Freescale Semiconductor MPC5200B - Manual (Page 588)

Overview
MPC5200B Users Guide, Rev. 1
Freescale Semiconductor 16-1
Chapter 16
XLB Arbiter
16.1 Overview
This document contains the following section:
• Section 16.1, Overview
• Section 16.2, XLB Arbiter Registers—MBAR + 0x1F00
16.1.1 Purpose
The purpose of the XLB Arbiter is to manage bus requests from the XLB masters (USB, PCI, BestComm, and e300 core), and determine
which master should be granted the bus at any one time. The arbiter employs both master prioritization and a fair-share LRU
(least-recently-used) algorithm to reduce access latency and starvation across all masters.
The XLB Arbiter consists of five functional blocks as shown below.
Figure 16-1. Block Diagram of XLB Arbiter
16.1.1.1 Prioritization
The prioritization block signals that a master is requesting the bus and which master has priority.
Priority is determined first by using the master priority level assigned by either the hardware-wired internal signals, or software-programmable
Master N Priority bits in the Arbiter Master N Priority Register depending on the Master Priority Enable bit for each master. Masters at the
same level of priority will be further sorted by a least recently used algorithm (LRU). Once a requesting master is identified as having priority
and is granted the bus, that master will be continue to be granted the bus if:
1. It is requesting the bus. The request must occur immediately after the required one clock de-assertion after a qualified bus grant, and
2. It is the highest priority device, and
3. There is no address retry assertion.
Prioritization
Bus Grant FSM
Configuration,
Status
, and
Interrupts
Watchdog
Slave Interface