Memory Interface
ARM DDI 0210C
Copyright © 2001, 2004 ARM Limited. All rights reserved.
3-21
The macrocell has an additional bus control signal,
nENIN
that enables the external
system to manually tristate the bus. In the simplest systems,
nENIN
can be tied LOW
and
nENOUT
can be ignored. In many applications, when the external data bus is a
shared resource, greater control might be required. In this situation,
nENIN
can be used
to delay when the external bus is driven.
Note
For backwards compatibility,
DBE
is also included. At the macrocell level,
DBE
and
nENIN
have almost identical functionality and in most applications one can be tied to
keep the data bus enabled.
The processor has another output control signal called
TBE
. This signal is usually only
used during test and must be tied HIGH when not in use. When driven LOW,
TBE
forces all tristateable outputs to high impedance, it is as though both
DBE
and
ABE
have been driven LOW, causing the data bus, the address bus, and all other signals
normally controlled by
ABE
to become high impedance.
Note
There is no scan cell on
TBE
. Therefore,
TBE
is completely independent of scan data
and can be used to put the outputs into a high impedance state while scan testing takes
place.
Table 3-6 lists the tristate control of the processor outputs.
Table 3-6 Tristate control of processor outputs
Processor output
ABE
DBE
TBE
A[31:0]
Yes
-
Yes
D[31:0]
-
Yes
Yes
nRW
Yes
-
Yes
LOCK
Yes
-
Yes
MAS[1:0]
Yes
-
Yes
nOPC
Yes
-
Yes
nTRANS
Yes
-
Yes
Содержание ARM7TDMI
Страница 6: ...Contents vi Copyright 2001 2004 ARM Limited All rights reserved ARM DDI 0210C ...
Страница 10: ...List of Tables x Copyright 2001 2004 ARM Limited All rights reserved ARM DDI 0210C ...
Страница 14: ...List of Figures xiv Copyright 2001 2004 ARM Limited All rights reserved ARM DDI 0210C ...
Страница 46: ...Introduction 1 26 Copyright 2001 2004 ARM Limited All rights reserved ARM DDI 0210C ...
Страница 120: ...Coprocessor Interface 4 18 Copyright 2001 2004 ARM Limited All rights reserved ARM DDI 0210C ...
Страница 142: ...Debug Interface 5 22 Copyright 2001 2004 ARM Limited All rights reserved ARM DDI 0210C ...
Страница 276: ...Differences Between Rev 3a and Rev 4 C 6 Copyright 2001 2004 ARM Limited All rights reserved ARM DDI 0210C ...
Страница 282: ...Glossary Glossary 6 Copyright 2001 2004 ARM Limited All rights reserved ARM DDI 0210C ...