Aim

• Make my first PCB with an FPGA
• Keep it super simple and cheap
• Configured by on-board FLASH or direct with a Raspberry Pi
• 6 PMODs, 2 buttons, 2 LEDs, FLASH for configuration bitstreams.

There is also an ECP5 version of this board: https://github.com/mattvenn/basic-ecp5-pcb

What a Lattice iCE40 FPGA needs

• A clock input. Has to be provided by an oscillator, it doesn't have a crystal driver.
• 1.2v core supply for the internal logic.
• 2.5v non volatile memory supply. Can be provided via a voltage drop over a diode from 3.3v.
• IO supply for the IO pins, different banks of IO can have different supplies. This design uses 3.3v for all banks.
• Get configured over SPI interface. This can be done directly by a microcontroller or a computer, or the bitstream can be programmed into some FLASH, and the FPGA will read it at boot. If FLASH isn't provided then the bitstream needs to be programmed at every power up or configuration reset.
• Decoupling capacitors for each IO bank.

PCB

• Schematic
• OHSPark project or Gerbers
• Cheap, 2 layer board. Probably not so good for fast signals.

BOM

• FPGA iCE40-HX4K-TQ144 (8k accessible with Icestorm tools)
• 3.3v reg TLV73333PDBVT
• 1.2v reg TLV73312PDBVT
• 12MHz oscillator SIT2001BI-S2-33E-12.000000G
• 16MB FLASH IS25LP016D-JBLE (optional).

Test

See the test program. This makes a nice pulsing effect on LED2, and LED1 is the slow PWM clock. The buttons increase or decrease pulsing speed.

make prog

Yosys and NextPNR are used to create the bitstream and then it's copied to the Raspberry Pi specified by PI_ADDR in the Makefile.

Fomu-Flash is used to flash the SPI memory, or program the FPGA directly.

HW V3: bfcbcf7

• fixed mistakenly removed pullups on io2/3 on SPI flash. This made the system vulnerable to noise while FPGA was reading flash.

HW V2: 444ba78

• removed a few unnecessary pullups on SPI lines - introduced a fault see HW V3.
• connect 4 general purpose pins between Pi & FPGA
• connect additional CS1 pin for SPI comms with Pi
• breakout all Pi pins on separate pins
• shrink board in y axis slightly for even holes and to fit larger Pi boards
• Allow an I2C connection between Pi & FPGA, broken out on pmod p3:
  • Place optional 4k7 pullup resistors R1&R2
  • Setup bitbanged I2C on Raspi on gpios 20 & 21 (pins 38 & 40)
• used FOMU programming pins so default FOMU programming app will work

HW V1: d9c7cdf

Errata

• didn't expose a 2nd CS pin so can't communicate with SPI without removing flash

My fork of Fomu-Flash is used for programming. Fomu-Flash should be able to reconfigure the pins used with -g, but I couldn't get this to work, so I hard-coded the right pins in this fork.

Inspiration

• https://github.com/OLIMEX/iCE40HX1K-EVB/blob/master/iCE40HX1K-EVB_Rev_B.pdf
• https://github.com/icebreaker-fpga/icebreaker/blob/master/hardware/v1.0e/icebreaker-sch.pdf
• icestick

Reference

• Family datasheet: http://www.latticesemi.com/view_document?document_id=49312
• Programming/FLASH spec: http://www.latticesemi.com/view_document?document_id=46502
• Pinout: http://www.latticesemi.com/view_document?document_id=49383