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.
Touchscreen Readout for ATOX=0
1/32K
ATO+1
ATO+1
ATO+1
Trigger
Conversions
0, 1, 2
Conversions
3, 4, 5
Conversions
6, 7
Touchscreen
Polarization
End of Conversion 2
New Touchscreen
Polarization
End of Conversion 5
New Touchscreen
Polarization
End of Conversion 7
Touchscreen
De-Polarization
Touchscreen Readout for ATOX=1
1/32K
ATO+1
ATO+1
ATO+1
ATO+1
Etc.
Trigger
Conversion 0
Conversion 1
Conversion 2
Conversion 3
Touchscreen
Polarization
End of Conversion 2
New Touchscreen
Polarization
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
Interupt Threshold for Pressure Application
Interupt Threshold for Pressure Removal
Current Source Inaccuracy
Condition
Min
Typ
Max
Unit
40
60
-
50
80
-
60
95
20
kOhm
kOhm
%
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
13892
Analog Integrated Circuit Device Data
Freescale Semiconductor
107
FREESCALE [ Freescale ]
