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| 型号： | LM12438CIWMX |
| 厂商： | NATIONAL SEMICONDUCTOR CORP |
| 元件分类： | 模拟信号调理 |
| 英文描述： | SPECIALTY ANALOG CIRCUIT, PDSO28 |
| 封装： | SOP-28 |
| 文件页数： | 68/80页 |
| 文件大小： | 1552K |
| 代理商： | LM12438CIWMX |
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70 Digital Interface (Continued)
74 I2C BUS INTERFACE
The I2C bus is a serial synchronous bus structure It is a
multi-master bus which means that more than one device
capable of controlling the bus can be connected to it The
bus uses 2 wires serial data (SDA) and serial clock (SCL)
to carry information between the devices connected to the
bus Both data and clock lines are bidirectional and are con-
nected to the positive power supply via a pull-up resistor
Each device is identified by a unique address whether it is a
microprocessorcontroller or a peripheral such as memory
keyboard data-converter or display Each device can oper-
ate as either transmitter or receiver depending on the func-
tion of the device In addition to transmitters and receivers
devices can also be considered as masters and slaves
when performing data transfer A master is the device that
initiates a data transfer on the bus and generates the clock
signals to permit that transfer At that time any device ad-
dressed is considered slave It should be apparent that the
I2C bus is not merely an interconnecting wire it embodies
comprehensive formats and procedures for addressing
transfer cycles start and stop clock generationsynchroni-
zation and bus arbitration The following discussion as-
sumes that the reader is familiar with the specification and
architecture of the I2C bus
The LM12434 and LM12 L 438’s I2C bus interface is se-
lected when the MODESEL1 and MODESEL2 pins have the
logic state of ‘‘10’’
Figure 18 shows a typical connection
diagram for the LM12434 and LM12 L 438 to the I2C bus
As was mentioned communication on the I2C bus is per-
formed on 2 lines SCL (serial clock) and SDA (serial data)
pins P5 and P4 are assigned to these lines The DAS oper-
ates as a slave on the I2C bus As a result the SCL line is
an input (no clock is generated by the LM12434 and
LM12 L 438) and the SDA line is a bi-directional serial data
path According to I2C bus specifications the DAS has a
7-bit slave address The four most significant bits of the
slave address are hard wired inside the LM12434 and
LM12 L 438 and are ‘‘0101’’ The three least significant
bits of the address are assigned to pins P3 – P1 Therefore
the LM12434 and LM12 L 438 I2C slave address is
0
1
0
1
P3
P2
P1
MSB
LSB
Tying the P3 – P1 pins to different logic levels allows up to
eight LM12434 and LM12 L 438’s to be addressed on a
single I2C bus
Figure 17 shows the timing diagram for the read and write
cycles for the LM12434 and LM12 L 438’s I2C interface
This timing diagram depicts the general relationship be-
tween the serial clock edges and the data bits It is not
meant to show guaranteed timing performance (See speci-
fication tables for parametric switching characteristics) The
DAS’s I2C interface timing parameters fully meet or exceed
the I2C bus specification Data transfer on the I2C bus is
byte oriented and the 16-bit data to be written to or read
from each register is transferred in two bytes
Write cycle
A write cycle is illustrated in
Figure 17a Com-
munication is initiated with a start condition generated by a
master (I2C bus specification) followed by a byte of the
DAS’s slave address with the readwrite bit (8th bit) being
‘‘0’’ indicating a write cycle will follow At the 9th SCL clock
pulse of the first data packet the DAS pulls the SDA line
low (‘‘0’’) to acknowledge that it has been addressed The
next byte is the address of the DAS register to be accessed
The format of this byte is three ‘‘0’s’’ (MSBs) followed by
four bits of register address (MSB first as shown) and a ‘‘0’’
as the last bit (LSB) After the DAS acknowledges the ad-
dress byte the 16-bit data proceeds in two bytes beginning
with the high order byte (MSB first) The direction of the
data in a write cycle is from master to DAS with acknowl-
edgement given by the DAS at the end of each byte The
cycle is completed by a stop condition generated by the
master
Readburst read cycle
The read and burst read cycles for
the I2C interface are combined in a single format A read
cycle is shown in
Figure 17b A read cycle starts the same
as a write with a slave address byte for write followed by a
register address byte After the register address byte is writ-
ten to the DAS the bus should be released without any stop
condition The master then applies a repeat start condition
followed by the DAS’s slave address but with the read
write bit being ‘‘1’’ indicating a read request from the mas-
ter The DAS (slave) acknowledges its address and begin-
ning with the next byte the direction of the data will be from
DAS to master The DAS starts to transmit the contents of
its register (addressed previously at second byte of the cy-
cle) synchronized with the clocks applied by the master An
even number of data bytes should be read from the DAS
(two bytes per register) At the end of each byte received
from the DAS the bus master generates an acknowledge
The DAS continues to repeat transmitting its register con-
tents as long as the master is transmitting clocks and ac-
knowledges at the end of each byte The DAS recognizes
the end of the transfer whenever the master does not ac-
knowledge at the end of an even numbered byte At this
point the master should generate a stop condition as re-
quired by the I2C bus specification Notice that the master
may read only one word (single read) or as many words (two
bytes each) as it needs using the read procedure
70
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