XC3S400-4FT256C Xilinx Inc, XC3S400-4FT256C Datasheet - Page 28

• Manufacturer Part Number: XC3S400-4FT256C
• Description: SEMI CONDUCTOR
• Manufacturer: Xilinx Inc

Specifications of XC3S400-4FT256C

• Case: BGA
• Dc: 04+

Spartan-3 FPGA Family: Functional Description
Block RAM automatically performs a bus-matching func-
tion. When data are written to a port with a narrow bus, then
read from a port with a wide bus, the latter port will effec-
tively combine “narrow” words to form “wide” words. Simi-
larly, when data are written into a port with a wide bus, then
read from a port with a narrow bus, the latter port will divide
“wide” words to form “narrow” words. When the data bus
width is eight bits or greater, extra parity bits become avail-
able. The width of the total data path (w) is the sum of the
DI/DO bus width and any parity bits (p).
The width selection made for the DI/DO bus determines the
number of address lines according to the relationship
expressed below:
Table 13: Port Aspect Ratios for Port A or B
Block RAM Data Operations
Writing data to and accessing data from the block RAM are
synchronous operations that take place independently on
each of the two ports.
The waveforms for the write operation are shown in the top
half of the
WE and EN signals enable the active edge of CLK, data at
the DI input bus is written to the block RAM location
addressed by the ADDR lines.
There are a number of different conditions under which data
can be accessed at the DO outputs. Basic data access
always occurs when the WE input is inactive. Under this
condition, data stored in the memory location addressed by
28
54
DI/DO Bus Width
(w – p bits)
16
32
1
2
4
8
Figure
13,
Figure
Bus Width (p bits)
DIP/DOP
14, and
0
0
0
1
2
4
Figure
15. When the
Total Data Path
Width (w bits)
18
36
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1
2
4
9
In turn, the number of address lines delimits the total num-
ber (n) of addressable locations or depth according to the
following equation:
The product of w and n yields the total block RAM capacity.
Equations (1) and (2) show that as the data bus width
increases, the number of address lines along with the num-
ber of addressable memory locations decreases. Using the
permissible DI/DO bus widths as inputs to these equations
provides the bus width and memory capacity measures
shown in
the ADDR lines passes through a transparent output latch
to the DO outputs. The timing for basic data access is
shown in the portions of
Figure 15
Data can also be accessed on the DO outputs when assert-
ing the WE input. This is accomplished using two different
attributes:
Choosing the WRITE_FIRST attribute, data is written to the
addressed memory location on an enabled active CLK edge
and is also passed to the DO outputs. WRITE_FIRST timing
is shown in the portion of
High.
Width (r bits)
ADDR Bus
Table
during which WE is Low.
14
13
12
11
10
9
r = 14 – [log(w–p)/log(2)]
13.
n = 2
Locations (n)
Addressable
Figure 13
r
16,384
No. of
8,192
4,096
2,048
1,024
512
Figure
DS099-2 (v2.2) May 25, 2007
13,
Product Specification
during which WE is
Figure
Block RAM
Capacity
16,384
16,384
16,384
18,432
18,432
18,432
(bits)
14, and
(1)
(2)
R