Circle is a C++ bare metal programming environment for the Raspberry Pi. It should be usable on all existing models (tested on model A+, B, B+, on Raspberry Pi 2, 3, 4, 400, 5 and on Raspberry Pi Zero), except on the Raspberry Pi Pico, which is not supported. Circle provides several ready-tested C++ classes and add-on libraries, which can be used to control different hardware features of the Raspberry Pi. Together with Circle there are delivered several sample programs, which demonstrate the use of its classes. Circle can be used to create 32-bit or 64-bit bare metal applications.
Circle includes bigger (optional) third-party C-libraries for specific purposes in addon/ now. This is the reason why GitHub rates the project as a C-language-project. The main Circle libraries are written in C++ using classes instead. That's why it is called a C++ programming environment.
With this release Circle initially supports the Raspberry Pi 5. There are many features, which are not available yet, but important features like USB and networking are supported. Please see the Circle documentation for more information on Raspberry Pi 5 support!
Circle comes with an USB serial CDC gadget now, which allows to communicate with a Circle application from a host computer via a serial interface without an USB serial adapter. This can be tested with the test/usb-serial-cdc-gadget.
The properties file library in addon/Properties supports section headers now.
A possible race condition in CTimer has been fixed, which could only occur with the KY-040 rotary encoder module driver.
Only the features with a "x" or other info are currently supported on the Raspberry Pi 5.
Circle supports the following features:
| Group | Features | Raspberry Pi 5 |
|---|---|---|
| C++ build environment | AArch32 and AArch64 support | AArch64 only |
| Basic library functions (e.g. new and delete) | x | |
| Enables all CPU caches using the MMU | x | |
| Interrupt support (IRQ and FIQ) | IRQ only | |
| Multi-core support (Raspberry Pi 2, 3 and 4) | x | |
| Cooperative non-preemtive scheduler | x | |
| CPU clock rate management | x | |
| Clang/LLVM support (experimental) | x | |
| Debug support | Kernel logging to screen, UART and/or syslog server | screen / UART |
| C-assertions with stack trace | x | |
| Hardware exception handler with stack trace | x | |
| GDB support using rpi_stub (Raspberry Pi 2 and 3) | ||
| Serial bootloader (by David Welch) included | x | |
| Software profiling support (single-core) | x | |
| QEMU support | ||
| SoC devices | GPIO pins (with interrupt, Act LED) and clocks | no clocks |
| Frame buffer (screen driver with escape sequences) | limited | |
| UART(s) (Polling and interrupt driver) | x | |
| System timer (with kernel timers) | x | |
| Platform DMA controller | memcopy only | |
| EMMC SD card interface driver | x | |
| SDHOST SD card interface driver (Raspberry Pi 1-3) | ||
| PWM output (2 channels) | ||
| PWM sound output (on headphone jack) | ||
| I2C master(s) and slave | masters only | |
| SPI0 master (Polling and DMA driver) | ||
| SPI1 auxiliary master (Polling) | ||
| SPI3-6 masters of Raspberry Pi 4 (Polling) | ||
| SMI master (experimental) | ||
| I2S sound output and input | ||
| HDMI sound output (without VCHIQ) | ||
| Hardware random number generator | x | |
| Watchdog device | x | |
| Official Raspberry Pi touch screen | ||
| VCHIQ interface and audio service drivers | ||
| BCM54213PE Gigabit Ethernet NIC of Raspberry Pi 4 | ||
| MACB / GEM Gigabit Ethernet NIC of Raspberry Pi 5 | x | |
| Wireless LAN access | x | |
| USB | Host controller interface (HCI) drivers | x |
| Standard hub driver (USB 2.0 only) | x | |
| HID class device drivers (keyboard, mouse, gamepad) | x | |
| Driver for on-board Ethernet device (SMSC951x) | ||
| Driver for on-board Ethernet device (LAN7800) | ||
| Driver for USB mass storage devices (bulk only) | x | |
| Driver for USB audio streaming devices (RPi 4 only) | x | |
| Drivers for different USB serial devices | x | |
| Audio class MIDI input support | x | |
| Touchscreen driver (digitizer mode) | x | |
| Printer driver | x | |
| MIDI gadget driver | ||
| Serial CDC gadget driver (experimental) | ||
| File systems | Internal FAT driver (limited function) | x |
| FatFs driver (full function, by ChaN) | x | |
| TCP/IP networking | Protocols: ARP, IP, ICMP, UDP, TCP | x |
| Clients: DHCP, DNS, NTP, HTTP, Syslog, MQTT | x | |
| Servers: HTTP, TFTP | x | |
| BSD-like C++ socket API | x | |
| Graphics | OpenGL ES 1.1 and 2.0, OpenVG 1.1, EGL 1.4 | |
| (not on Raspberry Pi 4) | ||
| uGUI (by Achim Doebler) | ||
| LVGL (by LVGL Kft) | x | |
| 2D graphics class in base library | ||
| Not supported | Bluetooth |
For building 64-bit applications (AArch64) see the next section.
