13892 datasheet(107/156 Pages) FREESCALE

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Analog Integrated Circuit Device Data

Freescale Semiconductor

107

13892

FUNCTIONAL DEVICE OPERATION

ADC SUBSYSTEM

Figure 30 shows how the ATO and ATOX settings determine the readout sequence. The ATO should be set long enough so

that the touch screen can be biased properly before conversions start.

Trigger

1/32K

ATO+1

Conversions

0, 1, 2

Conversions

3, 4, 5

Conversions

6, 7

End of Conversi on 2

New Touchscreen

Polarization

End of Conversion 5

New Touchscreen

Polarization

Touchscreen

Polarization

End of Conversion 7

Touchscreen

De-Polarization

Touchscreen Readout for ATOX=0

Trigger

1/32K

ATO+1

Conversion 0

Touchscreen

Polarization

Touchscreen Readout for ATOX=1

Conversion 1

Conversion 2

End of Conversion 2

New Touchscreen

Pol arization

Conversion 3

ATO+1

Etc.

Figure 30. Touch Screen Reading Timing

The main resistive touch screen panel characteristics are listed in Table 5. The switch matrix and readout scheme is designed

such that the on chip switch resistances are of no influence on the overall readout. The readout scheme however does not

account for contact resistances as present in the touch screen connectors. Therefore, the touch screen readings will have to be

calibrated by the user or in the factory where one has to point with a stylus the opposite corners of the screen.

When reading out the X-coordinate, the 10-bit ADC reading represents a 10-bit coordinate with '0' for a coordinate equal to

TSX2, and full scale '1023' when equal to TSX1. When reading out the Y-coordinate, the 10-bit ADC reading represents a 10-bit

coordinate with '0' for a coordinate equal to TSY2, and full scale '1023' when equal to TSY1. When reading the contact resistance

the 10-bit ADC reading represents the voltage drop over the contact resistance created by the known current source multiplied

by 2.

Table 97. Touchscreen Interface Characteristics

Parameter

Condition

Min

Typ

Max

Unit

Interupt Threshold for Pressure Application

kOhm

Interupt Threshold for Pressure Removal

kOhm

Current Source Inaccuracy

Over-temperature

The reference for the touch screen is TSREF and is powered from VCORE. In touch screen operation, TSREF is a dedicated

regulator. No other loads than the touch screen should be connected here. When the ADC performs non touch screen

conversions, the ADC does not rely on TSREF and the reference can be disabled. In applications not supporting touch screen

at all, the TSREF can be used as a low current general purpose regulator, or it can be kept disabled and the bypass capacitor

omitted. The operating mode of TSREF can be controlled with the TSREFEN bit in the same way as some other general purpose

regulators are controlled, see Linear Regulators.

COULOMB COUNTER

As indicated earlier on in this Section, the current into and from the battery can be read out through the general purpose ADC

as a voltage drop over the R1 sense resistor. Together with battery voltage reading, the battery capacity can be estimated. A

more accurate battery capacity estimation can be obtained by using the integrated Coulomb Counter.

The Coulomb Counter (or CC) monitors the current flowing in/out of the battery by integrating the voltage drop across the

battery current sense resistor R1, followed by an A to D conversion. The result of the A to D conversion is used to increase/

decrease the contents of a counter that can be read out by software. This function will require a 10

μF output capacitor to perform

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