Datasheet

Analog Integrated Circuit Device Data
81 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 0