Data Sheet
AD5263
Rev. F | Page 19 of 28
Daisy-Chain Operation
The serial data output (SDO) pin contains an open-drain
N-channel FET. This output requires a pull-up resistor in order
to transfer data to the SDI pin of the next package. This allows
for daisy-chaining several RDACs from a single processor serial
data line. The pull-up resistor termination voltage can be greater
than the VDD supply voltage. It is recommended to increase the
clock period when using a pull-up resistor to the SDI pin of the
following device because capacitive loading at the daisy-chain node
(SDO to SDI) between devices may induce time delay to subsequent
devices. Users should be aware of this potential problem to achieve
daisy-chained, a total of 20 bits of data is required. The first 10 bits,
section and bit map, go to U2 and the second 10 bits, with the
same format, go to U1. CS should be kept low until all 20 bits are
clocked into their respective serial registers. After this, CS is pulled
high to complete the operation and load the RDAC latch. Data
appears on SDO on the negative edge of the clock, thus making
it available to the input of the daisy-chained device on the rising
edge of the next clock.
03142-
046
AD5263
U2
SPI
U1
SDI
CLK
SDO
CLK
SDI
SDO
MOSI
VL
RP
2.2k
CS
CLK CS
CS
Figure 47. Daisy-Chain Configuration
I2C-COMPATIBLE 2-WIRE SERIAL BUS (DIS = 1)
In the I2C-compatible mode, the RDACs are connected to the
bus as slave devices.
slave address byte, consisting of a 7-bit slave address and a R/W
bit. The five MSBs are 01011 and the following two bits are
determined by the state of the AD0 and AD1 pins of the device.
AD0 and AD1 allow the user to place up to four of the I2C-
compatible devices on one bus.
The 2-wire I2C serial bus protocol operates as follows.
1. The master initiates a data transfer by establishing a
START condition, which is when a high-to-low transition
on the SDA line occurs while SCL is high (see
Figure 43).The following byte is the slave address byte, which consists
of the 7-bit slave address followed by an R/W bit. This R/W
bit determines whether data will be read from or written to
the slave device.
The slave whose address corresponds to the transmitted
address responds by pulling the SDA line low during the
ninth clock pulse (this is termed the acknowledge bit). At
this stage, all other devices on the bus remain idle while the
selected device waits for data to be written to or read from
its serial register. If the R/W bit is high, the master reads
from the slave device. If the R/W bit is low, the master
writes to the slave device.
2. In write mode, the second byte is the instruction byte. The
first bit (MSB) of the instruction byte is a don’t care. The
following two bits, labeled A1 and A0, are the RDAC
subaddress select bits.
The fourth MSB (RS) is the midscale reset. A logic high on
this bit moves the wiper of the selected channel to the center
tap where RWA = RWB. This feature effectively writes over
the contents of the register, so that when taken out of reset
mode, the RDAC remains at midscale.
The fifth MSB (SD) is the shutdown bit. A logic high causes
the selected channel to open circuit at Terminal A while
shorting the wiper to Terminal B. This operation yields
almost 0 in rheostat mode or 0 V in potentiometer
mode. This SD bit serves the same function as the SHDN
pin except that the SHDN pin reacts to active low. In
addition, the SHDN pin affects all channels, as opposed to
the SD bit, which affects only the channel being written to.
It is important to note that the shutdown operation does
not disturb the contents of the register. When brought out
of shutdown, the previous setting is applied to the RDAC.
The next two bits are O2 and O1. They are extra programmable
logic outputs that can be used to drive other digital loads,
logic gates, LED drivers, analog switches, etc.
The LSB is a don’t care bit (see the bit map in t
he I2C WriteAfter acknowledging the instruction byte, the last byte in write
mode is the data byte. Data is transmitted over the serial bus
in sequences of nine clock pulses (eight data bits followed by
an acknowledge bit). The transitions on the SDA line must
occur during the low period of SCL and remain stable