Pin-to-Pin Language Features

You do not need to specify a device in your ABEL-HDL source file. You can also omit pin numbers from signal declarations.

When you do not specify a device or pin numbers, you need to specify pin-to-pin attributes for declared signals, since the ABEL-HDL compiler cannot imply signal attributes from predetermined device attributes. If you do not specify signal attributes or other information (such as the dot extensions, which are described later), your design might not operate consistently if you later transfer it to a different target device.

Device-independence Vs. Architecture-independence

The requirement for signal attributes does not mean that a complex design must always be specified with a particular device in mind. You may still have to understand the differences between the specific device architectures that you might use, but you do not have to specify a particular device when describing your design.

The attributes and dot extensions provided in ABEL-HDL help you refine your design to work consistently when moving from one class of device architecture to another; for example from devices having inverted outputs to those with a particular kind of reset/preset circuitry. The more you refine your design using these language features, the more restrictive your design becomes in terms of the number of device architectures it is appropriate for. However, by using attributes and dot extensions carefully, you can avoid specifying a particular device type, and instead target your design to a more general class of device architectures.

Signal Attributes

Signal attributes remove ambiguities that occur when no specific device architecture is declared. If your design does not use device-related attributes (either implied by a device statement or expressed in an

ISTYPE statement), it may not operate the same way when targeted to different device architectures. See "Pin Declaration," "Node Declaration" and "Istype" in the Chapter "Language Reference."

Signal Dot Extensions

Signal dot extensions, like attributes, are a way you can more precisely describe the behavior of a circuit that may be targeted to different architectures. Dot extensions are applied to signals, and remove the ambiguities in equations.