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MPC563XM Reference Manual, Rev. 1
Freescale Semiconductor
191
Preliminary—Subject to Change Without Notice
The
waiting
state is used when the
hsel
signal is negated to the master port, indicating the master is
running valid cycles to a local slave other than the XBAR. In this case the max disables the slave port
decoder and holds
hresp
and
hready
negated.
The
stalled
state is used when the master makes a request to a slave port that is not immediately ready to
receive the request. In this case the state machine will direct the capture unit to send out the captured
address and control signals and will enable the slave port decoder to indicate a pending request to the
appropriate slave port.
The
steady state
state is used when the master port and slave port are in fully asynchronous mode, making
the XBAR completely transparent in the access. The state machine selects the appropriate slave’s
hresp,
hready
and
hrdata
to pass back to the master.
The
first cycle error response
and
second cycle error response
states are self explanatory. The XBAR
will respond with an error response to the master if the master tries to access an unimplemented memory
location through the XBAR (i.e. a slave port that does not exist).
8.4.3.5.2
Master Port State Machine Slave Swapping
The design of the master side state machine is fairly straight forward. The one real decision to be made is
how to handle the master moving from one slave port access to another slave port access. The approach
that was taken was to minimize or eliminate when possible any “bubbles” that would get inserted into the
access due to switching slave ports.
The state machine will not allow the master to request access to another slave port until the current access
being made is terminated. This prevents a single master from owning two slave ports at the same time (the
slave port it is currently accessing and the slave port it wishes to access next).
The state machine also maintains watch on the slave port the master is accessing as well as the slave port
the master wishes to switch to. If the new slave port is parked on the master then the master will be able
to make the switch without incurring any delays. The termination of the current access will also act as the
launch of the new access on the new slave port. If the new slave port is not parked on the master then the
master will incur a minimum one clock delay before it can launch its access on the new slave port.
This is the same for switching from the
busy, idle
or
waiting
state to actively accessing a slave port. If the
slave port is parked on the master the state machine will go to the
steady state
state and the access will
begin immediately. If the slave port is not parked on the master (serving another master, parked on another
master or in low power park mode) then the state machine will transition to the
stalled
state and at least a
one clock penalty will be paid.
8.4.4
Slave Port Functionality
8.4.4.1
General
Each slave port consists of a register slice, a bank of muxes and a state machine.
The register slice contains the registers associated with the specific slave port. The registers have a
quasi-IP bus interface at this level for reads and writes and the outputs feed directly into the state machine.
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