Python on Fomu¶
You can load MicroPython, a small Python implementation, onto Fomu as an ordinary RISC-V binary. A precompiled binary is located in the root of the Fomu workshop files.
dfu-util to load it:
dfu-util -D micropython-fomu.dfu
You should be greeted with a MicroPython banner and REPL:
This is a fully-functioning MicroPython shell. Try running some simple
commands such as
Fomu Python Extensions¶
Fomu’s MicroPython binary contains a few extended Python modules that
you can use to interact with some of the hardware. For example, the RGB
LED has some predefined modes you can access. These are all located
fomu module and access the
rgb block to change the
mode to the predefined
import fomu rgb = fomu.rgb() rgb.mode("error")
We can also look at some information from the SPI flash, such as the SPI ID. This ID varies between Fomu models, so it can be a good indication of what kind of Fomu your code is running on:
spi = fomu.spi() hex(spi.id())
If we look at the generated Fomu header files (to be found for instance in riscv-blink), we can see many, many memory-mapped registers. For example, the major, minor, and revision numbers all have registers:
#define CSR_VERSION_MAJOR_ADDR 0xe0007000 #define CSR_VERSION_MINOR_ADDR 0xe0007004 #define CSR_VERSION_REVISION_ADDR 0xe0007008 #define CSR_VERSION_MODEL_ADDR 0xe0007028
These are special areas of memory that don’t really exist. Instead, they
correspond to hardware. We can read these values using the
class. Read out the major, minor, and revision codes from your Fomu.
They may be different from what you see here:
import machine machine.mem32[0xe0007000] machine.mem32[0xe0007004] machine.mem32[0xe0007008]
CSR_VERSION_MODEL_ADDR contains a single character that
indicates what version of the hardware you have. We can convert this to
a character and print it out.
If you have a production board you will get
P as shown below;
If you have a hacker board you will get
H as shown below;
Memory-mapped RGB driver¶
The blinking LED is actually a hardware block from Lattice. It has control registers, and we can modify these registers by writing to memory in Fomu. Some of these registers control things such as the timing of the fade in and fade out pulses, and some control the level of each of the three colors.
There is a wrapper in Fomu’s MicroPython that simplifies the process of writing to these registers. The first argument is the register number, and the second argument is the value to write.
LEDDPWR registers, the second argument determines the
brightness, value ranges from 0 to 255.
Try changing the color of the three LEDs:
ADDR_RED_LED_PULSE_WIDTH = 0b0001 # LEDDPWRR ADDR_GREEN_LED_PULSE_WIDTH = 0b0010 # LEDDPWRG ADDR_BLUE_LED_PULSE_WIDTH = 0b0011 # LEDDPWRB rgb.write_raw(ADDR_RED_LED_PULSE_WIDTH, 255) # Red LED fully on rgb.write_raw(ADDR_GREEN_LED_PULSE_WIDTH, 14) # Green LED mostly off rgb.write_raw(ADDR_BLUE_LED_PULSE_WIDTH, 1) # Blue LED off
The color should change immediately. More information on these registers can be found in the ICE40 LED Driver Usage Guide.