0V

FS=Full-ScaleVoltage=V

REF

(1)

1LSB=V

/4096

REF

(1)

FS

1LSB

-

11...111

11...110

11...101

00...010

00...001

00...000

1LSB

InputVoltage

(V)

(2)

TSC2017

SBAS472 – DECEMBER 2009

www.ti.com

Touch Screen Settling

In some applications, external capacitors may be required across the touch screen to filter noise picked up by the

touch screen (that is, noise generated by the LCD panel or backlight circuitry). These capacitors provide a

low-pass filter to reduce the noise, but they also cause a settling time requirement when the panel is touched.

The settling time typically shows up as a gain error. The problem is that the input and/or reference has not

settled to its final steady-state value before the A/D converter samples the input(s) and provides the digital

output. Additionally, the reference voltage may continue to change during the measurement cycle.

To resolve these settling-time problems, the TSC2017 can be commanded to turn on the drivers only without

performing a conversion (see Table 3). Time can then be allowed, before the command is issued, to perform a

conversion. Generally, the time it takes to communicate the conversion command over the I2C bus is adequate

for the touch screen to settle.

Variable Resolution

The TSC2017 provides either 8-bit or 12-bit resolution for the A/D converter. Lower resolution is often practical

for measuring slow changing signals such as touch pressure. Performing the conversions at lower resolution

reduces the amount of time it takes for the A/D converter to complete its conversion process, which also lowers

power consumption.

8-Bit Conversion

The TSC2017 provides an 8-bit conversion mode (M = 1) that can be used when faster throughput is needed,

and the digital result is not as critical (for example, measuring pressure). By switching to the 8-bit mode, a

conversion result can be read by transferring only one data byte. The internal clock runs twice as fast at 4MHz.

The faster clock shortens each conversion by four bits and reduces data transfer time, which results in fewer

clock cycles and provides lower power consumption.

Conversion Clock and Conversion Time

The TSC2017 contains an internal clock, which drives the state machines inside the device that perform the

many functions of the part. This clock is divided down to provide a clock that runs the A/D converter. The

frequency of this clock is 4MHz clock for 8-bit mode, and 2MHz for the 12-bit mode.

Data Format

The TSC2017 output data are in straight binary format as shown in Figure 26. This figure shows the ideal output

code for the given input voltage and does not include the effects of offset, gain, or noise.

(1)

Reference voltage at converter: +REF – (–REF). See Figure 23.

(2)

Input voltage at converter, after multiplexer: +IN – (–IN). See Figure 23.

Figure 26. Ideal Input Voltages and Output Codes

18

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