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XRT79L71
PRELIMINARY
37
1-CHANNEL DS3/E3 CLEAR-CHANNEL FRAMERLIU COMBO - CC/HDLC ARCHITECTURE
REV. P2.0.0
2.2.2
The Motorola-Asynchronous Write-Cycle
If the Microprocessor Interface (of the XRT79L71) has been configured to operate in the Motorola-
Asynchronous Mode, then the Microprocessor should do all of the following, anytime it wishes to write a byte of
data into a register or some location within the Transmit LAPD Message buffer, the Transmit Cell Insertion
Memory or the Receive Cell Insertion Memory (within the XRT79L71).
1.
Place the address of the "target" register or buffer location (within the XRT79L71) on the Address Bus
input pins, A[14:0].
2.
While the C/P is placing the address value on the Address Bus, the Address Decoding circuitry (within
the user's system) should assert the CS (Chip Select) input pin of the XRT79L71, by toggling it "low".
This action enables further communication between the C/P and the XRT79L71 Microprocessor
Interface.
3.
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 XRT79L71.
4.
After allowing the data on the Address Bus pins to settle (by waiting the appropriate "Address Setup"
time), the C/P should toggle the ALE/AS input pin "high". This step causes the XRT79L71 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 XRT79L71) has now been selected.
5.
Afterwards, the C/P should indicate that this current bus cycle is a "Write" operation by toggling the
WR/R/W
(R/W) input pin "low".
6.
The C/P should then place the byte or word that it intends to write into the "target" register, on the bi-
directional data bus, D[7:0].
7.
Next, the C/P should initiate the bus cycle by toggling the RD/DS (Data Strobe) input pin "low". When
the XRT79L71 senses that the WRB_RW (R/W) input pin is "high" and that the RD/DS (Data Strobe)
input pin has toggled "low", it will enable the "input drivers" of the bi-directional data bus, D[7:0].
FIGURE 8. ILLUSTRATION OF THE BEHAVIOR OF MICROPROCESSOR INTERFACE SIGNALS, DURING A "MOTOROLA-
ASYNCHRONOUS" READ OPERATION.
ALE/AS
RD*/DS*
A[14:0]
CS*
D[7:0]
RDY/DTACK*
Not Valid
Valid Data
Address of Target Register
WR*/R/W*
Microprocessor places “target”
Address value on A[14:0]
Microprocessor Interface latches contents on
A[14:0] upon rising edge of AS*
Address Decoding
Circuitry asserts
CS*
Microprocessor keeps R/W* “high”
To denote READ Operation
Read Operation begins
Here
DTACK* toggles “l(fā)ow” to indicate
That valid data can be read from
D[7:0]
Read Operation is
Terminated Here