
XRT7245
DS3 UNI FOR ATM
á
PRELIMINARY
REV. 1.03
44
3.2.2.1.2
Programmed I/O Access in the
Motorola Mode
If the XRT7245 DS3 UNI is interfaced to a “Motorola-
type” μC/μP (e.g., the MC680X0 family, etc.); it
should be configured to operate in the “Motorola”
mode (by tying the “MOTO” pin to Vcc). Motorola-
type Programmed I/O “Read” and “Write” operations
are described below.
3.2.2.1.2.1
Whenever a “Motorola-type” μC/μP wishes to read
the contents of a register or some location within the
Receive LAPD Message or Receive OAM Cell Buffer,
(within the UNI device) it should do the following.
Assert the ALE_AS (Address-Strobe) input pin
by toggling it low. This step enables the Address
Bus input drivers, within the Microprocessor
Interface Block of the UNI IC.
Place the address of the “target” register (or
buffer location) within the UNI, on the Address
Bus input pins, A[8:0].
At the same time, the Address Decoding circuitry
(within the user’s system) should assert the CS*
(Chip Select) input pin of the UNI device, by tog-
gling it “l(fā)ow”. This action enables further commu-
nication between the μC/μP and the UNI Micro-
processor Interface block.
After allowing the data on the Address Bus pins
to settle (by waiting the appropriate “Address
Setup” time), the μC/μP should toggle the
The Motorola Mode Read Cycle
1.
2.
3.
4.
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
address of the register or buffer location (within
the UNI) has now been selected.
Further, the μC/μP should indicate that this cycle
is a “Read” cycle by setting the WRB_RW (R/W*)
input pin “high”.
Next the μC/μP should initiate the current bus
cycle by toggling the RdB_DS (Data Strobe) input
pin “l(fā)ow”. This step enables the bi-directional data
bus output drivers, within the XRT7245 DS3 UNI
device. At this point, the bi-directional data bus
output drivers will proceed to driver the contents
of the “Address” register onto the bi-directional
data bus, D[15:0].
After some settling time, the data on the “bi-
directional” data bus will stabilize and can be
read by the μC/μP The XRT7245 DS3 UNI will
indicate that this data can be read by asserting
the Rdy_Dtck (DTACK) signal.
After the μC/μP detects the Rdy_Dtck signal
(from the XRT7245 DS3 UNI) it will terminate the
Read Cycle by toggling the “RdB_DS” (Data
Strobe) input pin “high”.
5.
6.
7.
8.
Figure 10 presents a timing diagram which illustrates
the behavior of the Microprocessor Interface signals
during a “Motorola-type” Programmed I/O Read
Operation.
F
IGURE
9. B
EHAVIOR
OF
THE
M
ICROPROCESSOR
I
NTERFACE
S
IGNALS
,
DURING
AN
“I
NTEL
-
TYPE
” P
ROGRAMMED
I/O W
RITE
O
PERATION
.
ALE_AS
A[8:0]
CS*
D[15:0]
WRB_RW
Data to be Written
Address of Target Register
RdB_DS