National CP3BT26 manual Measuring Pen Force, RX2, RY2, Ryp, Compensation for Driver Resistance

CP3BT26

16.2.2Measuring Pen Force

Figure 27 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 16.2.3), and series resistance between the node of interest and the ADC is ig- nored because it has no significant effect.

		VCC
VCC	VCC	RY1
RX1	RY1	RZ
A	B	C
RX2	RY2	RX2
Sample X	Sample Y	Sample Z
TOUCH_CFG = 001	TOUCH_CFG = 010	TOUCH_CFG = 100
		DS188

Figure 27. 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:

RXP = RX1 + RX2

The value measured at node A is proportional to the ratio between the resistance to ground and the resistance of the X plate:

A RX2

------------ = ------------

2047 RXP

Solving for RX2, the resistance is:

RX2 = RXP ⋅		A 
		2047------------

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:

B RY2

------------ = ------------

2047 RYP

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

RYP = RY1 + RY2

The previous equation can be rewritten as:

------------B	=	RYP------------------------------–RY1
2047		RYP

Solving for RY1, the resistance is:

RY1 = RYP ⋅		1		B 
			– 	2047------------

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:

C	=	RX2
2047------------		RY1---------------------------------------------+ RZ + RX2

Solving for RZ, the resistance is:

		2047 – C
RZ = RX2	⋅ 	-----------------------C	– RY1

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

16.2.3Compensation 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:

	6		⋅ (AVCC – AGND)
 200-----------------------------+ 6 + 6

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:

70 mV ⋅ 2

-------------------------- = 230

610 uV

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:

 70 mV 

4096 –  --------------------= 3981

610 uV

Software scaling could be applied to this value if re- quired (as with technique 1, above), but no additional resolution is achieved.

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