SC16IS752_SC16IS762 All information provided in this document is subject to legal disclaimers. © NXP B.V. 2012. All rights reserved.
Product data sheet Rev. 9 — 22 March 2012 37 of 60
NXP Semiconductors
SC16IS752; SC16IS762
Dual UART with I
2
C-bus/SPI interface, 64-byte FIFOs, IrDA SIR
10.2 Addressing and transfer formats
Each device on the bus has its own unique address. Before any data is transmitted on the
bus, the master transmits on the bus the address of the slave to be accessed for this
transaction. A well-behaved slave with a matching address, if it exists on the network,
should of course acknowledge the master's addressing. The addressing is done by the
first byte transmitted by the master after the START condition.
An address on the network is seven bits long, appearing as the most significant bits of the
address byte. The last bit is a direction (R/W
) bit. A zero indicates that the master is
transmitting (‘write’) and a one indicates that the master requests data (‘read’). A complete
data transfer, comprised of an address byte indicating a ‘write’ and two data bytes is
shown in Figure 16
.
When an address is sent, each device in the system compares the first seven bits after
the START with its own address. If there is a match, the device will consider itself
addressed by the master, and will send an acknowledge. The device could also determine
if in this transaction it is assigned the role of a slave receiver or slave transmitter,
depending on the R/W
bit.
Each node of the I
2
C-bus network has a unique seven-bit address. The address of a
microcontroller is of course fully programmable, while peripheral devices usually have
fixed and programmable address portions.
When the master is communicating with one device only, data transfers follow the format
of Figure 16
, where the R/W bit could indicate either direction. After completing the
transfer and issuing a STOP condition, if a master would like to address some other
device on the network, it could start another transaction by issuing a new START.
Another way for a master to communicate with several different devices would be by using
a ‘Repeated START’. After the last byte of the transaction was transferred, including its
acknowledge (or negative acknowledge), the master issues another START, followed by
address byte and data without effecting a STOP. The master may communicate with a
number of different devices, combining ‘reads’ and ‘writes’. After the last transfer takes
place, the master issues a STOP and releases the bus. Possible data formats are
demonstrated in Figure 17
. Note that the repeated START allows for both change of a
slave and a change of direction, without releasing the bus. We shall see later on that the
change of direction feature can come in handy even when dealing with a single device.
Fig 16. A complete data transfer
S P
SDA
SCL
0 to 6
78
ACK
002aab046
START
condition
STOP
condition
address R/W
0 to 6
78
data
ACK
0 to 6
78
data ACK
