MC145572
7–4
MOTOROLA
The current
febe
count is maintained in BR4. The count in BR4 is incremented only when the re-
ceived
febe
bit is detected active (0) at the end of the superframe. When OR7(b1) is a 0, the
febe
counter does not wrap around when the count reaches $FF. When OR7(b1) is a 1, the
febe
counter
wraps around and continues counting from 0. Also, BR4 should be reset to 00 after Linkup is detected
during activation. This is done by the external microcontroller writing 00 to BR4. The count is increm-
ented when both Superframe Sync and Linkup in NR1(b1, b3) are set to a 1 and the received
febe
bit is a 0. Received
febe
is available in BR3(b4) and is a 0 when active.
The current
nebe
count is maintained in BR5. The count in BR5 is incremented only when the Com-
puted
nebe
bit is detected active (0) at the end of the superframe. The count is also incremented
once per superframe during loss of synchronization, i.e., if Superframe Sync, NR2(b1), drops to a
0 when Linkup, NR2(b3), is set to a 1. When OR7(b1) is a 0, the
nebe
counter does not wrap around
when the count reaches $FF. When OR7(b1) is a 1, the
nebe
counter wraps around and continues
counting from 0. Also, BR5 should be reset to 00 after Linkup is detected during activation. This is
done by the external microcontroller writing 00 to BR5. The count is incremented when both Super-
frame Sync and Linkup in NR1(b1, b3) are set to a 1 and when an error is detected in the received
crc
. A Computed
nebe
is active when the received
crc
does not exactly match the calculated
crc
on the received superframe data. The Computed
nebe
is available in BR3(b3) and is a 0 when a
crc
error has been detected.
crc
The MC145572 provides a mechanism where the outgoing
crc
can be corrupted. The transmitted
crc
is corrupted when BR8(b3) is set to a 1. The
crc
corruption is accomplished by inverting the
transmitted
crc
bits. See Table 7–1. The next two paragraphs are of particular interest to designers
of digital loop carrier system LULT and LUNT type line cards.
In NT mode operation, when it is desired to corrupt the outgoing
crc
, BR8(b3) should be set at the
end of reception of basic frame 4 and must be cleared at the end of reception of basic frame 8. This
inverts the outgoing
crc
in transmitted basic frames 4, 5, 6, and 7 of the current transmitted super-
frame. See Figure 7–2. When
crc
Corrupt mode, OR7(b2), is set to a 1, it is not necessary to clear
BR8(b3), since it is cleared automatically at the end of the transmitted superframe. This guarantees
that the corrupt
crc
will be transmitted only in the current superframe and that there will be a one–to–
one correspondence between the corrupt
crc
status received from a digital carrier system and the
corrupt
crc
transmitted on the U–interface. See
Section 7.7.
In LT mode operation, when it is desired to corrupt the outgoing
crc
, BR8(b3) should be set at the
end of reception of basic frame 8 and must be cleared at the end of reception of basic frame 8. This
inverts the outgoing
crc
in transmitted basic frames 1 through 8 of the current transmitted super-
frame. See Figure 7–3. When
crc
Corrupt mode, OR7(b2), is set to a 1, it is not necessary to clear
BR8(b3), since it is cleared automatically at the end of the transmitted superframe. This guarantees
that the corrupt
crc
will be transmitted only in the current superframe and that there will be a one–to–
one correspondence between the corrupt
crc
status received from a digital carrier system and the
corrupt
crc
transmitted on the U–interface. See
Section 7.7.
Table 7–1. Transmitted
crc
Configuration
ááááááááááááááááááááááááááá
ááááááááááááááááááááááááááá
ááááááááááááááááááááááááááá
á
á
áááááááááááááááááááááááááá
ááááááááááááááááááááááááááá
BR8(b3)
á
OR9(b2)
á
Effect on Transmitted
crc
ááá
ááá
0
ááá
ááá
X
No effect, transmitted
crc
is a good
crc
and far–end transceiver receives
á
á
Transmitted
crc
is corrupted only until the end of the current U–interface
returned to a 0 to stop the transmission of bad
crc
s.
á
á
Transmitted
crc
is continuously corrupted by inverting the
crc
symbols.
This causes the far–end transceiver to detect
crc
errors. BR8(b3) must be
1
1