MC34708 datasheet(82/159 Pages) FREESCALE

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Analog Integrated Circuit Device Data



Freescale Semiconductor

MC34708

Functional Block Description

7.7.3.8

Channel 8 Coin Cell Voltage

The voltage of the coin cell connected to the LICELL pin can be read on channel 8. Since the voltage range of the coin cell

exceeds the input voltage range of the ADC, the LICELL voltage is scaled as V(LICELL)*2/3. See .

7.7.3.9

Channel 9-11 ADIN9-ADIN11

There are 3 general purpose analog input channels that can be measured through the ADIN9-ADIN11 pins.

7.7.3.10 Channel 12-15 ADIN12-ADIN15

If the touch screen is not used, the inputs TSX1, TSX2, TSY1, and TSY2 can be used as general purpose inputs. They are

respectively mapped on ADC channels 12, 13, 14, and 15.

7.7.4

Touch Screen Interface

The touch screen interface provides all circuitry required for the readout of a 4-wire resistive touch screen. The touch screen X

plate is connected to TSX1 and TSX2, while the Y plate is connected to TSY1 and TSY2. A local supply TSREF will serve as a

reference. Several readout possibilities are offered.

If the touchscreen is not used, the inputs TSX1, TSX2, TSY1, and TSY2 can be used as general purpose inputs. They are

respectively mapped on ADC channels 12, 13, 14, and 15.

Touch Screen Pen detection bias can be enabled via the TSPENDETEN bit in the AD0 register. When this bit is enabled and a

pen touch is detected, the TSPENDET bit in the Interrupt Status 0 register is set and the INT pin is asserted - unless the interrupt

is masked. Pen detection is only active when TSEN is low.

The reference for the touch screen (Touch Bias) is TSREF and is powered from VCORE. During touch screen operation, TSREF

is a dedicated regulator. No loads other 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 is disabled.

The readouts are designed such that the on chip switch resistances are of no influence on the overall readout. The readout

scheme does not account for contact resistances, as present in the touch screen connectors. The touch screen readings will

have to be calibrated by the user or the factory, where one has to point with a stylus to the opposite corners of the screen. When

reading the X-coordinate, the 10-bit ADC reading represents a 10-bit coordinate, with ‘0’ for a coordinate equal to X-, and full

scale ‘1023’ when equal to X+. When reading the Y-coordinate, the 10-bit ADC reading represents a 10-bit coordinate, with ‘0’

for a coordinate equal to Y-, and full scale ‘1023’ when equal to Y+. When reading contact resistance, the 10-bit ADC reading

represents the voltage drop over the contact resistance created by the known current source, multiplied by 2.

The X-coordinate is determined by applying TSREF over the TSX1 and TSX2 pins, while performing a high-impedance reading

on the Y-plate through TSY1. The Y-coordinate is determined by applying TSREF between TSY1 and TSY2, while reading the

TSX1 pin. The contact resistance is measured by applying a known current into the TSY1 pin of the touch screen and through

the TSX2 pin, which is grounded. The voltage difference between the two remaining terminals TSY2 and TSX1 is measured by

the ADC, and equals the voltage across the contact resistance. Measuring the contact resistance helps determine if the touch

screen is touched with a finger or a stylus.

The TSSELx[1:0] allows the application processor to select its own reading sequence. The TSSELx[1:0] determines what is read

during the touch screen reading sequence, as shown in Table 75. The Touchscreen will always start at TSSEL0 and read up to

and including the channel set by TSSEL at the TSSTOP[2:0] bits. For example when TSSTOP[2:0] = 010, it will request the ADC

to read channels indicated in TSSEL0, TSSEL1, and TSSEL2. When TSSTOP[2:0] = 111, all eight addresses will be read.

Table 74. Coin Cell Voltage Reading Coding

Conversion Code

ADRESULTx[9:0]

Voltage at ADC input (V)

Voltage at LICELL (V)

1 111 111 110

2.400

3.6

1 000 000 000

1.200

1.8

0 000 000 000

0.000

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