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Multi-Layer AHB Crossbar Switch (XBAR)
MPC5644A Microcontroller Reference Manual, Rev. 6
Freescale Semiconductor
195
states inserted until the targeted slave port can service the master’s request. The latency in servicing the
request depends on each master’s priority level and the responding peripheral’s access time.
Since the XBAR appears to be just another slave to the master device, the master device will have no
knowledge of whether or not it actually owns the slave port it is targeting. While the master does not have
control of the slave port it is targeting it will simply be wait stated.
A master is given control of the targeted slave port only after a previous access to a different slave port has
completed, regardless of its priority on the newly targeted slave port. This prevents deadlock from
occurring when a master has an outstanding request to one slave port that has a long response time, has a
pending access to a different slave port, and a lower priority master is also making a request to the same
slave port as the pending access of the higher priority master.
Once the master has control of the slave port it is targeting, the master remains in control of that slave port
until it gives up the slave port by running an IDLE cycle or by leaving that slave port for its next access.
The master could also lose control of the slave port if another higher priority master makes a request to the
slave port; however, if the master is running a locked or fixed length burst transfer it retains control of the
slave port until that transfer is completed.
The XBAR will terminate all master IDLE transfers (as opposed to allowing the termination to come from
one of the slave busses). Additionally, when no master is requesting access to a slave port the XBAR will
drive IDLE transfers onto the slave bus, even though a default master may be granted access to the slave
port.
When a slave bus is being IDLEd by the XBAR it can park the slave port on the master port indicated by
the PARK bits in the SGPCR (Slave General Purpose Control Register). This can be done in an attempt to
save the initial clock of arbitration delay that would otherwise be seen if the master had to arbitrate to gain
control of the slave port. The slave port can also be put into low power park mode in attempt to save power.
9.2
XBAR registers
This section provides information on XBAR registers.
9.2.1
Register summary
There are two registers that reside in each slave port of the XBAR. These registers are IP bus compliant
registers. Read and write transfers both require two IP bus clock cycles. Read and writen operations can
be performed on these registers only in supervisor mode. Additionally, these registers can only be read
from or written to by 32-bit accesses.
The registers are fully decoded and an error response is returned if an unimplemented location is accessed
within the XBAR.
The slave registers also feature a bit, which when written with a 1, will prevent the registers from being
written to again. The registers will still be readable, but future write attempts will have no effect on the
registers and will be terminated with an error response.
The memory map for the XBAR program-visible registers is shown in
.
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Страница 52: ...Introduction MPC5644A Microcontroller Reference Manual Rev 6 52 Freescale Semiconductor...
Страница 56: ...Memory Map MPC5644A Microcontroller Reference Manual Rev 6 56 Freescale Semiconductor...
Страница 130: ...Device Performance Optimization MPC5644A Microcontroller Reference Manual Rev 6 130 Freescale Semiconductor...
Страница 204: ...Multi Layer AHB Crossbar Switch XBAR MPC5644A Microcontroller Reference Manual Rev 6 204 Freescale Semiconductor...
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