By whygee on Friday 2 October 2009, 05:48 - Electronics

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As one can guess from the past messages on this "*log", I have been slowly preparing custom FPGA boards as a background activity. It's not an easy thing and can be quite expensive. So I patiently gathered the necessary parts through online stores and eBay, looking for interesting deals.

Finally I have all the necessary parts for a cheap and repeatable prototype. Among others :

* A bunch of A3P250VQG100 : I got them from a really nice Canadian guy and I'll use this specific reference as the main target for the future works. I originally intended to target the A3P125 but I got more powerful for less money so why refuse ? :-D The A3P250 has enough logic for moderately complex stuff (though SRAM is really TIGHT) and can replace microcontrollers in many cases.

* QFP100 adapter boards : FUTURLEC has cheap and good proto boards. The tin makes soldering easy, just add some liquid flux, no need for additional solder.

With the help of Actel's docs and the schematics of other boards, including ACME's Colibri, I easily wired the power rails. The board is not recommended for high-speed signals but the goal is only to check the schematics for more ambitious boards (probably manufactured through FUTURLEC too, as their PCB pooling service looks great).

I created a small dumb VHDL design (8-bit counter with clock, reset and increment/decrement inputs) backed by a small board. The additional board also provides 3.3V from a battery, so I could avoid long wires from power supplies.

And in order to be programmed, the FPGA needs a JTAG interface. I soldered everything correctly but the JTAG/USB interface would refuse to work. After a small nap and many hypothesis, the problem was obvious : the JTAG signals were correctly wired but I forgot to wire the power supplies :-/ Obviously, when it's fixed, the things work considerably better... I'm amazed that it is the only error, considering my sleep deprivation :-D

No, really, it just works as expected. I may have finally become good at this, after all the failures and false starts of the past :-D It even reproduces the strange behaviour that I had seen in other designs : the pins are REALLY sensitive ! Don't forget the pull-down's ! I made a basic/passive anti-bounce (just a RC filter) but it is useless : a single clock push creates many strobes and the counter advances unpredictably. But I half-expected it and I did not even register the inputs in VHDL so it is naturally glitch-prone, so I don't care. It "works".

What does this mean ?

* When enough resources are gathered, complex things become easier. I have invested a lot of money and time in the past years just to get to that point and ... it feels good !

* Great things that were "possible" now become "available" for future projects. This includes YASEP and other (commercial ?) designs.

* FPGA are damn cool ! Actel's chips are certainly slower and less capable than other makers but their products make this little board possible and easy : once the power supplies and the JTAG are (hum, correctly) wired, the board can be plugged in other cheap prototypes. No need of external Flash chip, bootstrapping EPLD, or whatever...

Next in line : a parallel port interface (hooked to a computer) then the SRAM chips :-D Then I'll try to develop an embedded CPU design with Ethernet that could replace the Rabbit, PIC and AVR. Finally, I wish to create an Ethernet-based JTAG programmer that will replace the proprietary and USB-bound FlashPro3. This proprietary probe is not extremely expensive (Actel has wisely created even cheaper versions) but USB is such an annoyance !

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20200408:

This log foreshadows many developments for the decade, many of them are described in my projects list.

It's funny that the FUTURLEC link still works :-)

De-bouncing the A3P pins though is still an issue -_-