ad7879-1acbz-rl Analog Devices, Inc., ad7879-1acbz-rl Datasheet - Page 17

• Manufacturer Part Number: ad7879-1acbz-rl
• Description: Low Voltage Controller For Touch Screens
• Manufacturer: Analog Devices, Inc.

AUX/VBAT/GPIO PIN
Pin 1A (AUX/VBAT/GPIO) on the AD7879 can be programmed
as either an auxiliary input to the ADC, as a battery monitoring
input, or as a general-purpose digital input/output. To select the
auxiliary measurement, set the ADC channel address to 011.
To select a battery measurement, set the ADC channel address
to 010. To select the GPIO, set Bit 13 in Control Register 2
(Address 0x02) to 1.
AUXILIARY INPUT
The AD7879 has an auxiliary analog input, AUX. When selected,
the signal on the AUX pin (AUX/VBAT/GPIO) is connected
directly to the ADC input. This channel has a full-scale input
range from 0 V to V
011, and the result is stored in Register 0x0C.
BATTERY INPUT
The AD7879 can monitor battery voltages from 0.5 V to 5 V
when the BAT measurement is selected. Figure 28 shows a block
diagram of a battery voltage monitored through the VBAT pin.
The voltage to the V
maintained at the desired supply voltage via the dc-to-dc
regulator while the input to the regulator is monitored. This
voltage on VBAT is divided down by 4 internally, so that a 5 V
battery voltage is presented to the ADC as 1.25 V. To conserve
power, the divider circuit is on only during the sampling of a
voltage on VBAT. Note that the possible maximum input is 5 V.
The VBAT input is ADC Channel 010, and the result is stored
in Register 0x0C.
The maximum battery voltage that the AD7879 can measure
changes when a different reference voltage is used. The maxi-
mum voltage that is measurable is V
gives a full-scale output from the ADC. The battery voltage can
be calculated using the following formula:
V
BAT
0.5V TO 5V
BATTERY
Figure 28. Block Diagram of Battery Measurement Circuit
(V) = [(Register Value) × V
CC
CC
. The ADC channel addresses for AUX is
pin (V
CONVERTER
VBAT
DC-TO-DC
12kΩ
4kΩ
CC
/REF) of the AD7879 is
SW
0.125V TO 1.25V
CC
CC
× 4 because this voltage
× 4]/4095
V
CC
ADC
Rev. 0 | Page 17 of 36
LIMIT COMPARISON
The AUX measurement and the battery measurement can
be compared with high and low limits stored on-chip. An
out-of-limit result generates an alarm output at the INT pin
( PENIRQ / INT / DAV ) provided the INT function is enabled.
The high limit for both channels is stored in Register 0x04,
while the low limit is stored in Register 0x05.
After a measurement from either AUX or VBAT is taken, it
is compared with the high and low limits. The out-of-limit
comparison sets a status bit in Control Register 3. There are
separate status bits for both the high and low limits to indicate
which limit was exceeded. The interrupt sources can be masked
by clearing the corresponding enable bit in this register.
GPIO
The AD7879 has one general-purpose logic input/output pin,
GPIO (AUX/VBAT/GPIO). To enable the GPIO, set Bit 13 in
Control Register 2 to 1. If this bit it 0, then the AUX/VBAT
function is active on the pin. The other GPIO configuration bits
have no effect, if the GPIO is not enabled.
The GPIO data bit is located in Bit 12 of the Control Register 2.
Direction (Bit 11, Control Register2, Address 0x02)
Bit 11 sets the direction of the GPIO pin (AUX/VBAT/GPIO).
When GPIO DIR = 0, the pin is an output. Setting or clearing
bits in the GPIO data bit (Register 0x02[12]) outputs a value on
the GPIO pin.
When GPIO DIR = 1, the pin is an input. An input value on the
GPIO pin sets or clears the GPIO data bit (Register 0x02[12]).
GPIO data register bits are read-only when GPIO DIR = 1.
Polarity (Bit 10, Control Register 2, Address 0x02)
When GPIO POL = 0, the GPIO pin is active low. When GPIO
POL = 1, the GPIO pin is active high. How this bit affects the
GPIO operation also depends on the GPIO DIR bit.
If GPIO POL = 1 and GPIO DIR = 1, a 1 at the input pin sets
the corresponding GPIO data register bit to 1. A 0 at the input
pin clears the corresponding GPIO data bit to 0.
If GPIO POL = 1 and GPIO DIR = 0, a 1 in the GPIO data
register bit puts a 1 on the corresponding GPIO output pin. A 0
in the GPIO data register bit puts a 0 on the GPIO output pin.
If GPIO POL = 0 and GPIO DIR = 1, a 1 at the input pin sets
the corresponding GPIO data bit to 0. A 0 at the input pin clears
the corresponding GPIO data bit to 1.
If GPIO POL = 0 and GPIO DIR = 0, a 1 in the GPIO data
register bit puts a 0 on the corresponding GPIO output pin. A 0
in the GPIO data register bit puts a 1 on the GPIO output pin.
AD7879