ucb1100 NXP Semiconductors, ucb1100 Datasheet - Page 17

• Manufacturer Part Number: ucb1100
• Description: Advanced Modem/audio Analog Front-end
• Manufacturer: NXP Semiconductors

Philips Semiconductors
6.4 Touch Screen Interface
The UCB1100 contains a universal resistive touch screen interface
for 4 wire resistive touch screen, capable of performing both
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
UCB1100 is set in the standby 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 tbe floating, powered or grounded in the touch
screen switch matrix. The setting of each touch screen pin is
programmable by the power ts.. and ground ts.. bits in the touch
screen control register. Possible conflicting settings (grounding and
powering of a touch screen pin at the same time) are detected by
the UCB1100. In that case the UCB1100 will ground the touch
screen pin.
The UCB1100’s internal voltage reference (Vref) is as reference
voltage for the touch screen bias circuitry. 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.
1998 May 08
Advanced modem/audio analog front-end
Figure 17. Block Diagram of the Touch Screen Interface
adc_input_sel
ts.. ground
ts.. power
vssa3
tspx
analog mux
tsmx
tspy
tsmy
mux
vssa1
tsc_mod_sel
tsc_mod_sel
to adc input
touch screen
touch screen
current monitor
bias voltage
vdda1
tsc_bias_ena
SN00142
17
In addition to the measurements made 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 UCB1100 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. An 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). which can by used to activate the system around the
UCB1100 to start a touch screen readout 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.
Each of the four touch screen signals can be selected as input for
the built in 10 bit ADC, which is used to determine the voltage on the
selected touch screen pin. The flexible switch matrix and the multi
functional touch screen bias circuit enables the user of the UCB1100
to set each desired touch screen configuration.
The setting of the touch screen bias circuitry and the adc_input
multiplexer is determined by the setting of the tsc_mod_sel bits in
the touch screen control register according the following table.
Figure 18. Touch Screen Setup for Interrupt Detection
tsmy
Ï Ï Ï Ï Ï Ï Ï Ï Ï Ï Ï Ï
Î Î Î Î Î Î Î Î Î Î Î Î
Ï Ï Ï Ï Ï Ï Ï Ï Ï Ï Ï Ï
Î Î Î Î Î Î Î Î Î Î Î Î
Ï Ï Ï Ï Ï Ï Ï Ï Ï Ï Ï Ï
Vdda
Rint
comparator
tsmx
Xplate
tspx
Preliminary specification
comparator
UCB1100
SN00143
tspy