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TSC2301
SLAS371D – SEPTEMBER 2002 – REVISED AUGUST 2004
Reference
The TSC2301 has an internal voltage reference that can be set to 1.2 V or 2.5 V, through the reference control
register. This reference can also be set to automatically power down between conversions to save power, or
remain on to reduce settling time.
The internal reference voltage is only used in the single-ended mode for battery monitoring, temperature
measurement, and for utilizing the auxiliary inputs. Optimal touch screen performance is achieved when using a
ratiometric conversion, thus all touch screen measurements are done automatically in the differential mode.
An external reference can also be applied to the VREFIN pin, and the internal reference can be turned off.
Variable Resolution
The TSC2301 provides three different resolutions for the ADC: 8, 10, or 12 bits. Lower resolutions are often
practical for measurements such as touch pressure. 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.
Conversion Clock and Conversion Time
The TSC2301 contains an internal 8-MHz clock, which is used to drive the state machines inside the device that
perform the many functions of the part. This clock is divided down to generate the actual ADC conversion clock.
The division ratio for this clock is set in the ADC control register. The ability to change the conversion clock rate
allows the user to choose the optimal value for resolution, speed, and power. If the 8-MHz clock is used directly,
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.
Regardless of the conversion clock speed, the internal clock runs nominally at 8 MHz. The conversion time of the
TSC2301 is dependent upon several functions. While the conversion clock speed plays an important role in the
time it takes for a conversion to complete, a certain number of internal clock cycles is needed for proper
sampling of the signal. Moreover, additional times, such as the panel voltage stabilization time, can add
significantly to the time it takes to perform a conversion. Conversion time can vary depending upon the mode in
which the TSC2301 is used. Throughout this data sheet, internal and conversion clock cycles are used to
describe the times that many functions take. In considering the total system design, these times must be taken
into account by the user.
Touch Detect
The pen interrupt (PENIRQ) output function is detailed in
Figure 49. While in the touch screen monitoring mode,
the Y- driver is ON and connected to GND, the X+ input is connected through a pullup resistor to VDD, and the
PENIRQ output reflects the state of the X+ input. When the panel is touched, the X+ input is pulled to ground
through the touch screen and PENIRQ output goes LOW due to the current path through the panel to GND,
initiating an interrupt to the processor. During the measurement cycles for X- and Y-position, the X+ input is
disconnected from PENIRQ to eliminate any leakage current from the pullup resistor that might flow through the
touch screen, thus causing no errors.
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