C167CS datasheet(194/517 Pages) INFINEON

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C167CS

Derivatives

The External Bus Interface

User’s Manual

9-6

V2.0, 2000-07

Switching Between the Bus Modes

The EBC allows to switch between different bus modes dynamically, i.e. subsequent

external bus cycles may be executed in different ways. Certain address areas may use

multiplexed or demultiplexed buses or use READY control or predefined waitstates.

A change of the external bus characteristics can be initiated in two different ways:

Switching between predefined address windows automatically selects the bus mode

that is associated with the respective window. Predefined address windows allow to use

different bus modes without any overhead, but restrict their number to the number of

BUSCONs. However, as BUSCON0 controls all address areas, which are not covered

by the other BUSCONs, this allows to have gaps between these windows, which use the

bus mode of BUSCON0.

PORT1 will output the intra-segment address, when any of the BUSCON registers

selects a demultiplexed bus mode, even if the current bus cycle uses a multiplexed bus

mode. This allows to have an external address decoder connected to PORT1 only, while

using it for all kinds of bus cycles.

The usage of the BUSCON/ADDRSEL registers is controlled via the issued addresses.

When an access (code fetch or data) is initiated, the respective generated physical

address defines, if the access is made internally, uses one of the address windows

defined by ADDRSEL4 … 1, or uses the default configuration in BUSCON0. After

initializing the active registers, they are selected and evaluated automatically by

interpreting the physical address. No additional switching or selecting is necessary

during run time, except when more than the four address windows plus the default is to

be used.

Reprogramming the BUSCON and/or ADDRSEL registers allows to either change

the bus mode for a given address window, or change the size of an address window that

uses a certain bus mode. Reprogramming allows to use a great number of different

address windows (more than BUSCONs are available) on the expense of the overhead

for changing the registers and keeping appropriate tables.

Note: Be careful when changing the configuration for an address area that currently

supplies the instruction stream. Due to the internal pipelining, the first instruction

fetch that will use the new configuration depends on the instructions prior to the

configuration change. Special care is required when changing bits like BUSACT

or RDYEN, in order not to cut the instruction stream inadvertently.

Only change the other configuration bits after checking that the respective

application can cope with the intended modification(s).

It is recommended to change ADDRSEL registers only while the respective

BUSACT bit in the associated BUSCON register is cleared.

Switching from demultiplexed to multiplexed bus mode represents a special case.

The bus cycle is started by activating ALE and driving the address to Port 4 and PORT1

as usual, if another BUSCON register selects a demultiplexed bus. However, in the

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