Circle does not support 32-bit applications on the Raspberry Pi 5.
This describes building on PC Linux. See the file doc/windows-build.txt for information about building on Windows. If building for the Raspberry Pi 1 you need a toolchain for the ARM1176JZF core (with EABI support). For Raspberry Pi 2/3/4 you need a toolchain with Cortex-A7/-A53/-A72 support. A toolchain, which works for all of these, can be downloaded here. Circle has been tested with the version 12.2.Rel1 (arm-gnu-toolchain-12.2.rel1-x86_64-arm-none-eabi.tar.xz) from this website.
First edit the file Rules.mk and set the Raspberry Pi version (RASPPI, 1, 2, 3 or 4) and the PREFIX of your toolchain commands. Alternatively you can create a Config.mk file (which is ignored by git) and set the Raspberry Pi version and the PREFIX variable to the prefix of your compiler like this (don't forget the dash at the end):
RASPPI = 1
PREFIX = arm-none-eabi-
The following table gives support for selecting the right RASPPI value:
| RASPPI | Target | Models | Optimized for |
|---|---|---|---|
| 1 | kernel.img | A, B, A+, B+, Zero, (CM) | ARM1176JZF-S |
| 2 | kernel7.img | 2, 3, Zero 2, (CM3) | Cortex-A7 |
| 3 | kernel8-32.img | 3, Zero 2, (CM3) | Cortex-A53 |
| 4 | kernel7l.img | 4B, 400, CM4 | Cortex-A72 |
For a binary distribution you should do one build with RASPPI = 1, one with RASPPI = 2 and one build with RASPPI = 4 and include the created files kernel.img, kernel7.img and kernel7l.img. Optionally you can do a build with RASPPI = 3 and add the created file kernel8-32.img to provide an optimized version for the Raspberry Pi 3.
The configuration file Config.mk can be created using the configure tool too. Please enter ./configure -h for help on using it!
There are a number of configurable system options in the file include/circle/sysconfig.h. Please have a look into this file to learn, how you can configure Circle for your purposes. Some hardware configurations may require modifications to these options (e.g. using USB on the CM4).
Then go to the build root of Circle and do:
./makeall clean
./makeall
By default only the Circle libraries are built. To build a sample program after makeall go to its subdirectory and do make.
You can also build Circle on the Raspberry Pi itself (set PREFIX = (empty)) on Raspbian but you need some method to put the kernel.img file onto the SD(HC) card. With an external USB card reader on model B+ or Raspberry Pi 2/3/4 model B (4 USB ports) this should be no problem.
Circle supports building 64-bit applications, which can be run on the Raspberry Pi 3, 4 or 5. There are also Raspberry Pi 2 versions and the Raspberry Pi Zero 2, which are based on the BCM2837 SoC. These Raspberry Pi versions can be used too (with RASPPI = 3).
The recommended toolchain to build 64-bit applications with Circle can be downloaded here. Circle has been tested with the version 12.2.Rel1 (arm-gnu-toolchain-12.2.rel1-x86_64-aarch64-none-elf.tar.xz) from this website.
