XRT74L73 Datasheet, PDF (465/488 Pages) Exar Corporation – 3 CHANNEL, ATM UNI/PPP DS3/E3 FRAMING CONTROLLER

PRELIMINARY
XRT74L73
3 CHANNEL, ATM UNI/PPP DS3/E3 FRAMING CONTROLLER
REV. P1.0.1
FIGURE 213. HOW TO INTERFACE THE TERMINAL EQUIPMENT TO THE RECEIVE OVERHEAD DATA OUTPUT INTER-
FACE BLOCK FOR METHOD 2
E3_OH_In
RxOH
E3_OH_Enable_In
E3_Clk_In
Rx_Start_of_Frame
RxOHEnable
RxOutClk
RxOHFrame
Terminal Equipment
XRT74L73 E3 Framer
Method 2 Operation of the Terminal Equipment
If the Terminal Equipment intends to sample any
overhead data from the Inbound E3 data stream (via
the Receive Overhead Data Output Interface), then it
is expected to do the following.
1. Sample the state of the RxOHFrame signal (e.g.,
the Rx_Start_of_Frame input) on the falling edge
of the RxOutClk clock signal, whenever the RxO-
HEnable output signal is also sampled “High”.
2. Keep track of the number of times that the RxO-
HEnable signal has been sampled “High” since
the last time the RxOHFrame was also sampled
“High”. By doing this, the Terminal Equipment will
be able to keep track of which overhead bit is
being output via the RxOH output pin. Based
upon this information, the Terminal Equipment
will be able to derive some meaning from these
overhead bits.
3. Table 106 relates the number of RxOHEnable
output pulses (that have occurred since both the
RxOHFrame and the RxOHEnable pins were
both sampled “High”) to the E3 overhead bit that
is being output via the RxOH output pin.
TABLE 106: THE RELATIONSHIP BETWEEN THE NUMBER OF RXOHENABLE OUTPUT PULSES (SINCE RXOHFRAME
WAS LAST SAMPLED "HIGH") TO THE E3 OVERHEAD BIT, THAT IS BEING OUTPUT VIA THE RXOH OUTPUT PIN
NUMBER OF RXOHENABLE OUTPUT PULSES
THE OVERHEAD BIT BEING OUTPUT BY THE
XRT74L73
0 (Clock edge is coincident with RxOHFrame being detected “High”)
1
2
3
4
5
6
7
8
9
FA1 Byte - Bit 7
FA1 Byte - Bit 6
FA1 Byte - Bit 5
FA1 Byte - Bit 4
FA1 Byte - Bit 3
FA1 Byte - Bit 2
FA1 Byte - Bit 1
FA1 Byte - Bit 0
FA2 Byte - Bit 7
FA2 Byte - Bit 6
466