CP3UB26 datasheet(74/268 Pages) NSC

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15.2.2

Measuring Pen Force

Figure 14 shows equivalent circuits for the driver modes

used to measure the X, Y, and Z coordinates, in which Z rep-

resents pen force. In this discussion, the ohmic resistance

of the drivers is neglected (see Section 15.2.3), and series

resistance between the node of interest and the ADC is ig-

nored because it has no significant effect.

Figure 14.

Touchscreen Driver Modes

In the following examples, the ADC is assumed to operate

in single-ended mode to produce conversion values be-

tween 0 and 2047, however the same principles could be

extended to differential mode to recover the full range of the

ADC.

In Sample X mode, the X plate is driven between VCC and

ground, so that a value measured at node A on the TSY+ or

TSY- inputs is the center tap of a resistor-divider network.

The end-to-end resistance RXP of the X plate is:

The value measured at node A is proportional to the ratio

between the resistance to ground and the resistance of the

X plate:

Solving for RX2, the resistance is:

Similarly, in Sample Y mode the value measured at node B

on the TSX+ or TSX- inputs is proportional to the ratio be-

tween the resistance to ground and the resistance RYP of

the Y plate:

Because end-to-end resistance RYP of the Y plate is:

The previous equation can be rewritten as:

Solving for RY1, the resistance is:

Now that the resistance values RX2 and RY1 are known, it

is possible to calculate the value of the plate-to-plate con-

tact resistance, RZ, given the value measured at node C on

the TSX+ input in Sample Z mode. Node C is a tap in a re-

sistor-divider network composed of three resistors, such

that:

Solving for RZ, the resistance is:

The resistance RZ is proportional to the force of pen con-

tact.

15.2.3

Compensation for Driver Resistance

Plate resistances between opposite electrodes range from

100 ohms to 1k ohm. Because of the 6-ohm driver resis-

tance, some significant voltage drop will be experienced be-

tween, for example, TSX- and AGND. A 200-ohm plate will

drop:

With a 2.5V supply, this is 70 mV. A 12-bit ADC has 4096

possible values, so each value covers a range of 610 µV at

2.5V. A voltage drop of 70 mV across each of the low-ohmic

drivers reduces the number of available ADC values by:

This effective loss of resolution can be handled in a number

of ways.

1. The voltages on, for example, TSY+ and TSY- can be

sampled before sampling TSX+ and TSX-. Then, scal-

ing can be applied in software to convert the samples

to the full (4096-bit) range. This technique will not re-

cover any resolution, however it is worthy of some con-

sideration because touchscreen data is typically

passed to two applications:

Signature Analysis—only the raw data is required. No

absolute positioning is necessary.

Screen Overlay—for example, for cursor positioning.

In this application, a scaling or calibration is performed

to correctly overlay the touchscreen coordinates onto

the display. Because of this calibration, it is not even

necessary to sample TSY+ and TSY-.

2. The ADC has a positive voltage reference input which

can be internally connected to the TSY+ terminal. This

means that the number of available ADC values is in-

creased to:

Software scaling could be applied to this value if re-

quired (as with technique 1, above), but no additional

resolution is achieved.

DS188

VCC

RX1

RX2

Sample X

TOUCH_CFG = 001

VCC

RY1

RY2

Sample Y

TOUCH_CFG = 010

RX2

VCC

RY1

Sample Z

TOUCH_CFG = 100

RXP

RX1

RX2

2047

-------------

RX2

RXP

-------------

RX2

RXP

2047

-------------





2047

-------------

RY2

RYP

-------------

RYP

RY1

RY2

2047

-------------

RYP

RY1

–

RYP

-------------------------------

RY1

RYP

2047

-------------





–





2047

-------------

RX2

RY1

RX2

----------------------------------------------

RX2

2047

–

-----------------------









RY1

–

200

-----------------------------





AVCC

AGND

–

()

70 mV

610 uV

--------------------------

230

4096

70 mV

610 uV

--------------------





–

3981

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