NEC Barracuda Service Manual page 21

Barracuda PDA Maintenance
TOUCH SCREEN INTERFACE
The UCB1300 contains a universal resistive touch screen interface for 4-wire resistive touch screen, capable of
performing position, pressure and plate resistance measurements. In addition, the touch screen can be programmed to
generate interrupts when the touch screen is pressed. The last mode is also active when the UCB1300 is set in the
stand-by mode.
The touch screen interface connects to the touch screen by four wires: TSPX, TSMX, TSPY and TSMY. Each of
these pins can be programmed to be floating, powered or grounded in the touch screen switch matrix. The setting of
each touch screen pin is programmable through the touch screen control register. Possible conflicting settings
(grounding and powering of a touch screen pin at the same time) are detected by the UCB1300. In that case, the touch
screen pin will be grounded.
In position mode, opening the TS..gnd switch can take a long time. To avoid unpredictable delays after changing
the plates configuration, the touch screen interface should be programmed to pressure mode for the duration 1 SIB
frame before resuming a position measurement.
The UCB1300's internal voltage reference (Vref ) is used as reference voltage for the touch screen bias circuit.
This makes the touch screen biasing independent of supply voltage and temperature variations. Four low pass filters,
one on each touch screen terminal, are built in to minimize the noise coupled from the LCD into the touch screen
signals. An LCD typically generates large noise glitches on the touch screen, since they are closely coupled .The
influence of the glitches can nevertheless be minimized by performing measurements when the LCD is quiet. This
can be done by synchronizing the measurement and the video driver with the ADCSYNC pin.
In addition to the measurements mentioned above, the touch screen can also act as an interrupt source. In this
mode the X plate of the touch screen has to be powered and the Y plate has to be grounded. In this case the touch
screen is not biased by the active touch screen bias circuit, but by a resistor to VDDA1 . This configuration simply
biases the touch screen and the UCB1300 does not consume power unless the touch screen is touched. The voltage on
the X plate terminals drops if the screen is pressed. This voltage drop is detected by Schmitt-trigger circuits, of which
the outputs are connected to the interrupt control block. A touch screen interrupt is generated either when the touch
screen is pressed (falling edge enabled) or when the touch screen is released (rising edge enabled). It can be used to
activate the system around the UCB1300 to start a touch screen read-out sequence. The internal Schmitt-trigger
circuits are connected to the TSPX and TSMX signals after the built-in low pass filters. This reduces the number of
spurious interrupts, due to the coupling between the LCD screen and the touch screen sensors.
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