Freescale Semiconductor PowerQUICC MPC885 Reference Manual page 885

because the R/W request follows the slave port address in the I
must be placed in the lsb (bit 7) unless operating in reverse data mode; see
Register (I2MOD)."
To write to a slave, the master sends a write request (R/W = 0) along with either the target slave's address
or the general call (broadcast) address of all zeros, followed by the data to be written. To read from a slave,
the master sends a read request (R/W = 1) and the target slave's address. When the target slave
acknowledges the read request, the transfer direction is reversed, and the master receives the slave's
transmit buffers. If the receiver (master or slave) does not acknowledge each byte transfer in the ninth bit
frame, the transmitter signals a transmission error event (I2ER[TXE]). An I
shown in Figure 32-3.
SCL
SDA
Select master or slave mode for the controller using the I
master's start bit, I2COM[STR], to begin a transfer; setting a slave's I2COM[STR] activates the slave to
wait for a transfer request from a master.
If a master or slave transmitter's current TxBD[L] is set, transmission stops once the buffer is sent; that is,
I2COM[STR] must be set again to reactivate transfers. If TxBD[L] is zero, once the current buffer is sent,
the controller begins processing the next TxBD without waiting for I2COM[STR] to be set again.
The following sections further detail the transfer process.
2
32.3.1 I C Master Write (Slave Read)
If the MPC885 is the master, prepare the transmit buffers and BDs before initiating a write. Initialize the
first transmit data byte with the slave address and write request (R/W = 0).
If the MPC885 is the slave target of the write, prepare receive buffers and BDs to await the master's
request. Figure 32-4 shows the timing for a master write.
SDA
Note: Data and ACK are repeated n times.
Freescale Semiconductor
Start Condition
1 2 3 4 5 6
Data Byte
2
Figure 32-3. I C Transfer Timing
S
T
A
R
A
T
C
W K
Device Address
2
Figure 32-4. I C Master Write Timing
MPC885 PowerQUICC Family Reference Manual, Rev. 2
2
C bus specification, the R/W request bit
Section 32.4.1, "I2C Mode
2
C transfer timing diagram is
Stop Condition
7 8 9
A
C
K
2
C command register (I2COM[M/S]). Set the
S
T
O
A
P
C
K
Data Byte
2
I C Controller
32-3