A scratch RTOS-driven embedded application that measures humidity (RelH), pressure (hPa), and temperature (C) using a Raspberry Pi 3B+, BME280, and SSD1306 over I2C.
Using the BCM2835 (RPI 3B+ SoC) peripheral technical manual, SSD1306 datasheet,
and BME280 datasheet I wrote the entire kernel from scratch. This application
runs bare-metal on my Raspberry Pi 3B+ after the GPU bootloader firmware loads
kernel8.img into memory and executes it. I also used other resources to learn
and debug the Aarch64 architecture like QEMU, Anthropic's Claude, bztsrc's
raspi3-tutorial, and the ARM
architecture documentation.
Resources: (for other hobbyists)
- BCM2835 peripherals technical manual (RPI 3B+ SoC)
- Section 3 (pp 28-36) for
I2C - Section 6 (pp 89-105) for
GPIO - Section 13 (pp 175-177) for
UART0
- Section 3 (pp 28-36) for
- BCM2835 GPIOs
- SSD1306 datasheet (128x64 display)
- Section 8.1.5 (pp 19-21) for I2C operation
- Section 10 (pp 34-46) for commands
- Section 10.1.3 (pp 34-35) for addressing
- Application Note -> Section 3 (p 5) for software initialization
- BME280 datasheet
- Section 4.2.3 (pp 25-26) for integer compensation functions
- Section 5 (pp 26-31) for registers
- Section 6.2 (pp 32-33) for I2C operation
- bztsrc's raspi3-tutorial (bare-metal on RPI 3B+)
- Cortex A53 technical reference manual
Before starting, there are a few pieces of hardware we need:
- Raspberry Pi 3B+
- BME280 sensor + breakout board
- SSD1306 128x64 display + breakout board
- UART to USB dongle (optional, for serial debugging)
- Breadboard
- Jumper wires
All of these can easily be found on amazon for fairly cheap. The most expensive item is the Raspberry Pi, at $54 when I looked.
First, you need to download the Aarch64 toolchain
for your platform. I used WSL (linux), so I downloaded arm-gnu-toolchain-15.2.rel1-x86_64-aarch64-none-elf.tar.xz.
Make sure you download the aarch64-none-elf version so that it generates
ELF files, which are transformed into binary images.
Extract the download's folder, and move it into this repo's root folder
as toolchain. Alternatively, it can be placed anywhere and you can modify
the TOOLPREFIX environment variable (see Makefile).
Once your toolchain is established, you can run make to build kernel8.img.
-
Using Raspberry Pi Imager
- Format a micro-SD card using Raspberry Pi Imager. I would recommend choosing the "Raspberry Pi OS Lite (64-bit)" option.
- You should see a new drive called bootfs. Delete all files in here
except:
bootcode.bin,fixup.dat, andstart.elf - Copy
config.txtandkernel8.imgfrom here into bootfs
-
From Scratch
- Format a micro-SD card as FAT32
- Go to https://github.com/raspberrypi/firmware/tree/master/boot and
copy
bootcode.bin,fixup.dat, andstart.elfinto the formatted SD card - Copy
config.txtandkernel8.imgfrom here into the SD card
Your bootfs should look like this:
Plug your BME280 and SSD1306 into the breadboard. Then, connect the GPIO pins to the breadboard corresponding input pins for each module:
- Ground to the breadboard ground (then to each module)
- 3.3V to the breadboard positive (then to each module)
- GPIO2 SDA to the BME280 and SSD1306 SDA inputs
- GPIO3 SCL to the BME280 and SSD1306 SCL inputs
My BME280 also came with two extra pins: CSB and SD0.
- CSB is used for SPI, which I am not using and the datasheet said to connect to 3.3V in I2C operation
- SD0 changes the address of the I2C module if positive, so connect to ground for this
For UART, connect:
- Another GPIO ground to UART dongle ground
- GPIO UART TX to UART dongle RX
- GPIO UART RX to UART dongle TX
- NOTE: If the UART dongle comes with power pins (mine did), DO NOT connect them to the Raspberry Pi
Here's a good visual reference for Raspberry Pi 3B+ pins:
Now that everything is connected, you can plug in your micro-SD card and power cable into the Raspberry Pi and start it. It should immediately start showing an output on the SSD1306 if you did everything right.
Humidity (in relative humidity)
Pressure (in hPa/mbar)
Temperature (in degrees C)
