
á
PRELIMINARY
DS3 UNI FOR ATM
XRT7245
REV. 1.03
51
Each of these operations, within the Burst Access
are discussed below.
3.2.2.2.2.1.1
The Initial Read Operation
The initial read operation of a “Motorola-type” read
burst access is accomplished by executing a “Pro-
grammed I/O Read” cycle, as summarized below.
Execute a Single Ordinary (Programmed I/
O) Read Cycle, as described in steps A.1
through A.8 below.
A.
A.1
Assert the ALE_AS (AS*) input pin by toggling
it “l(fā)ow”. This step enables the “Address Bus”
input drivers (within the XRT7245 DS3 UNI)
within the UNI Microprocessor Interface Block.
A.2
Place the address of the “initial” target register
or buffer location (within the UNI), on the
Address Bus input pins, A[8:0].
A.3
At the same time, the Address-Decoding cir-
cuitry (within the user’s system) should assert
the CS* (Chip Select) input pins of the UNI by
toggling it “l(fā)ow”. This action enables further
communication between the μC/μP and the
UNI Microprocessor Interface block.
A.4
After allowing the data on the Address Bus
pins to settle (by waiting the appropriate
“Address Setup” time), the μC/μP should tog-
gle the ALE_AS input pin “high”. This step
causes the UNI device to latch the contents of
the Address Bus into its internal circuitry. At
this point, the “initial” address of the burst
access has now been selected.
A.5
Further, the μC/μP should indicate that this
cycle is a “Read” cycle by setting the
WRB_RW (R/W*) input pin “high”.
Figure 16 presents an illustration of the behavior of
the Microprocessor Interface Signals during the “ini-
tial” Read Operation, within a Burst I/O Cycle; for a
Motorola-type μC/μP
At the completion of this initial read cycle, the μC/μP
has read in the contents of the first register or buffer
location (within the XRT7245 DS3 UNI) for this par-
ticular burst access operation. In order to illustrate
how this “burst I/O cycle” works, the byte (or word) of
data, that is being read in Figure 16, has been la-
beled “Valid Data at Offset = 0x00”. This indicates
that the μC/μP is reading the very first register (or
buffer location) in this burst access.
The Subsequent Read Operations
The procedure that the μC/μP must use to perform
the remaining read cycles, within this Burst Access
operation, is presented below.
3.2.2.2.2.1.2
B.
Execute each subsequent Read Cycle, as
described in Steps B.1 through B.3, below.
B.1
Without toggling the ALE_AS input pin (e.g.,
keeping it “high”); toggle the RdB_DS (Data
Strobe) input pin “l(fā)ow”. This step accomplishes
the following.
a.
The UNI internally increments the “l(fā)atched
address” value (within the Microprocessor
Interface circuitry).
The output drivers of the “bi-directional”
data bus (D[15:0]) are enabled. At some
time later, the register or buffer location
corresponding to the “incremented”
b.
F
IGURE
16. B
EHAVIOR
OF
THE
M
ICROPROCESSOR
I
NTERFACE
S
IGNALS
,
DURING
THE
“I
NITIAL
” R
EAD
O
PERATION
OF A
B
URST
C
YCLE
(M
OTOROLA
T
YPE
P
ROCESSOR
).
ALE_AS
RDB_DS
A[8:0]
CS*
D[15:0]
Rdy_Dtck
Not Valid
Valid Data at Offset = 0x00
Address of “Initial” Target Register (Offset = 0x00)
WRB_RW