AT91SAM9G10-CU View Datasheet(PDF) - Atmel Corporation
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'AT91SAM9G10-CU' PDF : 730 Pages View PDF
11.6
Bus Interface Unit
The ARM926EJ-S features a Bus Interface Unit (BIU) that arbitrates and schedules AHB
requests. The BIU implements a multi-layer AHB, based on the AHB-Lite protocol, that enables
parallel access paths between multiple AHB masters and slaves in a system. This is achieved by
using a more complex interconnection matrix and gives the benefit of increased overall bus
bandwidth, and a more flexible system architecture.
The multi-master bus architecture has a number of benefits:
• It allows the development of multi-master systems with an increased bus bandwidth and a
flexible architecture.
• Each AHB layer becomes simple because it only has one master, so no arbitration or master-
to-slave muxing is required. AHB layers, implementing AHB-Lite protocol, do not have to
support request and grant, nor do they have to support retry and split transactions.
• The arbitration becomes effective when more than one master wants to access the same
slave simultaneously.
11.6.1
Supported Transfers
The ARM926EJ-S processor performs all AHB accesses as single word, bursts of four words, or
bursts of eight words. Any ARM9EJ-S core request that is not 1, 4, 8 words in size is split into
packets of these sizes. Note that the Atmel bus is AHB-Lite protocol compliant, hence it does not
support split and retry requests.
Table 11-7 gives an overview of the supported transfers and different kinds of transactions they
are used for.
Table 11-7. Supported Transfers
HBurst[2:0] Description
SINGLE
Single transfer
INCR4
INCR8
WRAP8
Four-word incrementing burst
Eight-word incrementing burst
Eight-word wrapping burst
Single transfer of word, half word, or byte:
• data write (NCNB, NCB, WT, or WB that has missed in DCache)
• data read (NCNB or NCB)
• NC instruction fetch (prefetched and non-prefetched)
• page table walk read
Half-line cache write-back, Instruction prefetch, if enabled. Four-word burst NCNB,
NCB, WT, or WB write.
Full-line cache write-back, eight-word burst NCNB, NCB, WT, or WB write.
Cache linefill
11.6.2
Thumb Instruction Fetches
All instructions fetches, regardless of the state of ARM9EJ-S core, are made as 32-bit accesses
on the AHB. If the ARM9EJ-S is in Thumb state, then two instructions can be fetched at a time.
11.6.3
Address Alignment
The ARM926EJ-S BIU performs address alignment checking and aligns AHB addresses to the
necessary boundary. 16-bit accesses are aligned to halfword boundaries, and 32-bit accesses
are aligned to word boundaries.
52 AT91SAM9G10
6462A–ATARM–03-Jun-09
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