
7-2
Bus Interface Unit and Write Buffer
Copyright 2000–2001 by LSI Logic Corporation. All rights reserved.
through the AHB interface for cache line fills and for initializing the tightly
coupled SRAMs. The AHB interface is also used to access code and
data that are not within the cacheable or tightly coupled memory address
regions.
When an AHB access occurs, the BIU and system controller handshake
to ensure that the ARM9E-S processor core is stalled until the access is
finished. If you are using the write buffer, you might be able to allow the
processor core to continue program execution. The BIU controls the write
buffer and related stall behavior.
7.2 AHB Bus Master Interface
The ARM946E-S implements a fully compliant AHB bus master interface
as defined in the
AMBA Specification, Rev 2.0
. See this document for a
detailed description of the AHB protocol.
7.2.1 About the AHB
The AHB architecture is based on separate cycles for address and data
rather than separate clock phases, as in the AMBA Advanced System
Bus (ASB). The address and control for an access are broadcast from
the rising edge of HCLK in the cycle before the data is expected to be
read or written. During this data cycle, the address and control for the
next transfer are driven out, providing a fully pipelined address
architecture.
When an access is in its data cycle, a slave can extend an access by
driving the HREADY signal LOW. This action stretches the current data
cycle, and the pipelined address and control for the next transfer also
stretches. With this system, all AHB masters and slaves sample
HREADY on the rising edge of HCLK to determine when an access is
complete and whether a new address can be sampled or driven out.