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MT8979 参数 Datasheet PDF下载

MT8979图片预览
型号: MT8979
PDF下载: 下载PDF文件 查看货源
内容描述: ISO- CMOS ST- BUS⑩家庭CEPT PCM 30 / CRC - 4成帧器和接口 [ISO-CMOS ST-BUS⑩ FAMILY CEPT PCM 30/CRC-4 Framer & Interface]
分类和应用: PC
文件页数/大小: 26 页 / 343 K
品牌: MITEL [ MITEL NETWORKS CORPORATION ]
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ISO-CMOS MT8979  
125µs  
CHANNEL  
31  
CHANNEL  
30  
CHANNEL  
0
CHANNEL  
0
CHANNEL  
31  
• • •  
Most  
Significant  
Bit (First)  
Least  
Significant  
Bit (Last)  
BIT  
0
BIT  
7
BIT  
6
BIT  
3
BIT  
2
BIT  
1
BIT  
5
BIT  
4
(8/2.048)µs  
Figure 7 - ST-BUS Stream Format  
the ERR bit can be used to evaluate the bit error  
rate of the line according to the CCITT  
Recommendation G.732 (see section on Frame  
Alignment Error Counter).  
selected as the clock source for the PBX) then the  
data rate at which the data is being written into the  
device on the line side may differ from the rate at  
which it is being read out on the ST-BUS side.  
Channel 19 contains the Phase Status Word (see  
Table 15), which can be used to determine the phase  
relationship between the ST-BUS frame pulse (F0i)  
and the rising edge of E8Ko. This information could  
be used to determine the long term trend of the  
received data rate, or to identify the direction of a  
slip.  
When the clocks are not phase-locked, two  
situations can occur:  
Case #1: If the data on the line side is being written  
in at a rate SLOWER than it is being read out on the  
ST-BUS side, the distance between the write pointer  
and the read pointer will begin to decrease over time.  
When the distance is less than two channels, the  
buffer will perform a controlled slip which will move  
the read pointers to a new location 34 channels  
away from the write pointer. This will result in the  
REPETITION of the received frame.  
Channel 20 contains the CRC error count (see Table  
16). This counter will wrap around once terminal  
count is achieved (256 errors). If the maintenance  
option is selected (bit 3 of MCW3) the counter is  
reset once per second.  
Case #2: If the data on the line side is being written  
in at a rate FASTER than it is being read out on the  
ST-BUS side, the distance between the write pointer  
and the read pointer will begin to increase over time.  
When the distance exceeds 42 channels, the elastic  
buffer will perform a controlled slip which will move  
the read pointer to a new location ten channels away  
from the write pointer. This will result in the LOSS of  
the last received frame.  
Channel 21 contains the Master Status Word 2 (see  
Table 17). This byte identifies the status of the CRC  
reframe and CRC sync. It also reports the Si bits  
received in timeslot 0 of frames 13 and 15 and  
the ninth and most significant bit (b ) of the 9-bit  
8
Phase Status Word.  
Elastic Buffer  
Note that when the device performs a controlled slip,  
the ST-BUS address pointer is repositioned so that  
there is either a 10 channel or 34 channel delay  
between the input CEPT frame and the output  
The MT8979 has a two frame elastic buffer at the  
receiver, which absorbs the jitter and wander in the  
received signal. The received data is written into the  
elastic buffer with the extracted E2i (2048 kHz) clock  
and read out of the buffer on the ST-BUS side with  
the system C2i (2048 kHz) clock (e.g., PBX system  
clock). Under normal operating conditions, in a  
synchronous network, the system C2i clock is  
phase-locked to the extracted E2i clock. In this  
situation every write operation to the elastic buffer is  
followed by a read operation. Therefore, underflow  
or overflow of data in the elastic buffer will not occur.  
ST-BUS frame. Since the buffer performs  
a
controlled slip only if the delay exceeds 42 channels  
or is less than two channels, there is a minimum  
eight channel hysteresis built into the slip  
mechanism. The device can, therefore, absorb eight  
channels or 32.5µs of jitter in the received signal.  
There is no loss of frame synchronization, multiframe  
synchronization or any errors in the signalling bits  
when the device performs a slip.  
If the system clock is not phase-locked to the  
extracted clock (e.g., lower quality link which is not  
4-167