MT9074
Advance Information
(TSLPD). The relative phase delay between the
system frame boundary and the transmit elastic
frame read boundary is measured every frame and
reported in the Transmit Slip Buffer Delay register-
(page 3H, address 17H). In addition the relative
offset between these frame boundaries may be
programmed by writing to this register. Every write to
Transmit Elastic Buffer Set Delay Word resets the
transmit elastic frame count bit TxSBMSB (address
17H, page 3H). After a write the delay through the
slip buffer is less than 1 frame in duration. Each write
operation will result in a disturbance of the transmit
DS1 frame boundary, causing the far end to go out of
sync. Writing BC (hex) into the TxSBDLY register
maximizes the wander tolerance before a controlled
slip occurs. Under normal operation no slips should
occur in the transmit path. Slips will only occur if the
input C4b clock has excess wander, or the Transmit
Elastic Buffer Set Delay Word register is initialized
too close to the slip pointers after system
initialization.
tolerance). The MT9074 will allow 92 usec (140 UI,
DS1 unit intervals) of wander and low frequency jitter
before a frame slip will occur.
When the C4b and the E1.5o clocks are not phase-
locked, the rate at which data is being written into the
slip buffer from the DS1 side may differ from the rate
at which it is being read out onto the ST-BUS. If this
situation persists, the delay limits stated in the
previous paragraph will be violated and the slip
buffer will perform a controlled frame slip. That is, the
buffer pointers will be automatically adjusted so that
a full DS1 frame is either repeated or lost. All frame
slips occur on frame boundaries.
The minimum delay through the receive slip buffer is
approximately 1 usec and the maximum delay is
approximately 249 uS. Figure 13 illustrates the
relationship between the read and write pointers of
the receive slip buffer (contiguous time slot
mapping). Measuring clockwise from the write
pointer, if the read page pointer comes within 8 usec
of the write page pointer a frame slip will occur,
which will put the read page pointer 157 usec from
the write page pointer. Conversely, if the read page
pointer moves more than 249 usec from the write
page pointer, a slip will occur, which will put the read
page pointer 124 usec from the write page pointer.
This provides a worst case hysteresis of 92 usec
peak = 142 U.I.
The two frame receive elastic buffer is attached
between the 1.544 Mbit/s DS1 receive side and the
2.048 Mbit/s ST-BUS side of the MT9074. Besides
performing rate conversion, this elastic buffer is
configured as a slip buffer which absorbs wander
and low frequency jitter in multi-trunk applications.
The received DS1 data is clocked into the slip buffer
with the E1.5o clock and is clocked out of the slip
buffer with the system C4b clock. The E1.5o
extracted clock is generated from, and is therefore
phase-locked with, the receive DS1 data. In the case
of Internal mode (pin BS/LS set low) operation, the
E1.5o clock may be phase-locked to the C4b clock by
an internal phase locked loop (PLL). Therefore, in a
single trunk system the receive data is in phase with
the E1.5o clock, the C4b clock is phase locked to the
E1.5o clock, and the read and write positions of the
slip buffer track each other.
The RSLIP and RSLPD status bits (page 3H,
address 13H, bits 7 and 6 respectively) give
indication of a receive slip occurrence and direction.
A maskable interrupt RxSLPI (page 1H, address
1BH, bit 0 - set high to mask) is also provided. RSLIP
changes state in the event of a slip. If RSLPD=0, the
slip buffer has overflowed and a frame was lost; if
RSLPD=1, an underflow condition occurred and a
frame was repeated
In a multi-trunk slave or loop-timed system (i.e.,
PABX application) a single trunk will be chosen as a
network synchronizer, which will function as
described in the previous paragraph. The remaining
trunks will use the system timing derived from the
synchronizer to clock data out of their slip buffers.
Even though the DS1 signals from the network are
synchronous to each other, due to multiplexing,
transmission impairments and route diversity, these
signals may jitter or wander with respect to the
synchronizing trunk signal. Therefore, the C1.50
clocks of non-synchronized trunks may wander with
respect to the C1.50 clock of the synchronizer and
the system bus. Network standards state that, within
limits, trunk interfaces must be able to receive error-
free data in the presence of jitter and wander (refer
to network requirements for jitter and wander
Slip Buffer in E1 mode
In E1 mode in addition to the elastic buffer in the jitter
attenuator(JA), another elastic buffer (two frames
deep) is present, attached between the receive side
and the ST-BUS (or GCI Bus) side of the MT9074 in
E1 mode. This elastic buffer is configured as a slip
buffer which absorbs wander and low frequency jitter
in multi-trunk applications. The received PCM 30
data is clocked into the slip buffer with the E1.5o
clock and is clocked out of the slip buffer with the
C4b clock. The E1.5o extracted clock is generated
from, and is therefore phase-locked with, the receive
PCM 30 data. In normal operation, the C4b clock will
be phase-locked to the E1.5o clock by a phase
locked loop (PLL). Therefore, in a single trunk
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