Pin-to-pin vs. Detailed Descriptions for Registered Designs

ABEL-HDL assignment operators can be used when writing high-level equations. The = operator specifies a combinational assignment, where the design is written with only the circuit's inputs and outputs in mind. The := operator specifies a registered assignment, where you must consider the internal circuit elements (such as output inverters, presets and resets) related to the memory elements (typically flip-flops). The semantics of these two assignment operators is discussed below.

Using := for Pin-to-pin Descriptions

The := implies a memory element is associated with the output defined by the equation. For example, the equation

Q1 := !Q1 # Preset;

implies that Q1 will hold its current value until the memory element associated with that signal is clocked (or unlatched, depending on the register type).

This equation is a pin-to-pin description of the output signal Q1. The equation describes the signal's behavior in terms of desired output pin values for various input conditions. Pin-to-pin descriptions are useful when describing a circuit that is completely architecture-independent.

Language elements that are useful for pin-to-pin descriptions are the := operator, and the .CLK, .OE, .FB, .CLR, .ACLR, .SET, .ASET and .COM dot extensions. These dot extensions help to resolve circuit ambiguities when describing architecture-independent circuits.

Pin-to-pin Descriptions

A pin-to-pin description of a device-based circuit is a description that refers only to the relationship between the device's input pins and corresponding values observed on the output pins. Pin-to-pin descriptions do not include information about macrocell configurations, flip-flop types, or the precise form of the logic programmed into the device.

The pin-to-pin example contains a complete pin-to-pin specification of a simple 2-bit counter with a reset.

Detailed Descriptions

Pin-to-pin designs are easier to move to alternative device types, and are easier to comprehend. However, detailed designs using ABEL dot extensions are often required to describe complex or device-specific features in a circuit. These device features are usually associated with flip-flops and cannot be described using pin-to-pin design descriptions.

The detailed example contains the same design as the example above, except using detailed methods to take advantage of devices that have a built-in register reset.

Mixed Descriptions

You can mix pin-to-pin and detailed description methods in the same design. The mixed example contains a source file where only the reset function is described using detailed design methods; the rest of the circuit is described pin-to-pin.