Keeping the cost down was a primary concern, yet we wanted to make a board that was genuinely useful.
To achieve this, Alan came up with a unique strategy:
1. Low cost FPGA with open source tool chain
2. Low cost ARM - to act as "system integrator" and glue.
3. On board Pi Zero - to host the open source FPGA tools - user supplies this at $5.00
Having settled on the ICE40HX4K - as the largest ICE40 available in an LQFP package - and thus more easily routed on a 2 layer board than a BGA part, we set about identifying the other key components.
Having seen James Bowman's J1a Forth Processor - a soft core running in an ICE40HX1K part, I wanted to add a fast (10nS) 16 bit wide SRAM - which in a TSSOP-44 package allows parts between 64K and 512K words. With this combination comes the ability to create a number of soft core processors, of which the 16 bit simple processor described in the "Nand to Tetris" course would seem a very suitable candidate.
Other applications include digital instrumentation that require large arrays of fast memory - such as digital oscilloscopes and logic analysers.
The lynchpin in this design is the use of a low cost ARM device, not only to provide USB communications, storage (in flash) of the FPGA bitfile, plus provide a series of ADCs and 5V tolerant GPIO pins and peripherals to augment the FPGA architecture.
We believe that if an FPGA board sells for around $30 then it will attract a fair bit of attention.
This was going to be tricky - so we set ourselves a BOM on a very tight budget of arount $10 based around the principal components - costed in 100 and 1000 off quantities:
100 off 1000 off
FPGA $5.56 $5.37
ARM $1.20 $1.00
SRAM $1.69 $1.22
PCB $0.88 $0.49
Connectors $0.50 $0.40
Remainder $0.50 $0.40
Sub Total $10.33 $8.88
LOP $2.58 $2.22
Total $12.91 $11.10
Pi Zero $5.00 $5.00
So in 1000 of quantity, the BOM and manufacturing (LOP = Labour, Operations Profit) comes to a total of around $11, which allows a selling price of about $30.
The customer provides their own Raspberry Pi or Pi Zero, from as little as $5.00.