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SS7646

8/21/2004 Rev.2.01

Battery Measurement

Another feature of the SS7646 is the battery

measurement ability. Figure 9 shows a block diagram

of a battery voltage monitored by the SS7646.

While the main power to the SS7646 via the

DC/DC regulator is maintained at the desired voltage,

the battery voltage can vary from 0.5V to 6V. The

SS7646 is able to monitor this battery voltage

divided down by 4 so that the battery voltage of 6V

becomes 1.5V to the ADC input. In order to minimize

the power consumption, the divider is only on during

the sampling phase of the ADC. See Table V for the

control bit configurations required to perform this

battery measurement.

Pressure Measurement

The SS7646 is also able to determine the pen or

finger touch through the pressure measurement with

s o m e s i m ple c a l c u l a t i o n s . T he p r e ss u r e

measurement is based on the contact resistance

between the X and Y plates. Figure 10 shows a block

diagram of the pressure measurement. There are two

recommended methods to perform the measurement.

Generally, the pressure measurement can be

accomplished sufficiently with 8-bit resolution mode,

but the following examples are shown with the

12-bit resolution mode.

Y+

Y-

X+

X-

Rtouch

Y+

Y-

X+

X-

Rtouch

Measure

X-Position

Measure

Z1-Position

Y+

Y-

X+

X-

Rtouch

Measure

Z2-Position

V

V

V

Figure 10. Block Diagram of Pressure Measurement

(1) The first method requires the user knowing the

X-plate resistance. Performing three touch screen

conversions, X-Position, Z1-Position and Z2-position,

then use the following equation to calculate the touch

resistance:

(2) The second method requires the user knowing both

the X-plate and Y-plate resistance. Performing three

touch screen conversions, X-Position, Y-Position and

Z1-position, then use the following equation to

calculate the touch resistance:

Pen Interrupt Request

The pen interrupt function is shown in Figure 11.

Normally the /PENIRQ is HIGH by connecting a

has been enabled (See Table V), Y- driver is ON and

connected to GND and /PENIRQ diode is connected

to X+ input. When the touch screen connected to the

SS7646 is touched, the X+ input is pulled to

ground through the touch screen and /PENIRQ will go

low, initiating an interrupt to the microprocessor.

During the X and Y plates measurement cycles, the

/PENIRQ diode will be internally connected to GND

and X+ input disconnected from the diode to eliminate

leakage current to the touch screen.

Y+

Y-

X+

/PENIRQ

ENABLE

100K

/PENIRQ

Figure 11. Block Diagram of Pen Interrupt Circuit

)

−

=

1

2

position

xplate

touch

Z

(Z

x

)

4096

(X

x

R

R

[]

4096

)

x Y

R

(

-

Z

(4096

x

4096

)

X

x

R

(

R

position

yplate

1

position

xplate

touch

1

)

−

=