
á
PRELIMINARY
DS3 UNI FOR ATM
XRT7245
REV. 1.03
53
“Address Bus” into its own 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
current bus cycle is a “Write” operation by tog-
gling the WRB_RW (R/W*) input pin “l(fā)ow”.
A.6
The μC/μP should then place the byte or word
that it intends to write into the “target” register,
on the bidirectional data bus, D[15:0].
A.7
Next, the μC/μP should initiate the bus cycle by
toggling the RdB_DS (Data Strobe) input pin
“l(fā)ow”. When the XRT7245 DS3 UNI device
senses that the WRB_RW input pin is “l(fā)ow”,
and that the RdB_DS input pin has toggled
“l(fā)ow” it will enable the “input drivers” of the bidi-
rectional data bus, D[15:0].
A.8
After waiting the appropriate amount of time,
for this newly placed data to settle on the bi-
directional data bus( e.g., the “Data Setup”
time) the UNI will assert the Rdy_Dtck
(DTACK) output signal.
A.9
After the mC/mC detects the Rdy_Dtck signal
(from the UNI) it should toggle the RdB_DS
input pin “high”. This action accomplishes two
things:
a.
It latches the contents of the bi-directional
data bus into the XRT7245 DS3 UNI
Microprocessor Interface block.
It terminates the “Write” cycle.
b.
Figure 18 presents a timing diagram which illustrates
the behavior of the Microprocessor Interface signals,
during the “Initial” write operation within a Burst
Access, for a “Motorola-type” μC/μP
At the completion of this initial write cycle, the μC/μP
has written a byte or word into the first register or
buffer location (within the XRT7245 DS3 UNI) for this
particular burst I/O access. In order to illustrate how
this “burst I/O cycle” works, the byte (or word) of data,
that is being written in Figure 18, has been labeled
“Data to be Written (Offset = 0x00).”
The Subsequent Write Operations
The procedure that the μC/μP must use to perform
the remaining write cycles, within this burst access
operation, is presented below.
3.2.2.2.2.2.2
B.
Execute each subsequent write cycle, as
described in Steps B.1 through B.3
B.1
Without toggling the ALE_AS (Address Strobe)
input pin (e.g., keeping it “high”); apply the
value of the next byte or word (to be written into
the UNI) to the bi-directional data bus pins,
D[15:0].
B.2
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).
F
IGURE
18. B
EHAVIOR
OF
THE
M
ICROPROCESSOR
I
NTERFACE
SIGNALS
,
DURING
THE
“I
NITIAL
” W
RITE
O
PERATION
OF A
B
URST
C
YCLE
(M
OTOROLA
-
TYPE
P
ROCESSOR
).
ALE_AS
A[8:0]
CS*
D[15:0]
RDB_DS
Rdy_Dtck
Data to be Written (Offset = 0x00)
Address of “Initial” Target Register (Offset = 0x00)