Detailed Circuit Descriptions

In contrast to a pin-to-pin description, the same circuit can be specified in a detailed form of design description, as follows:

Q1.CLK = Clock; "Register clocked from input
Q1.D = !Q1.Q # Preset; "D-type f/f used for register

In this form of the design, specifying the D input to a D-type flip-flop and specifying feedback directly from the register restricts the device architectures the design can be implemented in. Furthermore, the equations only describe the inputs to and feedback from the flip-flop, and do not provide any information regarding the configuration of the actual output pin. This means the design will operate quite differently when implemented in a device with inverted outputs (a simple P16R4 PAL device, for example), versus in a device with non-inverting outputs (such as an EP600).

To maintain the correct pin behavior using detailed equations, one additional language element, a 'buffer' (or its complement, 'invert') attribute, is required. The 'buffer' attribute ensures that the final implementation in a device has no inversion between the specified D-type flip-flop and the output pin associated with Q1. For example, add the following to the declarations section:

Q1 pin istype 'buffer';