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IceBreaker Tutorial

This tutorial will show you how to install FPGA development tools, synthesize a RISC-V core, compile and install programs and run them on a IceBreaker. The IceBreaker is equipped with a small FPGA (Ice40UP5k). With its 5K logic elements. It supports lower maxfreq than the Ice40HX1K of the IceStick, but it has many advantages:

  • it has enough space to fit a RISC-V RV32IMC core (femtorv32-gracilis)
  • it has built-in DSPs, that can do multiplies in one cycle (used by femtorv32-gracilis to implement RV32M)
  • it has 128 kB of built-in single ported RAM

If you want to connect stuff (small OLED screen, VGA, HDMI...), you will need to solder a couple of PMOD connectors (PMOD1A and PMOD1B, as on the photograph). Note that PMOD2 is shared with the buttons and LEDs of the separable right part of the board.

Install open-source FPGA development toolchain

Before starting, you will need to install the open-source FPGA development toolchain (Yosys, NextPNR etc...), instructions to do so are given here.

(Optional) configure femtosoc and femtorv32

Edit learn-fpga/FemtoRV/RTL/CONFIGS/icebreaker_config.v

This file lets you define what type of RISC-V processor you will create. For IceBreaker, we use the femtorv32-gracilis (RV32IMC).

The file learn-fpga/FemtoRV/RTL/CONFIGS/icebreaker_config.v also lets you select the device drivers present in the associated system-on-chip:

Device description comment
NRV_IO_LEDS 5 leds keep it
NRV_IO_UART serial connection through USB keep it
NRV_IO_SSD1351 small OLED screen (128x128) comment-out if you do not have it
NRV_IO_SSD1331 small OLED screen (96x64) comment-out if you do not have it
NRV_IO_MAX7219 led matrix (10x10) comment-out if you do not have it
NRV_MAPPED_SPI_FLASH flash mapped in memory space keep it

We activate the LEDs (for visual debugging) and the UART (to talk with the system through a terminal-over-USB connection). We use the 128 kbytes of single-ported RAM available in the Ice40up5K. If you do not have an OLED screen or a led matrix screen, you can comment out the corresponding lines in the file.

There are other options in the file. Normally you will not need to change them. If you want to know, they are listed below:

Option Description
NRV_FREQ Frequency of the processor in MHz
NRV_RAM Amount of RAM (128KB max)
NRV_RESET_ADDR Address where to jump at startup and reset (SPI flash + 64KB offset)
NRV_IO_HARDWARE_CONFIG Hardware register that can be queried by code to see configured devices
NRV_RUN_FROM_SPI_FLASH Do not initialize BRAM with firmware (firmware will be read from SPI flash)

The default value (25 MHz) corresponds to what is validated by Yosys/NextPNR. If you want you can try overclocking a bit, up to 30 - 35 MHz (but you may experience stability problems then). Note that frequency can only take some predefined values, listed in RTL/PLL/frequencies.txt. You can add your own value there if need be, but then you will need to use RTL/PLL/gen_pll.sh to re-generate the board-specific Verilog for the PLL.

Synthethize and program

You can now synthesize the design and send it to the device. Plug the device in a USB port, then:

$make ICEBREAKER

The first time you run it, it will download RISC-V development tools (takes a while).

Now, install a terminal emulator:

$sudo apt-get install python3-serial

(or sudo apt-get install screen, both work). To see the output, you need to connect to it (using the terminal emulator):

$make terminal

(if you installed screen instead of python3-serial, edit Makefile before accordingly. You may need also to change there ttyUSBnnn).

... but nothing happens. It is normal, we have not loaded any program in the system.

To exit, press <ctrl> ] (python-3-serial/miniterm), or <ctrl> a then '\' (screen).

Examples with the serial terminal (UART)

The directories FIRMWARE/EXAMPLES and FIRMWARE/ASM_EXAMPLES contain programs in C and assembly that you can run on the device. On the IceStick, only those that use 6K or less will work (list below).

To compile a program:

$cd FIRMWARE/EXAMPLES
$make hello.prog

The .prog target generates the program and sends it to the device's SPI flash, using iceprog. To see the result, use:

$cd ../..
$make terminal

There are several C and assembly programs you can play with (list below). To learn more about RISC-V assembly, see the RISC-V specifications, in particular the instruction set and the programmer's manual.

ASCII-art version of the Mandelbrot set, computed by a program in assembly (ASM_EXAMPLES/mandelbrot_terminal.S)

Program Description
ASM_EXAMPLES/blinker_shift.S the blinker program, using shifts
ASM_EXAMPLES/blinker_wait.S the blinker program, using a delay loop
ASM_EXAMPLES/test_serial.S reads characters from the serial over USB, and sends them back
ASM_EXAMPLES/mandelbrot_terminal.S computes the Mandelbrot set and displays it in ASCII art
EXAMPLES/hello.c displays a welcome message
EXAMPLES/sieve.c computes prime numbers

Examples with the LED matrix

For more fun, you can add an 8x8 led matrix. It is cheap (less than $1) and easy to find (just google search max7219 8x8 led matrix). Make sure pin labels (CLK,CS,DIN,GND,VCC) correspond to the image, then insert it in the PMOD1A connector of the IceStik as shown on the image (the image shows an IceStick instead of an IceBreaker, refer to the pin numbers that are the same for all PMODs).

You can also build a OysterPMOD (with both a led matrix and OLED screen using a single PMOD connector).

