Xilinx virtex-5 fpga - Page 31

Virtex-5 FPGA User Guide www.xilinx.com 31
UG190 (v5.0) June 19, 2009
Global Clocking Resources
Table 1-4 summarizes the attributes for the BUFGCTRL primitive.
BUFG
BUFG is simply a clock buffer with one clock input and one clock output. This primitive is
based on BUFGCTRL with some pins connected to logic High or Low. Figure 1-3 illustrates
the relationship of BUFG and BUFGCTRL. A LOC constraint is available for BUFG.
The output follows the input as shown in the timing diagram in Figure 1-4.
Table 1-4: BUFGCTRL Attributes
Attribute Name Description Possible Values
INIT_OUT Initializes the BUFGCTRL output to the specified
value after configuration. Sets the positive or
negative edge behavior. Sets the output level when
changing clock selection.
0 (default), 1
PRESELECT_I0 If TRUE, BUFGCTRL output uses the I0 input after
configuration
(1)
FALSE (default),
TRUE
PRESELECT_I1 If TRUE, BUFGCTRL output uses the I1 input after
configuration
(1)
FALSE (default),
TRUE
Notes:
1. Both PRESELECT attributes cannot be TRUE at the same time.
2. The LOC constraint is available.
X-Ref Target - Figure 1-3
Figure 1-3: BUFG as BUFGCTRL
X-Ref Target - Figure 1-4
Figure 1-4: BUFG Timing Diagram
IGNORE1
IGNORE0
CE1
CE0
S1
S0
I1
I0
O
BUFG
ug190_1_03_032206
V
DD
GND
V
DD
V
DD
V
DD
I
O
I
GND
GND
BUFG(I)
BUFG(O)
T
BCCKO_O
ug190_1_04_032206