Signal Description
5
2.0 Signal Description
Name
Type
Description
A[31:0]
OS8
Addresses. This is the processor address bus. If
addresses become valid during phase 2 of the cycle before the one to which they refer and
remain so during phase 1 of the referenced cycle. Their stable period may be controlled by
ALE
as described below. Refer to section "AC parameters" for timing diagrams.
ALE
(address latch enable) is HIGH, the
ABE
I
Address bus enable. This is an input signal which, when LOW, puts the address bus drivers
into a high impedance state.
ABE
must be tied HIGH when there is no system requirement
to turn off the address drivers.
ABORT
I
Memory
a requested access is not allowed. ARM60 can be configured to accept either early aborts for
compatibility with earlier processors or late aborts for greater flexibility.
ABORT
. This is an input which allows the memory system to tell the processor that
ALE
I
Address latch enable. This input is used to control transparent latches on the address outputs.
Normally the addresses change during phase 2 to the value required during the next cycle,
but for direct interfacing to ROMs they are required to be stable to the end of phase 2. Taking
ALE
LOW until the end of phase 2 will ensure that this happens. If the system does not
require address lines to be held in this way,
ALE
static, so
ALE
may be held LOW for long periods to freeze addresses.
must be tied HIGH. The address latch is
BIGEND
I
Big Endian configuration. When this signal is HIGH the processor treats bytes in memory as
being in Big Endian format. When it is LOW memory is treated as Little
Endian.
CPA
I
Coprocessor absent. A coprocessor which is capable of performing the operation that ARM60
is requesting (by asserting
nCPI
) should take
CPA
end of phase 1 of the cycle in which
nCPI
went LOW, ARM60 will
handshake and take the undefined instruction trap. If
ARM60 will busy-wait until
CPB
is LOW and then complete the coprocessor instruction.
LOW immediately. If
CPA
abort the coprocessor
is LOW and remains LOW,
is HIGH at the
CPA
CPB
I
Coprocessor busy. A coprocessor which is capable of performing the operation which
ARM60 is requesting (by asserting
nCPI
), but cannot commit to starting it immediately,
should indicate this by driving
CPB
HIGH. When the coprocessor is ready to start it should
take
CPB
LOW. ARM60 samples
CPB
at the end of phase 1 of each cycle in which
LOW.
nCPI
is
D[31:0]
I/
OS8
Data Bus. These are bidirectional signal paths which are used for data transfers between the
processor and external memory. During read cycles (when
must be valid before the end of phase 2 of the transfer cycle. During write cycles (when
is HIGH), the output data will become valid during phase 1 and remain valid throughout
phase 2 of the transfer cycle.
nRW
is LOW), the input data
nRW
DATA32
I
32 bit Data configuration. When this signal is HIGH the processor can access data in a 32 bit
address space using address lines
A[31:0]
. When it is LOW the processor can access data from
a 26 bit address space using
A[25:0]
. In this latter configuration the address lines
not used. Before changing
DATA32
, ensure that the processor is not about to access an
address greater that 0x3FFFFFF in the next cycle.
A[31:26]
are
Table 1: Signal Description