Xilinx Constraints Guide

IOB (IOB)
The IOB constraint:
• Is a basic mapping and synthesis constraint.
Chapter 4: Xilinx Constraints
• Indicates which flip-flops and latches can be moved into the IOB/ILOGIC/OLOGIC.
The mapper supports a command line option (-pr i | o | b | off) that allows flip-flop
or latch primitives to be pushed into the input IOB (i), output IOB (o), or input/output
IOB (b) on a global scale. The IOB constraint, when associated with a flip-flop or latch,
tells the mapper to pack that instance into an IOB type component if possible. The IOB
constraint has precedence over the mapper -pr command line option. However, IOB
constraints do not have precedence over LOC constraints.
XST considers the IOB constraint as an implementation constraint, and therefore
propagates it in the generated NGC file. XST also duplicates the flip-flops and latches
driving the Enable pin of output buffers, so that the corresponding flip-flops and latches
can be packed in the IOB.
• TRUE
Allows the flip-flop or latch to be pulled into an IOB
• FALSE
Indicates not to pull it into an IOB
• AUTO
Used by XST only. XST takes into account timing constraints and automatically
decides to push or not to push flip-flops into IOBs
• FORCE
Architecture Support
Applicable Elements
Requires that the flip-flop or latch be pulled into an IOB, otherwise an error is
given. FORCE produces an error only if the register has I/O connections and cannot
be packed in the IOB.
Applies to FPGA devices. Does not apply to CPLD devices.
• Non-INFF/OUTFF flip-flop and latch primitives
• Registers
Propagation Rules
Applies to the design element to which it is attached.
Syntax Examples
The following examples show how to use this constraint with particular tools or
methods. If a tool or method is not listed, you cannot use this constraint with it.
Schematic Syntax
• Attach to a flip-flop or latch instance or to a register
• Attribute Name
IOB
Constraints Guide
UG625 (v. 13.2) July 6, 2011 www.xilinx.com 141