RPM-Based PWM Fan Controller
Datasheet
Chapter 3 Communications
3.1
System Management Bus Interface Protocol
The EMC2301 communicates with a host controller, such as an SMSC SIO, through the SMBus. The
SMBus is a two-wire serial communication protocol between a computer host and its peripheral
devices. A detailed timing diagram is shown in
Stretching of the SMCLK signal is supported;
however, the EMC2301 will not stretch the clock signal.
T
LOW
T
HIGH
T
HD:STA
T
SU:STO
SMCLK
T
HD:STA
T
RISE
T
FALL
T
HD:DAT
T
SU:DAT
T
SU:STA
SMDATA
T
BUF
P
S
S - Start Condition
S
P - Stop Condition P
Figure 3.1 SMBus Timing Diagram
3.1.1
SMBus Start Bit
The SMBus Start bit is defined as a transition of the SMBus Data line from a logic ‘1’ state to a logic
‘0’ state while the SMBus Clock line is in a logic ‘1’ state.
3.1.2
SMBus Address and RD / WR Bit
The SMBus Address Byte consists of the 7-bit client address followed by a RD / WR indicator bit. If
this RD / WR bit is a logic ‘0’, then the SMBus Host is writing data to the client device. If this RD / WR
bit is a logic ‘1’, then the SMBus Host is reading data from the client device.
The SMBus address is set at 0101_111(r/w)b.
3.1.3
SMBus Data Bytes
All SMBus Data bytes are sent most significant bit first and composed of 8-bits of information.
3.1.4
SMBus ACK and NACK Bits
The SMBus client will acknowledge all data bytes that it receives (as well as the client address if it
matches and the ARA address if the ALERT# pin is asserted). This is done by the client device pulling
the SMBus Data line low after the 8th bit of each byte that is transmitted.
The Host will NACK (not acknowledge) the data received from the client by holding the SMBus data
line high after the 8th data bit has been sent.
Revision 1.1 (10-12-09)
12
SMSC EMC2301
DATASHEET
SMSC [ SMSC CORPORATION ]
