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PRELIMINARY
XRT79L71
242
REV. P2.0.0
1-CHANNEL DS3/E3 CLEAR-CHANNEL FRAMERLIU COMBO - CC/HDLC ARCHITECTURE
The Two Methods of Extracting out Overhead Data from the Receive Overhead Data Output Interface
block
There are two methods that can be used to extract the overhead data from the Receive Overhead Data Output
Interface block. One Method is referred to as Method 1 or the RxOHClK Method, and the other method is
referred to as Method 2 or the RxClk/RxOHEnable Method.
Each of these methods is described in
considerable detail below.
4.3.5.1
Operating the Receive Overhead Data Output Interface Block using Method 1 - The
RxOHClk Method
This particular method is referred to as the RxOHClk method for the following reasons.
a. The System-Side Terminal Equipment will use the RxOHClk clock output signal from the Receive
Overhead Data Output Interface block to sample and latch the overhead data via the RxOH output pin.
b. The Receive Overhead Data Output Interface block will update the data via the RxOH output pin upon
the falling edge of the RxOHClk clock output signal.
If Method 1 is used, the System-Side Terminal Equipment will need to interface to the following Receive
Overhead Data Output Interface pins.
RxOHFrame
A8
O
Receive Overhead Data Output Interface block - Start of Frame Indicator
Output pin:
The exact approach to used to sample this output pin depends upon whether
extract data from the Receive Overhead Data Output Interface block, as
described below and in the following sections.
If Method 1 is used:
The XRT79L71 will pulse this output pin "High" for one period of RxOHClk coin-
cident to whenever the Receive Overhead Data Output Interface block outputs
the very first overhead bit of the most recently received DS3 frame. This output
pin will be "Low" at all other times. The Receive Overhead Data Output Inter-
face block will update this output pin, upon the falling edge of RxOHClk.
Hence, the System-Side Terminal Equipment should be designed or configured
to sample the data, at this pin, upon the rising edge of RxOHClk.
If Method 2 is used:
The Receive Overhead Data Output Interface block will assert the RxOHEnable
output pin for one RxClk period whenever the data, residing on the RxOH out-
put pin has become stable and is safe for sampling. In this case, the user
should design or configure the System-Side Terminal Equipment to sample and
latch both the RxOH and the RxOHFrame output pins upon the falling edge of
RxClk coincident to whenever the RxOHEnable output pin is sampled "High".
RxOHEnable
B7
O
Receive Overhead Data - Enable Output Pin:
This particular output pin is only used if Method 2 is employed, (see
Section4.3.5.2 below). The Receive Overhead Data Output Interface will assert this
signal (e.g., pulse it "High") for one RxClk period, coincident to whenever it is
safe for the System-Side Terminal Equipment to sample the overhead bit that is
being output via the RxOH output pin.
This output pin will remain "Low" at all other times.
If Method 2 is used, then design or configure the System-Side Terminal Equip-
ment such that it will sample and latch the RxOH output from the XRT79L71,
upon the falling edge of RxClk, coincident to whenever the RxOHEnable output
pin is sampled "High".
TABLE 31: LIST AND DESCRIPTION OF THE PINS ASSOCIATED WITH THE RECEIVE OVERHEAD DATA OUTPUT
INTERFACE BLOCK
SIGNAL NAME
PIN/BALL #TYPE
DESCRIPTION