HDL References

Most online info about Hardware Description Languages (HDLs) is pretty poor. I've collected here the best references I've found, generally aimed at Verilog. If you are learning the language, jump straight to Verilog-2001 over Verilog-1995. It'll save you a lot of work. (Sorry, no clue about SystemVerilog yet.)

The coding style guides are not about mere aesthetics or convention, but show how to write correctly synthesizable code. It'll save you a lot of headaches.

• The Sutherland HDL Verilog 2001 Quick Reference Guide (original link)
  Once you know the language, this is my go-to reference.
• The Verilog Golden Reference Guide (original link)
  A Verilog-1995 reference from Doulos.
  This one has more context, rules, and tips on the usage of the language. Still relevant for Verilog-2001.
• The Precision RTL Synthesis Style Guide (original link)
  Also covers VHDL.
• The Actel Coding Style Guide (original link)
  Covers both VHDL and Verilog.
• Signed Arithmetic in Verilog 2001 -- Opportunities and Hazards (original link)
  Included here because, at least under Quartus, the approach this paper advocates for adder/subtractors is WRONG!
  Simply denote your operands and result as signed, and the carry bit as unsigned, and add them all up together: Sum = A + B + cin.
• Sunburst Design
  There's a pile of interesting-looking papers on advanced topics here. Many about SystemVerilog.
• Quick Reference for Verilog HDL (original link)
  A good little guide from AMBIT Design Systems. It's for Verilog-1995, but it explains some things (like where variable assignments are legal) that are common with Verilog-2001.
• Coding Guidelines for Datapath Synthesis (original link)
  Short little document from Synopsys that clearly explains how to deal with signed/unsigned arithmetic in datapaths. Mostly Verilog, with some VHDL.