7 memory and bus access errors, 8 bus arbitration – Cirrus Logic EP93xx User Manual

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DS785UM1

2-9

ARM920T Core and Advanced High-Speed Bus (AHB)

EP93xx User’s Guide

A write data bus is used to move data from the master to a slave, while a read data bus is
used to move data from a slave to the master. Every transfer consists of:

•

An address and control cycle

•

One or more cycles for the data.

In normal operation a master is allowed to complete all the transfers in a particular burst
before the arbiter grants another master access to the bus. However, in order to avoid
excessive arbitration latencies, it is possible for the arbiter to break up a burst, and, in such
cases, the master must re-arbitrate for the bus in order to complete the remaining transfers in
the burst.

2.2.7 Memory and Bus Access Errors

There are several possible sources of access errors:

•

Reads to reserved or undefined register memory addresses will return indeterminate

data. Writes to reserved or undefined memory addresses are generally ignored, but this
behavior is not guaranteed. Many register addresses are not fully decoded, so aliasing
may occur. Addresses and memory ranges listed as Reserved should not be accessed;
access behavior to these regions is not defined

•

Access to non-existent registers or memory may result in a bus error

•

Any access to the APB control register space will complete normally, as these devices

have no means of signaling an error

•

Access to non-existent AHB or APB registers may result in a bus error, depending on the

device and nature of the error. Device specific access rules are defined in the device
descriptions

•

External memory access is controlled by the Static Memory Controller (SMC) or the

Synchronous Dynamic RAM (SDRAM) controller. In general, access to non-existent
external memory will complete normally, with reads returning random false data.

2.2.8 Bus Arbitration

The arbitration mechanism is used to ensure that only one master has access to the bus that
it controls at any one time. The Arbiter performs this function by observing a number of
different requests to use the bus, and then deciding which is currently the highest priority
master requesting the bus.

The arbitration scheme can be broken down into three main areas:

•

The main AHB system bus Arbiter

•

The SDRAM slave interface Arbiter

•

The EBI bus Arbiter