CP3BT23_14 Datasheet, PDF (113/324 Pages) Texas Instruments – CP3BT23 Reprogrammable Connectivity Processor with Bluetooth and Dual CAN Interfaces

CP3BT23
www.ti.com
SNOSCX3A – JULY 2013 – REVISED JANUARY 2014
Similarly, in Sample Y mode the value measured at node B on the TSX+ or TSXinputs is proportional to
the ratio between the resistance to ground and the resistance RYP of the Y plate:
B / 2047 = RY2 / RYP
(5)
Because end-to-end resistance RYP of the Y plate is:
RYP = RY1 + RY2
(6)
The previous equation can be rewritten as:
B / 2047 = (RYP - RY1) / RYP
(7)
Solving for RY1, the resistance is:
RY1 = RYP x (1 - (B / 2047))
(8)
Now that the resistance values RX2 and RY1 are known, it is possible to calculate the value of the plate-
to-plate contact resistance, RZ, given the value measured at node C on the TSX+ input in Sample Z
mode. Node C is a tap in a resistor-divider network composed of three resistors, such that:
C / 2047 = RX2 / (RY1 + RZ + RX2)
(9)
Solving for RZ, the resistance is:
RZ = RX2 x [(2047 - C) / (C - RY1)]
(10)
The resistance RZ is proportional to the force of pen contact.
16.2.3 Compensation for Driver Resistance
Plate resistances between opposite electrodes range from 100 ohms to 1k ohm. Because of the 6-ohm
driver resistance, some significant voltage drop will be experienced between, for example, TSXand AGND.
A 200-ohm plate will drop:
(6/(200 + 6 + 6)) / (AVCC - AGND)
(11)
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 x 2) / 610 µV = 230
(12)
This effective loss of resolution can be handled in a number of ways.
1. The voltages on, for example, TSY+ and TSYcan be sampled before sampling TSX+ and TSX-. Then,
scale to the full (4096-bit) range. This technique will not re-cover any resolution, however it is worthy of
some consideration 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 increased to:
4096 - (70 mV / 610 µV) = 3981
(13)
Software scaling could be applied to this value if required (as with technique 1, above), but no
additional resolution is achieved.
3. By extension, the ADC negative voltage reference can be internally connected to the TSYterminal, to
recover the full 4096 values.
The Global Configuration Register (ADCGCR) provides the flexibility to implement any of these
techniques.
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12-Bit Analog to Digital Converter 113