C8051F120/1/2/3/4/5/6/7
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Rev. 1.2
20.3. SMBus Transfer Modes
The SMBus0 interface may be configured to operate as a master and/or a slave. At any particular time, the interface
will be operating in one of the following modes: Master Transmitter, Master Receiver, Slave Transmitter, or Slave
Receiver. See Table 20.1 for transfer mode status decoding using the SMB0STA status register. The following mode
descriptions illustrate an interrupt-driven SMBus0 application; SMBus0 may alternatively be operated in polled
mode.
20.3.1. Master Transmitter Mode
Serial data is transmitted on SDA while the serial clock is output on SCL. SMBus0 generates a START condition and
then transmits the first byte containing the address of the target slave device and the data direction bit. In this case the
data direction bit (R/W) will be logic 0 to indicate a "WRITE" operation. The SMBus0 interface transmits one or
more bytes of serial data, waiting for an acknowledge (ACK) from the slave after each byte. To indicate the end of the
serial transfer, SMBus0 generates a STOP condition.
20.3.2. Master Receiver Mode
Serial data is received on SDA while the serial clock is output on SCL. The SMBus0 interface generates a START
followed by the first data byte containing the address of the target slave and the data direction bit. In this case the data
direction bit (R/W) will be logic 1 to indicate a "READ" operation. The SMBus0 interface receives serial data from
the slave and generates the clock on SCL. After each byte is received, SMBus0 generates an ACK or NACK depend-
ing on the state of the AA bit in register SMB0CN. SMBus0 generates a STOP condition to indicate the end of the
serial transfer.
A
A
A
S
W
P
Data Byte
Data Byte
SLA
S = START
P = STOP
A = ACK
W = WRITE
SLA = Slave Address
Received by SMBus
Interface
Transmitted by
SMBus Interface
Interrupt
Interrupt
Interrupt
Interrupt
Figure 20.4. Typical Master Transmitter Sequence
Data Byte
Data Byte
A
N
A
S
R
P
SLA
S = START
P = STOP
A = ACK
N = NACK
R = READ
SLA = Slave Address
Received by SMBus
Interface
Transmitted by
SMBus Interface
Interrupt
Interrupt
Interrupt
Interrupt
Figure 20.5. Typical Master Receiver Sequence
Summary of Contents for C8051F120
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