2.3.3.8 Macros for Specific I/O Standards

There are different macro types for any I/O standard or feature that determine the required VCCI and VREF voltages for an I/O. The generic buffer macros require the LVTTL standard with slow slew rate and 24 mA-drive strength. LVTTL can support high slew rate but this should only be used for critical signals.

Most of the macro symbols represent variations of the six generic symbol types:

• CLKBUF: Clock Buffer
• HCLKBUF: Hardwired Clock Buffer
• INBUF: Input Buffer
• OUTBUF: Output Buffer
• TRIBUF: Tristate Buffer
• BIBUF: Bidirectional Buffer

Other macros include the following:

• Differential I/O standard macros: The LVDS and LVPECL macros either have a pair of differential inputs (for example, INBUF_LVDS) or a pair of differential outputs (for example, OUTBUF_LVPECL).
• Pull-up and pull-down variations of the INBUF, BIBUF, and TRIBUF macros. These are available only with TTL and LVCMOS thresholds. They can be used to model the behavior of the pull-up and pull-down resistors available in the architecture. Whenever an input pin is left unconnected, the output pin will either go high or low rather than unknown. This allows users to leave inputs unconnected without having the negative effect on simulation of propagating unknowns.
• DDR_REG macro. It can be connected to any I/O standard input buffers (that is, INBUF) to implement a double data rate register. Designer software will map it to the I/O module in the same way it maps the other registers to the I/O module.

The followings tables list all the available macro names differentiated by I/O standard, type, slew rate, and drive strength.