For software engineers wanting to get into hardware design, the most important thing to remember is that it isn't software, despite feeling a bit like writing software. Writing instructions and describing hardware are very different, regardless of how much a HDL may try to hide that difference with similar looking constructs.
As someone who had done both hardware and software for decades, I'd forgotten how much I had internalized until I saw a programmer friend try to optimize the parts count on a circuit with an arduino, 8 LEDs and 8 resistors.
He knew that the resistor was to limit current, and he knew about parallel and series circuits, so he just used one resistor on the other side of the LEDs instead of 8 of them... and then wondered why the LED brightness changed with the number of lit LEDs.
Hardware isn't the same as software... it's easy to forget that sometimes. 8)
A big difference is also that things are sequential at the gate level. Everything happens at the same time. It’s almost like a massive multithreaded program. Also, propagation delays can make you pull your hair out if you aren’t aware of them when debugging. There’s also a possibility of unclean signals (not Vcc or GND, transition bouncing, etc.).
One level of abstraction further down, Gates are analog, not digital. If you don't clock things right, you could end up with huge currents going through two transistors trying to drive something opposite ways, and then lose the chip, or battery life, or an intermittent glitch.
Indeed. The best description that I’ve heard of the process is “work out the hardware you want to exist to solve the problem, and then write the HDL code which will synthesize into that hardware”. In verilog/vhdl there are a lot of ways you can write the HDL which will give correct outputs but synthesize into hardware which is garbage (too big or won’t meet timing). You have to learn the specific patterns to write which hint to the synthesis tool what you would like it to synthesize.
I'm hoping to talk about this in the next installment where I actually talk about the design. It's been really interesting getting a feel for how the processes are different. It's something I've been aware of before from talking to hardware engineers, but I think I'm only truly understanding it now that I'm doing it myself :)
