External Bus Interface
MPC561/MPC563 Reference Manual, Rev. 1.2
9-8
Freescale Semiconductor
9.5
Bus Operations
This section provides a functional description of the system bus, the signals that control it, and the bus
cycles provided for data transfer operations. It also describes the error conditions, bus arbitration, and reset
operation.
The MPC561/MPC563 generates a system clock output (CLKOUT). This output sets the frequency of
operation for the bus interface directly. Internally, the MPC561/MPC563 uses a phase-lock loop (PLL)
circuit to generate a master clock for all of the MPC561/MPC563 circuitry (including the bus interface)
which is phase-locked to the CLKOUT output signal.
All signals for the MPC561/MPC563 bus interface are specified with respect to the rising edge of the
external CLKOUT and are guaranteed to be sampled as inputs or changed as outputs with respect to that
edge. Since the same clock edge is referenced for driving or sampling the bus signals, the possibility of
clock skew could exist between various modules in a system due to routing or the use of multiple clock
lines. It is the responsibility of the system to handle any such clock skew problems that could occur.
9.5.1
Basic Transfer Protocol
The basic transfer protocol defines the sequence of actions that must occur on the MPC561/MPC563 bus
to perform a complete bus transaction. A simplified scheme of the basic transfer protocol is illustrated in
.
Figure 9-3. Basic Transfer Protocol
The basic transfer protocol provides for an arbitration phase and an address and data transfer phase. The
address phase specifies the address for the transaction and the transfer attributes that describe the
transaction. The data phase performs the transfer of data (if any is to be transferred). The data phase may
transfer a single beat of data (four bytes or less) for nonburst operations, a 4-beat burst of data (4 x 4 bytes),
an 8-beat burst of data (8 x
2 bytes) or a 16-beat burst of data (16 x 1 bytes).
RETRY
Retry
1
Low
I
In the case of regular transaction, this signal is driven
by the slave device to indicate that the
MPC561/MPC563 must relinquish the ownership of
the bus and retry the cycle.
O
When an external master owns the bus and the
internal MPC561/MPC563 bus initiates access to the
external bus at the same time, this signal is used to
cause the external master to relinquish the bus for one
clock to solve the contention.
Table 9-1. MPC561/MPC563 BIU Signals (continued)
Signal Name
Pins
Active
I/O
Description
Arbitration
Address Transfer
Data Transfer
Termination
Summary of Contents for MPC561
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