Advance Information
MT9074
updated by each end of packet (closing flag)
received and therefore should be read when an end
of packet is received so that the next packet does not
overwrite the registers.
the 1.544 Mhz clock. No phase drift or wander can
exist between the two signals - therefore no slips will
occur. The delay through the transmit elastic buffer is
then fixed, and is a functions of the relative mapping
between the DSTi channels and the DS1 timeslots.
These delays vary with the position of the channel in
the frame. For example, DS1 timeslot 1 sits in the
elastic buffer for approximately 1 usec and DS1
timeslot 24 sits in the elastic buffer for approximately
32 usec.
Slip Buffers
Slip Buffer in T1 mode
In T1 mode MT9074 contains two sets of slip buffers,
one on the transmit side, and one on the receive
side. Both sides may perform a controlled slip. The
mechanisms that govern the slip function are a
function of backplane timing and the mapping
between the ST-BUS channels and the DS1
channels. The slip mechanisms are different for the
transmit and receive slip buffers. The extracted 1.544
Mhz clock (E1.5o) and the internally generated
transmit 1.544 Mhz clock are distinct. Slips on the
transmit side are independent from slips on the
receive side.
If the system 4.096 Mhz clock (C4b) is externally
generated (pin BS/LS high), the transmit 1.544 Mhz
clock is phase locked to it, but the PLL is designed to
filter jitter present in the C4b clock. As a result phase
drift will result between the two signals. The delay
through the transmit elastic buffer will vary in
accordance with the input clock drift, as well as being
a function of the relative mapping between the DSTi
channels and the DS1 timeslots. If the read pointers
approach the write pointers (to within approximately
1 usec) or the delay through the transmit buffer
exceeds 218 usecs a controlled slip will occur. The
contents of a single frame of DS1 data will be
skipped or repeated; a maskable interrupt (masked
by setting bit 1 - TxSLPI high in Interrupt Mask Word
Zero - page 1H, address 1bH) will be generated, and
the status bit TSLIP (page 3H, address 17H) of MSB
Transmit Slip Buffer register will toggle. The direction
of the slip is indicated by bit 6 of the same register
The transmit slip buffer has data written to it from the
near end 2.048 Mb/s stream. The data is clocked out
of the buffer using signals derived from the transmit
1.544 Mhz clock. The transmit 1.544 Mhz clock is
always phase locked to the DSTi 2.048 Mb/s stream.
If the system 4.096 Mhz clock (C4b) is internally
generated (pin BS/LS low), then it is hard locked to
0 uS
Write
Pointer
Read Pointer
4 uS
92 uS
Read Pointer
221 uS
Wander Tolerance
512 Bit
Elastic
Store
62 uS
188 uS
92 uS
96 uS
Read Pointer
129 uS
Read Pointer
Read Vectors
Minimum Delay
Frame 0
Frame 1
Frame 0
Frame 1
Write Vectors
Frame 0
Frame 1
Read Vectors - Maximum Delay
Figure 12 - Read and write pointers in the transmit slip buffers
27
MITEL [ MITEL NETWORKS CORPORATION ]
