XRT7250 datasheet(64/463 Pages) EXAR

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DS3/E3 FRAMER IC

XRT7250

REV. 1.1.1

indicates that the µC/µP is reading the very first regis-

ter (or buffer location) in this burst access operation.

2.2.2.2.1.1.2

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.

B.0

Execute each subsequent Read Cycles, as

described in steps 1 through 3 below.

B.1

Without toggling the ALE_AS input pin (e.g.,

keeping it "Low"), toggle the RD_DS input pin

"Low". This step accomplishes the following.

a. The Framer will internally increments the latched

address value (within the Microprocessor Inter-

face circuitry).

b. The output drivers of the bi-directional data bus,

D[7:0] are enabled. At some time later, the regis-

ter or buffer location corresponding to the incre-

mented latched address value will be driven onto

the bi-directional data bus.

B.2

Immediately after the Read Strobe pin toggles

"Low" the Framer IC will toggle the RDY_DTCK

(READY) output pin "Low" to indicate its NOT

READY status.

B.3

After some settling time, the data on the bi-

directional data bus will stabilize and can be

read by the µC/µP. The XRT7250 DS3/E3

Framer will indicate that this data is ready to be

read by toggling the RDY_DTCK (READY) sig-

nal "High".

B.4

After the µC/µP detects the RDY_DTCK signal

(from the XRT7250 DS3/E3 Framer), it can

then terminates the Read cycle by toggling the

RD_DS (Read Strobe) input pin "High".

For subsequent read operations, within this burst cy-

cle, the µC/µP simply repeats steps 1 through 3, as il-

lustrated in Figure 30.

In addition to the behavior of the Microprocessor In-

terface signals, Figure 30 also illustrates other points

regarding the Burst Access Operation.

a. The Framer internally increments the address

value, from the original latched value shown in

Figure 29. This is illustrated by the data, appear-

ing on the data bus, (for the first read access)

being labeled Valid Data at Offset = 0x01 and that

for the second read access being labeled Valid

Data at Offset = 0x02.

b. The Framer performs this address incrementing

process even though there are no changes in the

Address Bus Data, A[8:0].

2.2.2.2.1.1.3

Terminating the Burst Access

Operation

The Burst Access Operation will be terminated upon

the rising edge of the ALE_AS input signal. At this

point the Framer will cease to internally increment the

latched address value. Further, the µC/µP is now free

to execute either a Programmed I/O access or to start

another Burst Access Operation with the XRT7250

DS3/E3 Framer.

2.2.2.2.1.2

The Intel-Mode Write Burst Access

Whenever an Intel-type µC/µP wishes to write data in-

to a contiguous range of addresses, then it should do

the following.

FIGURE 30. BEHAVIOR OF THE MICROPROCESSOR INTERFACE SIGNALS, DURING SUBSEQUENT READ OPERATIONS

WITHIN THE

BURST I/O CYCLE

ALE_AS

RD_DS

A[8:0]

D[15:0]

RDY_DTCK

Not Valid

Valid Data at Offset =0x01

WR_RW

Not Valid

Valid Data at Offset =0x02

Address of “Initial” Target Register (Offset = 0x00)

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