Detailed Descriptions: Designing for Macrocells

One way to understand the difference between pin-to-pin and detailed description methods is to think of detailed descriptions as macrocell specifications. A macrocell is a block of circuitry normally (but not always) associated with a device's I/O pin. The following diagram illustrates a typical macrocell associated with signal Q1:

Detailed descriptions are written for the various input ports (the diagram shows them with dot extension labels) of the macrocell. Note that the macrocell shown features a configurable inversion between the Q output of the flip-flop and the output pin labeled Q1. If this inverter is used (or if a device is selected that features a fixed inversion), then the behavior seen on the Q1 output pin will be inverted from the logic applied to or observed on the various macrocell ports, including the feedback port Q1.q.

Pin-to-pin descriptions, on the other hand, allow you to describe your circuit in terms of the behavior expected on an actual output pin, regardless of the architecture of the underlying macrocell. The following diagram illustrates the pin-to-pin concept:

When pin-to-pin descriptions are written in ABEL-HDL, the "generic macrocell" shown above is synthesized from whatever type of macrocell actually exists in the target device.