Connections between the STM32 and the iCE40

Introduction

The BlackIce board has a pretty powerful STM32 microcontroller. By default, it's used to program the FPGA bitstream and then just sit idle, but it can be used for any kind of application that needs a processor and some custom hardware.

So if we want to use it for that, we need to know which options are available to exchange data between the STM32 and the FPGA.

In the table below, the STM32 Pin column contains the name that is assigned to the pin in the BlackIce schematic. The iCE40 Name column contains the name that has been assigned in the examples/common/blackice-ii.pcf file of the BlackIce-II GitHub repo. This way, you can use the pin name straight in your Verilog design.

Shared Signals between the STM32 and the FPGA

Schematic Signal	STM32 Pin	iCE40 Name	Special Board Function	Comments
SS#	PA15	PMOD[52]	SPI FPGA bitstream download	At bootup, configured as SPI slave by FPGA. After that, usable as GPIO between STM32 and FPGA.
SCK	PB3	PMOD[53]	SPI FPGA bitstream download	At bootup, configured as SPI slave by FPGA. After that, usable as GPIO between STM32 and FPGA.
MISO	PB4	PMOD[54]	SPI FPGA bitstream download	At bootup, configured as SPI slave by FPGA. After that, usable as GPIO between STM32 and FPGA.
MOSI	PB5	PMOD[55]	SPI FPGA bitstream download	At bootup, configured as SPI slave by FPGA. After that, usable as GPIO between STM32 and FPGA.
QCS#	PA2	QSPICSN	High-speed QUAD SPI interface	Usable as GPIO, or as QUADSPI slave.
QCK	PA3	QSPICK	High-speed QUAD SPI interface	Usable as GPIO, or as QUADSPI slave.
QD0	PB1	QSPIDQ[0]	High-speed QUAD SPI interface	Usable as GPIO, or as QUADSPI slave.
QD1	PB0	QSPIDQ[1]	High-speed QUAD SPI interface	Usable as GPIO, or as QUADSPI slave.
QD2	PA7	QSPIDQ[2]	High-speed QUAD SPI interface	Usable as GPIO, or as QUADSPI slave.
QD3	PA6	QSPIDQ[3]	High-speed QUAD SPI interface	Usable as GPIO, or as QUADSPI slave.
RX	PA10	PMOD[2]	UART RX	Driven by the CH340 with serial data received from USB2.
TX	PA10	PMOD[3]	UART TX	At bootup, driven by the STM32. After that, it's usable as a GPIO between STM32 and FPGA.
DIG16	PD2	PMOD[51]	Switch SW2.1, SDCard	Make sure switch is open when using as GPIO or SDCard to avoid short circuit!
DIG17	PC10	PMOD[50]	Switch SW2.2, SDCard	Make sure switch is open when using as GPIO or SDCard to avoid short circuit!
DIG18	PC11	PMOD[49]	Switch SW1.2, SDCard	Make sure switch is open when using as GPIO or SDCard to avoid short circuit!
DIG19	PC12	PMOD[48]	Switch SW1.1, SDCard	Make sure switch is open when using as GPIO or SDCard to avoid short circuit!
B1	PC8	B1	Button S3, SDCard	Don't press button open when using as GPIO or SDCard to avoid short circuit!
B2	PC9	B2	Button S4, SDCard	Don't press button open when using as GPIO or SDCard to avoid short circuit!
DBG1	PC13-TAMPER-RTC	DBG1
DONE	PB7	DONE
RESET	PB6	<CRESET_B>	iCE40 global reset	This pin is used by the STM32 to completely reset the iCE40. It cannot be used for anything else.

Conclusion

For lower data rates, the most obvious solution is to use an SPI interface on the same IO pins that are used to load the bitstream. This also has the additional advantage that you can reuse major parts of the iceboot firmware.

For very high data rates, the quad SPI pins are the best option.