![](http://datasheet.mmic.net.cn/Exar-Corporation/XRT79L71IB-F_datasheet_100145/XRT79L71IB-F_464.png)
XRT79L71
PRELIMINARY
449
1-CHANNEL DS3/E3 CLEAR-CHANNEL FRAMERLIU COMBO - CC/HDLC ARCHITECTURE
REV. P2.0.0
frame being applied to the "TxNib[3:0]" input pins. Once the XRT79L71 detects the rising edge of the input at
its "TxFrameRef" input pin, it will begin to generate and transmit a new E3 frame.
NOTES:
1.
In this particular mode, the System-Side Terminal Equipment is controlling the start of "Frame Generation" and is
referred to as the "Frame Master".
Since the XRT79L71 does not control nor dictate the instant that it will
generate a new E3 frame, but is driven by the "System-Side" Terminal Equipment, it is referred to as the "Frame
Slave".
2.
If the XRT79L71 is configured to operate in "Mode 5", then it is imperative that the "Tx_Start_of_Frame" or
"TxFrameRef" signal is synchronized to the "TxInClk" input clock signal.
Failure to do this will result in the
transmission of erred E3 data to the remote terminal equipment.
The Transmit Payload Data Input Interface block's handling of E3 Overhead Bytes when configured to
operate in the "Nibble-Parallel" Mode
In contrast to the DS3 Framing formats (which are "bit-oriented" framing formats), the E3, ITU-T G.832 framing
format is a "byte-oriented" framing format. As a consequence, there will be cases in which the Transmit
Payload Data Input Interface (within the XRT79L71) will be processing an "E3 overhead nibble", and the
"TxOH_Ind" output pin (in this case) DOES have meaning. In "Mode 5" Operation, the XRT79L71 will pulse its
"TxOH_Ind" output pin "HIGH" one nibble-period prior to the instant that it will process a given "Overhead"
nibble within the outbound E3 frame.
Since the "TxOH_Ind" output pin of the XRT79L71 is electrically
connected to the "E3_Overhead"_Ind" input pin (of the System-Side Terminal Equipment); whenever the
XRT79L91 device pulses its "TxOH_Ind" output pin "HIGH", it will also drive the "E3_Overhead_Ind" input pin
(of the System-Side Terminal Equipment) "HIGH". Whenever the "System-Side Terminal Equipment" detects
this pin toggling "high" it should delay the transmission of the very next E3 payload nibble by one "TxNibClk"
clock period.
NOTE:
Since the E3, ITU-T G.832 Frame consists of overhead bytes (in lieu of overhead nibbles), whenever the
"TxOH_Ind" output pin (of the XRT79L71) pulses "high" it will do so for four (4) consecutive nibble-periods (when
processing the FA1 and FA2 bytes) and it will do so for two (2) consecutive bit-periods, when processing the
remaining five (5) overhead bytes. Therefore, for the E3, ITU-T G.832 framing format, whenever the "System-Side
Terminal Equipment" detects the "TxOH_Ind" output pin being pulled "high", it is expected to (1) continuously
sample the state of the "TxOH_Ind" output pin with each rising edge of "TxNibClk" and (2) to NOT transmit an E3
payload bit (to the Transmit Payload Data Input Interface block) until it samples the "TxOH_Ind" output pin toggling
"low".
The Frequency of TxNibClk for E3, Nibble-Parallel Mode Operation
In contrast to that for the DS3 framing formats, for E3 Applications (both ITU-T G.832 and ITU-T G.751 framing
formats) the frequency of the TxNibClk clock signal is exactly one-fourth of the frequency of the "TxInClk"
signal.