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Virtex-4 SelectIO Primitives
These five generic primitive names represent most of the available differential I/O
standards:
•
IBUFDS (input buffer)
•
IBUFGDS (clock input buffer)
•
OBUFDS (output buffer)
•
OBUFTDS (3-state output buffer)
•
IOBUFDS (input/output buffer)
IBUF and IBUFG
Signals used as inputs to Virtex-4 devices must use an input buffer (IBUF). The generic
Virtex-4 IBUF primitive is shown in Figure 6-17.
IBUF/IBUFG
I (Input)
From device pad
O (Output)
into FPGA
ug070_6_17_071904
Figure 6-17:
Input Buffer (IBUF/IBUFG) Primitives
The IBUF and IBUFG primitives are the same. IBUFGs are used when an input buffer is
used as a clock input. In the Xilinx software tools, an IBUFG is automatically placed at
clock input sites.
OBUF
An output buffer (OBUF) must be used to drive signals from Virtex-4 devices to external
output pads. A generic Virtex-4 OBUF primitive is shown in Figure 6-18.
OBUF
I (Input)
From FPGA
O (Output)
to device pad
ug070_6_18_071904
Figure 6-18: Output Buffer (OBUF) Primitive
Virtex-4 User Guide
UG070 (v1.5) March 21, 2006
www.xilinx.com
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