:1] is set to the default values listed in Table 3. Any function

decoded as shown in Table 4 (excluding the setup command) has no access to these four configuration bits.

Table 4. Converter Function Select (CFS) Information

INPUT TO

A[2:0]

SER/DFR

+REF

–REF = –IN

ADC = +IN

X-DRIVERS

Y-DRIVERS

DESCRIPTION

0h

Don't care

VDD

GND

TEMP1

All OFF

All OFF

Measure TEMP1

1h

1 (single-ended)

VDD

GND

X+

All OFF

All ON

Measure Y position

1h

0 (differential mode)

Y+

Y–

X+

All OFF

All ON

Measure Y position

Used as noise filter

2h

—

—

—

All OFF

All OFF

bypass

3h

1 (single-ended)

VDD

GND

X+

X– ON

Y+ ON

Measure Z1 position

3h

0 (differential mode)

Y+

X–

X+

X– ON

Y+ ON

Measure Z1 position

4h

1 (single-ended)

VDD

GND

Y–

X– ON

Y+ ON

Measure Z2 position

4h

0 (differential mode)

Y+

X–

Y–

X– ON

Y+ ON

Measure Z2 position

5h

1 (single-ended)

VDD

GND

Y+

All ON

All OFF

Measure X position

5h

0 (differential mode)

X+

X–

Y+

All ON

All OFF

Measure X position

6h

Don't care

VDD

GND

AUX

All OFF

All OFF

Measure AUX

7h

Don't care

VDD

GND

TEMP2

All OFF

All OFF

Measure TEMP2

(1)

Bit 2 is the SER/DFR control bit for all commands except for the setup command.

(2)

Use the setup command to configure the touch screen controller or access the software reset function.

MODE—The mode bit sets the resolution of the A/D converter. With this bit low, the next conversion has 12 bits

of resolution; with this bit high, the next conversion has eight bits of resolution.

SER/DFR—The SER/DFR bit controls the reference mode: either single-ended (high) or differential (low). The

differential mode is also referred to as the ratiometric conversion mode and is preferred for X-Position,

Y-Position, and Pressure-Touch measurements for optimum performance. The reference is derived from the

voltage at the switch drivers, which is almost the same as the voltage to the touch screen. In this case, a

reference voltage is not needed because the reference voltage to the A/D converter is the same as the voltage

across the touch screen. In single-ended mode, the converter reference voltage is always the difference between

the VREF and GND pins (see Table 4 and Figure 24 through Figure 26, for further information).

If X-Position, Y-Position, and Pressure-Touch are measured in the single-ended mode, then VDD is used as the

reference.

NOTE: The differential mode can only be used for X-Position, Y-Position, and Pressure-Touch measurements.

All other measurements require the single-ended mode.

Copyright © 2008, Texas Instruments Incorporated

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