
5.7.1.2 Conversion Clock and Conversion Time
TSC2117
Low-Power Audio Codec With Embedded miniDSP, Stereo Class-D
Speaker Amplifier, and Smart Four-Wire Touch-Screen Controller
www.ti.com
SLAS550A – APRIL 2009 – REVISED JUNE 2009
Data Format
The TSC2117 output data is in unsigned binary format and can be read from two 8-bit registers over the
SPI interface.
Voltage Reference
The TSC2117 has an internal voltage reference that can be set to 1.25 V or 2.5 V through the reference
control register (page 3/regiser 6) .
The internal reference voltage should only be used in the single-ended mode for battery monitoring, for
temperature measurement, and for using the auxiliary inputs.
The TSC2117 is designed to allow use with an external voltage reference (page 3/regiser 6). In many
systems, a 2.5-V reference is supplied; however, this device supports a reference voltage up to the AVDD
level. The external reference should be a low-noise signal and accordingly, depending on the application,
it might be good to provide some R-C filtering at the VREF pin.
This voltage reference should only be used in the single-ended mode for measuring the auxiliary inputs
(AUX1, AUX2, and VBAT). Optimal touch-screen performance is achieved when using a ratiometric
conversion; thus, all touch-screen measurements are done automatically in the ratiometric mode.
Variable Resolution
The TSC2117 provides three different resolutions for the ADC: 8, 10, or 12 bits. Lower resolutions are
often practical for measurements such as system voltages. Performing the conversions at lower resolution
reduces the amount of time it takes for the ADC to complete its conversion process, which lowers power
consumption. The ADC resolution can be programmed by writing to page 3/register 2, bits D6–D5.
The TSC2117 contains an internal oscillator, which is used to drive the state machines inside the device
that perform the many functions of the part. MCLK is also available as a high-frequency clock source. The
clock source (internal or MCLK) is selected by writing to page 3/register 16, bit D7. This clock is divided
down to provide a clock to run the SAR ADC. The division ratio for this clock is set by writing to
page 3/register 2, bits D4–D3. The ability to change the conversion clock rate allows the user to choose
the optimal value for resolution, speed, and power. If the internal oscillator is used for the conversion
clock, the ADC is limited to 8-bit resolution; using higher resolutions at this speed does not result in
accurate conversions. Using a 4-MHz conversion clock is suitable for 10-bit resolution; 12-bit resolution
requires that the conversion clock run at 1 or 2 MHz.
To avoid asynchronous issues, the system should use the same value for both page 3/register 16, bit 7
and page 3/register 17, bit 7.
APPLICATION INFORMATION
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