Xilinx 7 Series User Manual page 39

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Other capabilities of BUFGCTRL are:
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Table 2-4
Table 2-4: BUFGCTRL Attributes
Notes:
1. Both PRESELECT attributes cannot be TRUE at the same time.
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.
the relationship of BUFG and BUFGCTRL. The LOC constraint is available for manually
placing the BUFG location. See the Constraints Guide for more information.
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7 Series FPGAs Clocking Resources User Guide
UG472 (v1.5) July 13, 2012
Before time event 1, output O uses input I0.
At time T , before the rising edge at time event 1, both CE0 and S0 are
BCCCK_CE
deasserted Low. At about the same time, both CE1 and S1 are asserted High.
At time T , after time event 3, output O uses input I1. This occurs after a High
BCCKO_O
to Low transition of I0 (event 2) followed by a High to Low transition of I1.
At time event 4, IGNORE1 is asserted.
At time event 5, CE0 and S0 are asserted High while CE1 and S1 are deasserted Low.
At T , after time event 6, output O has switched from I1 to I0 without
BCCKO_O
requiring a High to Low transition of I1.
Pre-selection of the I0 and I1 inputs are made after configuration but before device
operation.
The initial output after configuration can be selected as either High or Low.
Clock selection using CE0 and CE1 only (S0 and S1 tied High) can change the clock
selection without waiting for a High to Low transition on the previously selected
clock.
summarizes the attributes for the BUFGCTRL primitive.
Attribute Name
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.
PRESELECT_I0 If TRUE, BUFGCTRL output will use the I0 input
after configuration
PRESELECT_I1 If TRUE, BUFGCTRL output will use the I1 input
after configuration
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Global Clocking Resources
Description
(1)
(1)
Possible Values
0 (default), 1
FALSE (default),
TRUE
FALSE (default),
TRUE
Figure 2-5 illustrates
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