
MOTOROLA
Chapter 32. I2C Controller
32-5
Part V. The Communications Processor Module
Figure 32-5. I
2
C Master Read Timing
A master read performed by the MPC860 occurs as follows:
1. Set the masterÕs I2COM[STR] to initiate the read. The transfer starts when the
SDMA channel loads the transmit FIFO with data and the I
2
C bus is not busy.
2. The slave detects a start condition on SDA and SCL.
3. After the Þrst byte is shifted in, the slave compares the received data to its slave
address. If the slave is an MPC860, the address is programmed in its I
2
C address
register (I2ADD).
Ñ If a match is found and the slave is ready, then the slave acknowledges the request
and begins sending on the clock pulse after the acknowledge. If the slave is an
MPC860, it is ready when its transmit FIFO has been loaded by the SDMA
channel (the transmit buffers and BDs have been prepared and I2COM[STR] has
been set).
Ñ If a match is found but the slave is not ready, the read request is not
acknowledged and the transaction is aborted. If the slave is an MPC860, a
maskable transmission error interrupt is triggered to allow software to prepare
data for transmission on the next try.
Ñ If a mismatch occurs, the slave ignores the message and searches for a new start
condition.
4. The master acknowledges each byte sent as long as an overrun does not occur. If the
master receiver fails to acknowledge a byte, the slave aborts transmission. For a
slave MPC860, the abort generates a maskable interrupt. A maskable interrupt is
also issued after a complete buffer is sent or after an error. If an underrun occurs, the
MPC860 slave sends ones until a stop condition is detected.
32.3.4 I
2
C Multi-Master Considerations
The I
2
C controller supports a multi-master conÞguration, in which the I
2
C controller must
alternate between master and slave modes. The I
2
C controller supports this by
implementing I
2
C master arbitration in hardware. However, due to the nature of the I
2
C bus
and the implementation of the I
2
C controller, certain software considerations must be made.
SDA
DATA BYTE
DEVICE ADDRESSR
S
T
O
P
S
T
A
R
T
N
O
A
C
K
A
C
K
NOTE: AFTER THE
N
TH DATA BYTE, THE MASTER DOES NOT ACKNOWLEDGE THE
Содержание MPC860 PowerQUICC
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