XPT2046 Ver la hoja de datos (PDF) - Unspecified

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XPT2046

Touch Screen Controller

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XPT2046 Touch Screen Controller

Simplified Diagram of Differential Reference

This situation can be remedied as shown in Figure 8 By setting the SER/DFR bit low, the +REF and –REF inputs

are connected directly to Y+ and Y–, respectively, which makes the analog-to-digital conversion ratiometric. The

result of the conversion is always a percentage of the external resistance, regardless of how it changes in relation

to the on-resistance of the internal switches. Note that there is an important consideration regarding power

dissipation when using the ratiometric mode of operation (see the Power Dissipation section for more details).

As a final note about the differential reference mode, it must be used with +VCC as the source of the +REF

voltage and cannot be used with VREF. It is possible to use a high-precision reference on VREF and single-ended

reference mode for measurements which do not need to be ratiometric. In some cases, it is possible to power the

converter directly from a precision reference. Most references can provide enough power for the XPT2046,but

might not be able to supply enough current for the external load (such as a resistive touch screen).

VCC

YP

XP

IN + REF +

Converter

IN- REF -

YN

GND

Figure 8. Simplified Diagram of Differential Reference (SER/DFR low,

Y switches enabled,X+ is analog input)

Touch Screen Settling

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

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

low-pass filter to reduce the noise, but cause a settling time requirement when the panel is touched that typically

shows up as a gain error. There are several methods for minimizing or eliminating this issue. The problem is that

the input and/or reference has not settled to the final steady-state value prior to the ADC sampling the input(s)and

providing the digital output. Additionally, the reference voltage may still be changing during the measurement

cycle. Option 1 is to stop or slow down the XPT2046 DCLK for the required touch screen settling time. This

allows the input and reference to have stable values for the Acquire period (3 clock cycles of the XPT2046; see

Figure 12). This works for both the single-ended and the differential modes.Option 2 is to operate the XPT2046 in

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