FemtoSOC configuration

Now we need to activate hardware support for the led matrix (and deactivate the UART). To do that, configure devices in FemtoRV/RTL/CONFIGS/icestick_config.v as follows:

...
`define NRV_IO_MAX7219    // Mapped IO, 8x8 led matrix
...

Then you need to re-synthethize and send the bitstream to the IceStick:

$make ICEBREAKER

Now you can compile the hello world program (FIRMWARE/EXAMPLES/hello.c). Edit it and uncomment the following line:

femtosoc_tty_init();

This line automatically redirects printf() to the configured device (here the led matrix). Now compile the program and send it to the device:

$cd FIRMWARE/EXAMPLES
$make hello.prog

When the led matrix is configured, printf() is automatically redirected to the scroller display routine. The sieve.c program will also behave like that.

There are other examples that you can play with:

Program Description
ASM_EXAMPLES/test_led_matrix.S display two images on the led matrix in ASM
EXAMPLES/life_led_matrix.c Game of life on a 8x8 toroidal world

If you want to write your own program: in C, you first need to switch the display on using MAX7219_init(), then you can use the function MAX7219(col,data) where col is the column index in 1..8 (and not 0..7 !!!), and data an 8-bit integer indicating which led should be lit. Take a look at FIRMWARE/EXAMPLES/life_led_matrix.c for reference.

Examples with the OLED screen

With its 64 pixels, our led matrix is somewhat limited and lacks colors... Let us generate more fancy graphics. For this, you will need a SSD1351 128x128 oled display. It costs around $15 (there exists cheaper screens, such as 240x240 IPS screens driven by the ST7789, but they really do not look as good, and they are not compatible, believe me the SSD1351 is worth the price). Make sure you get one of good quality (if it costs less than $5 then I'd be suspicious, some users reported failures with such low-cost versions). Got mine from Waveshare. Those from Adafruit were reported to work as well.

These little screens need 7 wires. The good news is that no soldering is needed, just get a 2x6 pins connector such as the one on the image, connect the wires as shown to the connector, then the connector to the IceStick. If the colors of the wires do not match, use the schematic on the right to know wich wire goes where (the image shows an IceStick instead of an IceBreaker, refer to the pin numbers that are the same for all PMODs).

You can also build a OysterPMOD (with both a led matrix and OLED screen using a single PMOD connector).

Now you need to reconfigure icebreaker_config.v as follows:

...
`define NRV_IO_SSD1351    // Mapped IO, 128x128x64K OLed screen
...

Then you need to re-synthethize and send the bitstream to the IceStick:

$make ICEBREAKER

Let us compile a test program:

$ cd FIRMWARE/EXAMPLES
$ make gfx_test.prog

If everything goes well, you will see an animated colored pattern on the screen. Note that the text-mode demos (hello.c and sieve.c) still work and now display text on the screen. There are other programs that you can play with:

(The black diagonal stripes are due to display refresh, they are not visible normally).

Program Description
ASM_EXAMPLES/test_OLED.S displays an animated pattern.
ASM_EXAMPLES/mandelbrot_OLED.S displays the Mandelbrot set.
EXAMPLES/cube.c displays a rotating 3D cube.
EXAMPLES/mandelbrot.c displays the Mandelbrot set (C version).
EXAMPLES/riscv_logo.c a rotozoom with the RISCV logo (back to the 90's).
EXAMPLES/spirograph.c rotating squares.
EXAMPLES/gfx_test.c displays an animated pattern (C version).
EXAMPLES/gfx_demo.c demo of graphics functions(old chaps, remember EGAVGA.bgi ?).
EXAMPLES/test_font_OLED.c test font rendering.
EXAMPLES/sysconfig.c displays femtosoc and femtorv configurations.

The LIBFEMTORV32 library includes some basic font rendering, 2D polygon clipping and 2D polygon filling routines. Everything fits in the available 6kbytes of memory !

Storing stuff on the SPI Flash

Is it all we can do with an IceStick ? No we can do more ! Let us see how to port a Y2K demo called ST-NICCC. Such 3D graphics cannot be rendered in real time by our femto-machine (and the 8 MHz 68000 of the Atari ST could not either !), so it does render a precomputed stream of 2D polygons stored in a file. The file weights 640Kb, and remember, we only have 6Kb, so what can we do ? It would be possible to wire a SDCard adapter and store the file there, but there is a much easier solution: the IceStick stores the configuration of the FPGA in a flash memory, and there is plenty of unused room in it: if it does not fit in one chip, we can overflow in the neighborhing chip !. This flash memory is a tiny 8-legged chip, that talks to the external world using a serial protocol (SPI). In fact we are already using it ! It is where our programs are stored, and FemtoRV32 directly executes them from there. We are also going to store some data there.

Let us copy the data to the SPI flash:

$ iceprog -o 1M FIRMWARE/EXAMPLES/DATA/scene1.bin

This copies the data starting from a 1Mbytes offset (the lower addresses are used to store the configuration of the FPGA and to store our programs, so do not overwrite them). The data file scene1.bin is the original one, taken from the ST_NICCC demo.

Now you can compile the demo program and send it to the IceStick:

$ cd FIRMWARE/EXAMPLES
$ make ST_NICCC_spi_flash.prog

Can we do more with this tiny system ? Yes, we can do raytracing !

$ cd FIRMWARE/EXAMPLES
$ make tinyraytracer.prog

It will display the classical shiny spheres and checkboard scene. It takes around 3 minutes to do so, be patient ! Our processor does not have a FPU yet (it is probably possible to fit one on the IceBreaker, we'll try to do that in the future).