
Signal Descriptions
ARM DDI 0192A
Copyright ARM Limited 1997, 1998, 2000. All rights reserved.
A-5
A.2
Coprocessor interface signals
The coprocessor interface signals are listed in Table A-2.
Table A-2 Coprocessor interface signal descriptions
Name
Type
Description
CPCLK
Out
Coprocessor clock.
This clock controls the operation of the coprocessor interface.
CPDATA[31:0]
In/out
Coprocessor data bus.
Data is transferred to and from the coprocessor using this bus. Data is valid on the falling
edge of
CPCLK
.
CPDBE
In
Coprocessor data bus enable.
This signal when HIGH, indicates that the coprocessor intends to drive the coprocessor
data bus,
CPDATA.
If the coprocessor interface is not to be used then this signal must be
tied LOW.
CPnWAIT
Out
Coprocessor not wait.
The coprocessor clock
CPCLK
is qualified by
CPnWAIT
to allow the ARM720T to
control the transfer of data on the coprocessor interface.
CPTESTREAD
In
Coprocessor test read.
This signal can be used for test of a coprocessor, if attached, and must only be used with
the ARM720T held in reset. When HIGH, it enables
Data Bus
(DB) to be driven on to
CPDATA
, and must be held LOW. It must never be asserted at the same time as
CPTESTWRITE
.
CPTESTWRITE
In
Coprocessor test write.
This signal can be used for test of a coprocessor, if attached, and must only be used with
the ARM720T held in reset. When HIGH, it enables DB to be driven on to
CPDATA
,
and must be held LOW. It must never be asserted at the same time as
CPTESTREAD
.
EXTCPA
In
External coprocessor absent.
A coprocessor that is capable of performing the operation that ARM720T is requesting,
by asserting
nCPI
takes
EXTCPA
LOW immediately. If
EXTCPA
is HIGH at the end of
the low phase of the cycle in which
nCPI
went LOW, ARM720T aborts the coprocessor
instruction and takes the undefined instruction trap. If
EXTCPA
is LOW and remains
LOW, ARM720T busy-waits until
EXTCPB
is LOW, and then completes the
coprocessor instruction.
EXTCPB
In
External coprocessor busy.
A coprocessor that is capable of performing the operation that ARM720T is requesting,
by asserting
nCPI
, but cannot commit to starting it immediately, indicates this by driving
EXTCPB
HIGH. When the coprocessor is ready to start it takes
EXTCPB
LOW.
ARM720T samples
EXTCPB
at the low phases of each cycle in which
nCPI
is LOW.