DS92UT16TUF Datasheet, PDF (18/86 Pages) National Semiconductor (TI) – UTOPIA-LVDS Bridge for 1.6 Gbps Bi-directional Data Transfers

8.0 UTOPIA Interface Operation
(Continued)
nected Port are defined by the UCSPL register. If a bit
location in the UCSPL register is set, then that sub-port is
connected.
In Figure 11, the registers are set as follows: UCPL3 =
UCPL2 = UCPL1 = 0xF, UCPL0 = 0xFD, and UCSPL = 0xEF.
With bit 1 of UCPL0 cleared, then Port 1 is not connected.
This means that none of the eight sub-ports of Port 1 are
connected. So Port 1 Sub-port 7, Port 1 Sub-port 6, Port 1
Sub-port 5, Port 1 Sub-port 4, Port 1 Sub-port 3, Port 1
Sub-port 2, Port 1 Sub-port 1, and Port 1 Sub-port 0 are not
connected. Port 1 will therefore, not be polled.
With bit 7 of UCSPL cleared, then sub-port 7 is not con-
nected. This means that sub-port 7 for all possible 31 ports is
not connected. So Port 31 Sub-port 7, Port 30 Sub-port 7,
Port 29 Sub-port 7,......and Port 0 Sub-port 7 are not con-
nected.
Therefore, clearing a bit in the UCPL3–UCPL0 registers will
disconnect 8 possible PHY port locations and clearing a bit
in the UCSPL register will disconnect 31 possible PHY port
locations.
FIGURE 11. Connected Port and Connected Sub-Port Usage
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9.0 MTB Queue Configuration
9.1 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 Section 6.3.5 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 mecha-
nism, 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 12. These are recommended
maximum thresholds only.
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