
PRELIMINARY
XRT86SH221
92
REV. P1.0.5
SDH-TO-PDH FRAMER/MAPPER WITH INTEGRATED 21-CHANNEL E1 SH LIU
FRAME COUNTERS AND TIMING GENERATION
Receive Frame and Multiframe counters and timing generators provide timing for frame and multiframe
alignment, CRC-4 check, signaling extraction, facility data link extraction, yellow alarm, and all the timing for
per-channel parameter fetch. The data extracted from this timing is placed into the appropriate internal storage
elements for the microprocessor to access. The information extracted is not valid unless the receive module
has achieved valid synchronization.
CRC-4 VERIFICATION
The CRC verification is performed by calculating the 4-bit CRC checksum for each incoming sub-multiframe
and comparing this result to the received CRC remainder bits in the subsequent sub-multiframe. The CRC
errors are accumulated over one second intervals. Optionally, a CRC frame resync can be initiated when 915
or more CRC errors occur in one second. The number of CRC errors accumulated during the previous second
is available by reading the E1 Receive Synchronization Bit Error Counter.
ALARM AND ERROR INDICATION
The Alarm indication logic examines the incoming E1 data for alarm conditions. When the change of an alarm
condition is detected, corresponding bits are set in the Alarm and Error Status Register. The Alarm and Error
Interrupt Enable Register is used to select the events that generate interrupts on the microprocessor interrupt
pin when their state changes.
LOF (Red Alarm) Defect/Alarm
When DEFDET=1, the Loss Of Frame defect is detected when "FASC" (in framing control register) consecutive
incorrect frame alignment signals have been received (default is 3 to comply with G.706). It is cleared when 2
consecutive FAS's are detected.
When DEFDET = 0, The red alarm is detected by monitoring the occurrence of Loss Of Frame (LOF) over a 4
ms interval. An LOF valid flag will be posted on the interval when one or more LOF occurred during the
interval. Each interval with a valid LOF flag increments a flag counter which declares RED alarm when 25 valid
intervals have been accumulated. An interval without valid LOF flag decrements the flag counter. The Red
alarm is removed when the counter reaches zero.
TRANSMIT SLIP BUFFERING
The Voyager-Lite has two-frame (512 bits) elastic stores. This store can be enabled or disabled via
programming bits SB_ENB in the Slip Buffer Control and Status Register (SBCSR). If the elastic buffer either
fills or empties, a controlled slip will occur. If the buffer empties and a read occurs, then a full frame of data will
be repeated and a status bit will be updated. If the buffer fills and a write comes, then a full frame of data will be
deleted and another status bit will be set. If the slip buffer is bypassed (SB_ENB[1:0] = 00 or 11), the slip buffer
is used as a regular JA buffer. If SB_ENB = 2, the slip buffer is put into a FIFO mode. In the FIFO mode, the
slip buffer is acting like a standard first-in-first-out storage. A fixed read and write latency is maintained in a
programmable fashion controlled by the FIFO Latency Register. However, the user should assume the
responsibility to phase lock the input clock to the receive clock to avoid either overrun or under-run. A Slip
Buffer Control & Status register is used to control the slip buffer operations and control interrupts and report its
status.