DS92UT16 Datasheet, PDF (24/111 Pages) Texas Instruments – DS92UT16TUF UTOPIA-LVDS Bridge for 1.6 Gbps Bi-directional Data Transfers

DS92UT16
OBSOLETE
SNOS992E – JANUARY 2002 – REVISED APRIL 2013
MTB Queue Configuration
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SINGLE BRIDGE MTB CONFIGURATION
The Multi-port Traffic Buffer is a 160 cell linked-list buffer that is shared across as many as 31 Port queues.
There is a single queue per MPHY address.
In the up-bridge direction, a per queue flow control protocol (described in Flow Control) prevents queue overflow.
Each Port has a programmable upper fill threshold. Should any queue reach this upper threshold, back-pressure
is applied over the serial link, via the flow control mechanism, to the far end (transmitting) device. The
transmitting device uses the normal UTOPIA flow control handshaking to prevent any more cells being
transferred to that MPHY and thus prevents overflow.
With link-list buffers, each queue may be over-assigned memory space, working on the assumption that not
every queue will back up simultaneously. To accommodate the rare occasions where the buffer as a whole
approaches full but individual queues are below their full threshold, the device also compares the overall buffer
fill against a threshold. Should the overall buffer approach overflow, the flow control mechanism provides a global
‘halt’ command to ensure that no cells will be lost.
The MTB Queue Threshold, MTBQT30–MTBQT0 registers define the maximum size in PDU cells of each of the
31 queues. If all 31 queues are being used it is recommended that the threshold be left at the default of 4 cells. If
less than 31 queues are in use then the queue thresholds may be raised if required. The recommended
maximum queue thresholds are given in Table 11. These are recommended maximum thresholds only.
It is further recommended that any queue that is not being used is set with a threshold of zero. When a queue
has reached its programmed threshold the device flow control mechanism will prevent the far end device from
accepting cells for that MPHY address. Therefore, by setting the threshold of an unused queue to zero, it
prevents the UTOPIA interface of the far end device from accepting cells for that MPHY address by either, not
asserting the CLAV for that MPHY address when in PHY Mode, or not selecting that MPHY address when in
ATM mode.
Also, note that setting a threshold of zero will cause the corresponding Queue Full bit in the
MTBQFL3–MTBQFL0 registers to be continuously set for that queue.
Number of
Queues in Use
31
30
29
28
27
26
25
24
23
22
21
20
19
18
17
16
Table 11. Recommended Maximum MTB Queue Thresholds
Recommended
Threshold
4
4
5
5
5
6
6
7
7
8
9
10
10
11
12
4
Number of
Queues in Use
15
14
13
12
11
10
9
8
7
6
5
4
3
2
1
Recommended
Threshold
15
16
18
20
23
26
29
34
39
47
58
74
100
100
154
24
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