Touch Screen Settling

Touch Detect

TSC2008

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.

There are two ways to resolve this issue. Option 1 is to stop or slow down the TSC2008 SCLK for the required

touch screen settling time. This option allows the input and reference to have stable values for the Acquire period

(three clock cycles of the TSC2008; see Figure 30). This option works for both the single-ended and the

differential modes. Option 2 is to operate the TSC2008 in the differential mode only for the touch screen

measurements and command the TSC2008 to remain on (touch screen drivers ON) and not go into power-down

(PD0 = 1). Several conversions are made, depending on the settling time required and the TSC2008 data rate.

Once the required number of conversions have been made, the processor commands the TSC2008 to go into its

power-down state on the last measurement. This process is required for X-Position, Y-Position, and Z-Position

measurements.

value of either 50k

Ω (default) or 90kΩ (which allows the total resistance from X+ to Y– to be as high as 30kΩ).

Write command '1010' (setup command) followed by data '1xx0' sets the pull-up resistor to 90k

Ω. NOTE: The

first three bits must be '0's and the select bit is the last bit. To change the pull-up resistor back to 50k

Ω, issue

write command '1010' followed by data '0xx0'.

An example for the Y-position measurement is detailed in Figure 27. The PENIRQ output is pulled high by an

internal pull-up resistor. While in power-down mode with PD0 = 0, the Y– driver is on and connected to GND,

and the PENIRQ output is connected to the X+ input. When the panel is touched, the X+ input is pulled to ground

through the touch screen, and PENIRQ output goes low because of the current path through the panel to GND,

initiating an interrupt to the processor. During the measurement cycle for X-, Y-, and Z-Position, the X+ input is

disconnected from the PENIRQ pull-down transistor to eliminate any pull-up resistor leakage current from flowing

through the touch screen, thus causing no errors.

If the last command byte written to the TSC2008 contains PD0 = 1, the pen-interrupt output function is disabled

and cannot detect when the panel is touched. In order to re-enable the pen-interrupt output function under these

circumstances, a command byte must be written to the TSC2008 with PD0 = 0.

If the last command byte contains PD0 = 0, then the pen-interrupt function is enabled at the end of a conversion.

The end of conversion (EOC) occurs on the rising edge of PENIRQ.

In both cases previously listed, it is recommended that whenever the host writes to the TSC2008, the master

processor masks the interrupt associated to PENIRQ. This masking prevents false triggering of interrupts when

the PENIRQ line is disabled in the cases previously listed.

Copyright © 2008, Texas Instruments Incorporated

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Product Folder Link(s): TSC2008