XC3S400-4FT256C Xilinx Inc, XC3S400-4FT256C Datasheet - Page 42

• 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
Table 24: BUFGMUX Select Mechanism
The two clock inputs can be asynchronous with regard to
each other, and the S input can change at any time, except
for a short setup time prior to the rising edge of the presently
selected clock (I0 or I1). Violating this setup time require-
ment can result in an undefined runt pulse output.
The BUFG clock buffer primitive drives a single clock signal
onto the clock network and is essentially the same element
as a BUFGMUX, just without the clock select mechanism.
Similarly, the BUFGCE primitive creates an enabled clock
buffer using the BUFGMUX select mechanism.
Each BUFGMUX buffers incoming clock signals to two pos-
sible destinations:
1. The vertical spine belonging to the same side of the die
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— top or bottom — as the BUFGMUX element in use.
The two spines — top and bottom — each comprise
four vertical clock lines, each running from one of the
BUFGMUX elements on the same side towards the
center of the die. At the center of the die, clock signals
S Input
0
1
O Output
I0 Input
I1 Input
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2. The clock input of either DCM on the same side of the
Use either a BUFGMUX element or a BUFG (Global Clock
Buffer) element to place a Global input in the design. For the
purpose of minimizing the dynamic power dissipation of the
clock network, the Xilinx development software automati-
cally disables all clock line segments that a design does not
use.
A global clock line ideally drives clock inputs on the various
clocked elements within the FPGA, such as CLB or IOB
flip-flops or block RAMs. A global clock line also optionally
drives combinatorial inputs. However, doing so provides
additional loading on the clock line that might also affect
clock jitter. Ideally, drive combinatorial inputs using the sig-
nal that also drives the input to the BUFGMUX or BUFG ele-
ment.
For more details, refer to the “Using Global Clock
Resources” chapter in UG331.
reach the eight-line horizontal spine, which spans the
width of the die. In turn, the horizontal spine branches
out into a subsidiary clock interconnect that accesses
the CLBs.
die — top or bottom — as the BUFGMUX element in
use.
DS099-2 (v2.2) May 25, 2007
Product Specification
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