80KSBR200 Datasheet, PDF (28/172 Pages) Integrated Device Technology – sRIO SERIAL BUFFER FLOW-CONTROL DEVICE

IDT 80KSBR200
Notes
Advanced Datasheet*
4.6 Waterlevel and Watermarks
In the SerB, the "watermark" is a programmable event trigger threshold for the data level associated with a queue. The
term "waterlevel" refers to the actual data level within a queue, regardless of whether a "watermark" is used. When the
waterlevel in the queue reaches the watermark, an event will take place, depending upon the configuration. The waterlevel
and watermarks are used primarily as an indicator to control data flow within a queue.
The waterlevels are different than the PAF and PAE flags, because PAE/PAF flags deal in increments of total memory
space (1/256th of the queue total) and do not deal with actual data counts. Waterlevels actually count packets or bytes as
designated.
4.6.1 Waterlevel Controls
Waterlevels are primarily controlled in the configuration registers. The following items are available for controlling the
waterlevels:
◆ _{Data in queue is held in packets.}
◆ _{Waterlevel - A counter that holds the actual data level in a queue. The count will be in packets.}
◆ _{Watermark - This word holds the trigger point for the waterlevel. When the waterlevel reaches this point, the}
flag will be set. The flag may cause other events to occur (doorbells, interrupts, etc.)
◆ _{A single packet at a time will be sent in their original sizes. Packets will continue to be sent, until the waterlevel}
drops below the watermark. The remaining data will be held in the queue.
◆ _{Space Available - This is a word in the waterlevel register that indicates how much space is remaining in the}
queue. The space is in packets. The value of the counter is the total capacity of the queue minus the number
of bytes already consumed.
It should be noted that when operating as a waterlevel master, the "master mode" only affects the queue output. It is
still possible to receive write commands on the queue input.
4.6.2 Example Uses of Waterlevels
There are several possible uses of waterlevels and watermarks. A few of the suggested applications are as follows.
Stable Data Level in Queue
This application allows the steady maintenance of a data level in a queue. As data is received, it is stored in the queue
until the data level reaches the watermark. Upon the waterlevel reaching the watermark, for every packet received, the
queue will transmit an equivalent data item. The following items set up this scenario:
◆ _{The queue is set up to be a master}
◆ _{The waterlevel is programmed to count packets}
◆ _{The watermark is set to the desired number of packets to be held within the queue at all times}
◆ _{The queue sits idle when the packet within the queue is less than the watermark. Packets are received, but}
not transmitted
◆ _{If the waterlevel reaches or exceeds the watermark, the queue will transmit enough packets to bring the}
waterlevel back below the watermark.
PPS Specific Use of this Scenario
The basestation application that uses the PPS requires that there be a specific timed delay between the SerB input and
output packets. The delay is dependent upon the system requirements, but once the system is configured, it remains fixed.
It could be any designated delay, but the maximum in the TI DSP application is 10ms. The quantity of packets that would
accumulate within the designated time frame would be dependent upon how many RF cards are used in the basestation.
The PPS issues packets to the SerB on a stable time interval, meaning that by using the watermark to designate a
quantity of packets, a time interval can be derived from the total. Using the watermark to trigger packet transmissions, the
SerB may be used as a programmed packet delay.
In the typical PPS application, all packets will be identical in length and at equal time intervals. Usually the PPS will
reform packets to all be equal in size regardless of the number of antennas, but in some (rare) PPS applications that have
multiple antennas, it may be possible for the PPS to send packets of various sizes to the SerB. This should cause no prob-
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March 19, 2007
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