XRT72L50 Datasheet, PDF (423/471 Pages) Exar Corporation – SINGLE CHANNEL DS3/E3 FRAMER IC WITH HDLC CONTROLLER

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XRT72L50
SINGLE CHANNEL DS3/E3 FRAMER WITH HDLC CONTROLLER
REV. 1.2.1
Clearing the FERF Condition
The Receive E3 Framer block will clear the FERF condition once it has received a User-Selectable number of
E3 frames is either 3 or 5 depending upon the value that has been written into Bit 4 (Rx FERF Algo) of the Rx
E3 Configuration/Status Register, as discussed above.
Whenever the Receive E3 Framer clears the FERF status, then it will do the following:
1. Generate a Change in the FERF Status Interrupt to the Microprocessor.
2. Clear the Bit 0 (RxFERF) within the Rx E3 Configuration & Status register, as depicted below.
RxE3 Configuration & Status Register 2 (Address = 0x11)
BIT 7
BIT 6
BIT 5
BIT 4
BIT 3
BIT 2
BIT 1
BIT 0
Rx LOF Algo RxLOF
RxOOF
RxLOS
RxAIS
RxPld Unstab
Rx
TMark
RxFERF
R/W
RO
RO
RO
RO
RO
RO
RO
0
0
0
0
0
0
0
0
6.3.2.7 Error Checking of the Incoming E3 Frames
The Receive E3 Framer block performs error-checking on the incoming E3 frame data that it receives from the
Remote Terminal Equipment. It performs this error-checking by computing the BIP-8 value of an incoming E3
frame. Once the Receive E3 Framer block has obtained this value, it will compare this value with that of the EM
byte that it receives, within the very next E3 frame. If the locally computed BIP-8 value matches the EM byte of
the corresponding E3 frame, then the Receive E3 Framer block will conclude that this particular frame has
been properly received. The Receive E3 Framer block will then inform the Remote Terminal Equipment of this
fact by having the Local Terminal Equipment Transmit E3 Framer block send the Remote Terminal an E3 frame,
with the FEBE bit-field, within the MA byte, set to “0”.
This procedure is illustrated in Figure 184 and Figure 185.
Figure 184 illustrates the Local Receive E3 Framer receiving an error-free E3 frame. In this figure, the locally
computed BIP-8 value of 0x5A matches that received from the Remote Terminal, within the EM byte-field.
Figure 185 illustrates the subsequent action of the Local Transmit E3 Framer block, which will transmit an E3
frame to the Remote Terminal, with the FEBE bit-field set to “0” . This signaling indicates that the Local
Receive E3 Framer has received an error-free E3 frame.
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