Altera Hardware Description Language

AHDL is a proprietary digital Hardware Description Language (HDL) from Altera Corporation for programming their Complex Programmable Logic Devices (CPLD) and Field Programmable Gate Arrays (FPGA). This language has an Ada programming language-like syntax and similar operation to VHDL or Verilog. It is supported by Altera's Quartus and Max+ series of compilers.

By default Altera tools expect the AHDL source files to have a .tdf extension (Text Design Files).

A disadvantage of AHDL is that it is proprietary. An advantage of AHDL is that all language constructs are synthesizable. AHDL is to Verilog much as assembly language is to a higher-level programming language: in AHDL, you have more control but less high-level support.

Example

% a simple AHDL up counter, released to public domain 13 November 2006 %
% [block quotations achieved with percent sign] %
% like c, ahdl functions must be prototyped %

% PROTOTYPE:
 FUNCTION COUNTER (CLK)
        RETURNS (CNTOUT[7..0]); %

% function declaration, where inputs, outputs, and
bidirectional pins are declared %
% also like c, square brackets indicate an array %

SUBDESIGN COUNTER
(
        CLK             :INPUT;
        CNTOUT[7..0]    :OUTPUT;
)

% variables can be anything from flip-flops (as in this case),
tri-state buffers, state machines, to user defined functions %

VARIABLE
        TIMER[7..0]: DFF;

% as with all hardware description languages, think of this
 less as an algorithm and more as wiring nodes together %

BEGIN
        DEFAULTS

                TIMER[].prn = VCC; %  this takes care of d-ff resets %
                TIMER[].clrn = VCC;
        END DEFAULTS;

        TIMER[].d = TIMER[].q + H"1";
END;