XRT72L56 Datasheet, PDF (75/486 Pages) Exar Corporation – SIX CHANNEL DS3/E3 FRAMER IC WITH HDLC CONTROLLER

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SIX CHANNEL DS3/E3 FRAMER IC WITH HDLC CONTROLLER
PRELIMINARY
XRT72L56
REV. P1.1.2
A.0 Execute a Single Ordinary (Programmed I/
O) Read Cycle, as described in steps A.1
through A.7 below.
A.1 Place the address of the initial-target register or
buffer location (within the Framer) on the
Address Bus input pins A[11:0].
A.2 While the µC/µP is placing this address value
onto the Address Bus, the Address Decoding
circuitry (within the user's system) should
assert the CS input pin of the Framer, by tog-
gling it "Low". This step enables further com-
munication between the µC/µP and the Framer
Microprocessor Interface block.
A.3 Assert the ALE_AS (Address Latch Enable) pin
by toggling it "High". This step enables the
Address Bus input drivers, within the Micropro-
cessor Interface block of the Framer.
A.4 After allowing the data on the Address Bus pins
to settle (by waiting the appropriate Address
Data Setup time), the µC/µP should then toggle
the ALE_AS pin "Low". This step latches the
contents, on the Address Bus pins, A[11:0], into
the XRT72L56 DS3/E3 Framer Microprocessor
Interface block. At this point, the initial address
of the burst access has now been selected.
NOTE: The ALE_AS input pin should remain "Low" for the
remainder of this Burst Access operation.
A.5 Next, the µC/µP should indicate that this cur-
rent bus cycle is a Read Operation by toggling
the RD_DS (Read Strobe) input pin "Low".
This action also enables the bi-directional data
bus output drivers of the Framer device. At this
point, the bi-directional data bus output drivers
will proceed to drive the contents of the
addressed register onto the bi-directional data
bus, D[7:0].
A.6 Immediately after the µC/µP toggles the Read
Strobe signal "Low", the Framer device will tog-
gle the RDY_DTCK (READY) output pin "Low".
The Framer device does this in order to inform
the µC/µP that the data (to be read from the
data bus) is NOT READY to be latched into the
µC/µP.
A.7 After some settling time, the data on the bi-
directional data bus will stabilize and can be
read by the µC/µP. The XRT72L56 DS3/E3
Framer will indicate that this data is ready to be
read, by toggling the RDY_DTCK (Ready) sig-
nal "High".
A.8 After the µC/µP detects the RDY_DTCK signal
(from the XRT72L56 DS3/E3 Framer IC), it can
then will terminate the Read cycle by toggling
the RD_DS (Read Strobe) input pin "High".
Figure 29 presents an illustration of the behavior of
the Microprocessor Interface Signals, during the initial
Read Operation, within a Burst I/O Cycle for an Intel-
type µC/µP.
FIGURE 29. BEHAVIOR OF THE MICROPROCESSOR INTERFACE SIGNALS, DURING THE INITIAL READ OPERATION OF A
BURST CYCLE (INTEL TYPE PROCESSOR)
ALE_AS
A(11:0)
CS
D(7:0)
RD_DS
WR_R/W
RDY_DTCK
Address of "Initial" Target Register (Offset = 0x00)
Not Valid
Valid Data of
Offset = 0x00
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 XRT72L56 DS3/E3 Framer) for
this particular burst I/O access operation. In order to
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