Xilinx V2.1 - Manual (Page 106)

106 Xilinx Development System
Xilinx System Generator v2.1 Reference Guide
FIFO
The Xilinx FIFO block implements a First-In-First-Out memory
queue.
Values presented at the module’s data-input port is written to the
next available empty memory location when the write-enable input
is one. The memory full status output port is assserted to one when
no unused locations remain in the module’s internal memory. The
percent full output port indicates the percentage of internal memory
in use, represented with user-specified precision. By asserting the
read-enable input port, data can be read out of the FFO via the data
output port (dout) in the order in which they were written. The
memory-empty status output (empty) indicates that no more data reside in the
memory.
The FIFO can be implemented either using distributed or block RAM. If distributed
memory is selected, the maximum depth of the FIFO is 256. If block RAM is used, the
maximum depth is 64K words.
Block Parameters Dialog Box
The block parameters dialog box can be invoked by double-clicking the icon in your
Simulink mode.
Figure 3-72: FIFO block parameters dialog box
Parameters specific to the FIFO block are:
•
Depth : specifies the number of words that can be stored.
•
Bits of Precision to Use for Percent Full Signal : specifies the
number of bits that will be output from the %full port. The binary point for this
unsigned output is always at the top of the word. Thus, if the Bits of
Precision is set to one, the output can take on two values: 0.0 and 0.5, the latter
indicating that the FIFO is at least 50% full. Given two bits of precision, the
possible output values are 0.00, 0.25, 0.50 and 0.75.