Crossbar Switch (XBAR)
MPC5565 Microcontroller Reference Manual, Rev. 1.0
7-8
Freescale Semiconductor
7.3
Functional Description
This section describes the functionality of the XBAR in more detail.
7.3.1
Overview
The main goal of the XBAR is to increase overall system performance by allowing multiple masters to
communicate concurrently with multiple slaves. To maximize data throughput, it is essential to keep
arbitration delays to a minimum.
This section examines data throughput from the point of view of masters and slaves, detailing when the
XBAR stalls masters, or inserts bubbles on the slave side.
7.3.2
General Operation
When a master makes an access to the XBAR from an idle master state, the access is taken immediately
by the XBAR. If the targeted slave port of the access is available (that is, the requesting master is currently
granted ownership of the slave port), the access is immediately presented on the slave port. It is possible
to make single clock (zero wait state) accesses through the XBAR by a granted master. If the targeted slave
port of the access is busy or parked on a different master port, the requesting master receives wait states
until the targeted slave port can service the master request. The latency in servicing the request depends
on each master’s priority level and the responding slave’s access time.
Because the XBAR appears to be simply another slave to the master device, the master device has no
indication that it owns the slave port it is targeting. While the master does not have control of the slave port
it is targeting, it is wait-stated.
A master is given control of a 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 the following conditions:
•
Outstanding request to slave port A that has a long response time
•
Pending access to a different slave port B
•
Lower priority master also makes a request to the different slave port B.
28
Reserved, must be cleared.
29–31
PARK
Park. Used to determine which master port this slave port parks on when no masters are actively making
requests. PCTL must be set to 00.
000 Park on master port 0
001 Park on master port 1
010 Park on master port 2
011 Invalid value
100 Invalid value
101 Invalid value
110 Invalid value
111 Invalid value
Table 7-5. XBAR_SGPCR
n
Field Descriptions (continued)
Field
Description
Содержание MPC5565
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