There are distro-provided toolchains on certain Linux platforms (e.g. g++-aarch64-linux-gnu on Ubuntu or gcc-c++-aarch64-linux-gnu on Fedora), which may work with Circle and can be a quick way to use it, but you have to test this by yourself. If you encounter problems (e.g. no reaction at all, link failure with external library) using a distro-provided toolchain, please try the recommended toolchain (see above) first, before reporting an issue.
First edit the file Rules.mk and set the Raspberry Pi architecture (AARCH, 32 or 64) and the PREFIX64 of your toolchain commands. The RASPPI variable has to be set to 3, 4 or 5 for AARCH = 64. Alternatively you can create a Config.mk file (which is ignored by git) and set the Raspberry Pi architecture and the PREFIX64 variable to the prefix of your compiler like this (don't forget the dash at the end):
AARCH = 64
RASPPI = 3
PREFIX64 = aarch64-none-elf-
The configuration file Config.mk can be created using the configure tool too. Please enter ./configure -h for help on using it!
Then go to the build root of Circle and do:
./makeall clean
./makeall
By default only the Circle libraries are built. To build a sample program after makeall go to its subdirectory and do make.
Copy the Raspberry Pi firmware (from boot/ directory, do make there to get them) files along with the kernel*.img (from sample/ subdirectory) to a SD(HC) card with FAT file system.
It is now always recommended to copy the file config32.txt (for 32-bit mode) or config64.txt (for 64-bit mode) from the boot/ directory to the SD(HC) card and to rename it to config.txt there. These files are especially required to enable FIQ use on the Raspberry Pi 4. Furthermore the additional file armstub7-rpi4.bin (for 32-bit mode) or armstub8-rpi4.bin (for 64-bit mode) is required on the SD card then. Please see boot/README for information on how to build these files.
Finally put the SD(HC) card into the Raspberry Pi.
- include: The common header files, most class headers are in the include/circle/ subdirectory.
- lib: The Circle class implementation and support files (other libraries are in subdirectories of lib/).
- sample: Several sample applications using Circle in different subdirectories. The main function is implemented in the CKernel class.
- addon: Contains contributed libraries and samples (has to be build manually).
- app: Place your own applications here. If you have own libraries put them into app/lib/.
- boot: Do make in this directory to get the Raspberry Pi firmware files required to boot.
- doc: Additional documentation files.
- test: Several test programs, which test different features of Circle.
- tools: Tools for building Circle and for using Circle more comfortable (e.g. a serial bootloader).
The following C++ classes were added to Circle:
Base library
- CMACBDevice: Driver for MACB/GEM Ethernet NIC of Raspberry Pi 5
- CSouthbridge: Driver for the RP1 multi-function device of the Raspberry Pi 5
USB library
- CUSBSerialHostDevice: Generic host driver for USB serial devices (was: CUSBSerialDevice)
- CUSBSubSystem: USB sub-system of the Raspberry Pi 5
USB gadget library
- CUSBCDCGadget: USB serial CDC gadget
- CUSBCDCGadgetEndpoint: Endpoint of the USB serial CDC gadget
The available Circle classes are listed in the file doc/classes.txt. If you have Doxygen installed on your computer you can build a class documentation in doc/html/ using:
./makedoc
At the moment there are only a few classes described in detail for Doxygen.
- Standard library support
- Dynamic memory management and the "new" operator
- DMA buffer requirements
- Serial bootloader support
- Multi-core support
- USB plug-and-play
- Debugging support
- JTAG debugging
- QEMU support
- Eclipse IDE support
- About real-time applications
- cmdline.txt options
- Screen escape sequences
- Keyboard escape sequences
- Clang support
- Memory layout
- Naming conventions
- Known issues
Raspberry Pi is a trademark of Raspberry Pi Ltd.
Linux is a trademark of Linus Torvalds.
PS3 and PS4 are registered trademarks of Sony Computer Entertainment Inc.
Windows, Xbox 360 and Xbox One are trademarks of the Microsoft group of companies.
Nintendo Switch is a trademark of Nintendo.
Khronos and OpenVG are trademarks of The Khronos Group Inc.
OpenGL ES is a trademark of Silicon Graphics Inc.
The micro:bit brand belongs to the Micro:bit Educational Foundation.
HDMI is a registered trademark of HDMI Licensing Administrator, Inc.
