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21.5.2. Read Sequence (Master)
During a read sequence, an SMBus master reads data from a slave device. The master in this transfer will
be a transmitter during the address byte, and a receiver during all data bytes. The SMBus interface gener-
ates the START condition and transmits the first 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 (READ). Serial data is then
received from the slave on SDA while the SMBus outputs the serial clock. The slave transmits one or more
bytes of serial data.
If hardware ACK generation is disabled, the ACKRQ is set to ‘1’ and an interrupt is generated after each
received byte. Software must write the ACK bit at that time to ACK or NACK the received byte.
With hardware ACK generation enabled, the SMBus hardware will automatically generate the ACK/NACK,
and then post the interrupt.
It is important to note that the appropriate ACK or NACK value should be
set up by the software prior to receiving the byte when hardware ACK generation is enabled.
Writing a ‘1’ to the ACK bit generates an ACK; writing a ‘0’ generates a NACK. Software should write a ‘0’
to the ACK bit for the last data transfer, to transmit a NACK. The interface exits Master Receiver Mode
after the STO bit is set and a STOP is generated. The interface will switch to Master Transmitter Mode if
SMB0DAT is written while an active Master Receiver. Figure 21.6 shows a typical master read sequence.
Two received data bytes are shown, though any number of bytes may be received. Notice that the ‘data
byte transferred’ interrupts occur at different places in the sequence, depending on whether hardware ACK
generation is enabled. The interrupt occurs
before
the ACK with hardware ACK generation disabled, and
after
the ACK when hardware ACK generation is enabled.
Interrupts with Hardware ACK Enabled (EHACK = 1)
S
SLA
R
A
Data Byte
A
Data Byte
N
P
Interrupts with Hardware ACK Disabled (EHACK = 0)
Received by SMBus
Interface
Transmitted by
SMBus Interface
S = START
P = STOP
A = ACK
N = NACK
R = READ
SLA = Slave Address
Figure 21.6. Typical Master Read Sequence
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SILABS [ SILICON LABORATORIES ]
