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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 and on Raspberry Pi Zero), except on the Compute Module 4, which is not fully supported. The status of the Raspberry Pi 400 is currently unknown. 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 named a C++ programming environment.

Release 43.2

This intermediate release comes with new drivers and improvements in detail. First a number of USB serial adapters with different chips (CH341, CP2102, PL2303, FTDI) is supported now. These drivers are primarily intended to be used with serial client-server protocols, because a handshake (flow control) is not implemented so far. Only the tested combinations of USB vendor/device ID have been added to the drivers. If you discover, that your adapter with different IDs is working too, please let us know! The USB vendor/device ID combination has to be added in GetDeviceIDTable() at the bottom of the respective file lib/usb/usbserial*.cpp.

These new USB serial drivers allow to provide serial client libraries and sample programs, which work with the RTK.GPIO board and the micro:bit computer. Please follow the given links for details.

There is an important change because of the updated firmware version, which is recommended for Circle. For FIQ support on the Raspberry Pi 4 in AArch32 mode a specific ARM stub file armstub7-rpi4.bin is required now. You will need a config.txt file for 32-bit operation now too for this configuration, as it already has been the case for 64-bit operation. Please read the file boot/README for further information.

The USB mouse driver supports a mouse wheel and up to five buttons now. sample/10-usbmouse has been updated to demonstrate this. This required a small modification to the mouse API. If your application uses a mouse, you probably have to add the nWheelMove (int) parameter to your mouse handler's prototype.

The Raspberry Pi 4 supports multiple displays. This can be used in Circle to provide more than one CBcmFrameBuffer and CScreenDevice instances now. If you want to use this feature, you have to add the setting max_framebuffers=2 to your config.txt file on the SD card.

The LittlevGL project has been renamed by its authors to LVGL. Because the support for this project in Circle has been updated to the new version 7.6.1, the respective directory addon/lvgl and the class CLGVL now follows this convention. This support enables USB plug-and-play now.

A number of fixes has been applied for USB plug-and-play support on the Raspberry Pi 1-3.

Some time has been spent to provide build support on Windows. As described in doc/windows-build.txt, it is possible to build all Circle libraries and samples without modification now on this platform.

Finally some information about JTAG debugging in AArch64 mode on the Raspberry 3 and 4 have been added. Please see doc/debug-jtag.txt.

The 43rd Step

This release adds USB plug-and-play (USB PnP) support to Circle. It has been implemented for all USB device drivers, which can be subject of dynamic device attachments or removes, and for a number of sample programs. Existing applications have to be modified to support USB PnP, but this is not mandatory. An existing application can continue to work without USB PnP unmodified. Please see the file doc/usb-plug-and-play.txt for details on USB PnP support and sample/README for information about which samples are USB PnP aware!

USB PnP requires the system option USE_USB_SOF_INTR to be enabled in include/circle/sysconfig.h on the Raspberry Pi 1-3 and Zero. Because it has proved to be beneficial for most other applications too, it is enabled by default now. Rarely it may be possible, that your application has disadvantages from it. In this case you should disable this option and go back to the previous setting (e.g. if you need the maximum network performance).

An important issue has been fixed throughout Circle, which affected the alignment of buffers used for DMA operations. These buffers must be aligned to the size of a data-cache line (32 bytes on Raspberry 1 and Zero, 64 bytes otherwise) in base address and size. In some cases your application may need to be updated to meet this requirement. For example this applies to the samples 05-usbsimple, 06-ethernet and 25-spidma. Please see the file doc/dma-buffer-requirements.txt for details!

Another problem in the past was, that the output to screen or serial device affected the IRQ timing of applications. There is the system option REALTIME, which already improved this timing. Unfortunately it was not possible to use low- or full-speed USB devices (e.g. USB keyboard) on the Raspberry Pi 1-3 and Zero, when this option was enabled. Now this is supported, when the system option USE_USB_SOF_INTR is enabled together with REALTIME.

The new class CWriteBufferDevice can be used to buffer the screen or serial output in a way, that writing to these devices is still possible at IRQ_LEVEL, even when the option REALTIME is defined. Using the new class CLatencyTester it is demonstrated in the new sample/40-irqlatency, how this affects the IRQ latency of the system. Please read the README file of this sample for details!


Circle supports the following features:

Group Features
C++ build environment AArch32 and AArch64 support
Basic library functions (e.g. new and delete)
Enables all CPU caches using the MMU
Interrupt support (IRQ and FIQ)
Multi-core support (Raspberry Pi 2, 3 and 4)
Cooperative non-preemtive scheduler
CPU clock rate management
Debug support Kernel logging to screen, UART and/or syslog server
C-assertions with stack trace
Hardware exception handler with stack trace
GDB support using rpi_stub (Raspberry Pi 2 and 3)
Serial bootloader (by David Welch) included
Software profiling support (single-core)
QEMU support
SoC devices GPIO pins (with interrupt, Act LED) and clocks
Frame buffer (screen driver with escape sequences)
UART(s) (Polling and interrupt driver)
System timer (with kernel timers)
Platform DMA controller
EMMC SD card interface driver
SDHOST SD card interface driver (Raspberry Pi 1-3)
PWM output (2 channels)
PWM sound output (on headphone jack)
I2C master(s) and slave
SPI0 master (Polling and DMA driver)
SPI1 auxiliary master (Polling)
SPI3-6 masters of Raspberry Pi 4 (Polling)
I2S sound output
Hardware random number generator
Official Raspberry Pi touch screen
VCHIQ interface and audio service drivers
BCM54213PE Gigabit Ethernet NIC of Raspberry Pi 4
Wireless LAN access (experimental)
USB Host controller interface (HCI) drivers
Standard hub driver (USB 2.0 only)
HID class device drivers (keyboard, mouse, gamepad)
Driver for on-board Ethernet device (SMSC951x)
Driver for on-board Ethernet device (LAN7800)
Driver for USB mass storage devices (bulk only)
Drivers for different USB serial devices
Audio class MIDI input support
Printer driver
File systems Internal FAT driver (limited function)
FatFs driver (full function, by ChaN)
TCP/IP networking Protocols: ARP, IP, ICMP, UDP, TCP
Clients: DHCP, DNS, NTP, HTTP, Syslog, MQTT
Servers: HTTP, TFTP
BSD-like C++ socket API
Graphics OpenGL ES 1.1 and 2.0, OpenVG 1.1, EGL 1.4
(not on Raspberry Pi 4)
uGUI (by Achim Doebler)


For building 64-bit applications (AArch64) see the next section.

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 9.2-2019.12 (gcc-arm-9.2-2019.12-x86_64-arm-none-eabi.tar.xz) from this website.

First edit the file and set the Raspberry Pi version (RASPPI, 1, 2, 3 or 4) and the PREFIX of your toolchain commands. Alternatively you can create a 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):

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, (CM3) Cortex-A7
3 kernel8-32.img 3, (CM3) Cortex-A53
4 kernel7l.img 4B 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.

Then go to the build root of Circle and do:

./makeall clean

By default only the latest sample (with the highest number) is build. The ready build kernel.img file should be in its subdirectory of sample/. If you want to build another sample 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 or 4. There are also Raspberry Pi 2 versions, which are based on the BCM2837 SoC. These Raspberry Pi versions can be used too.

The recommended toolchain to build 64-bit applications with Circle can be downloaded here. Circle has been tested with the version 9.2-2019.12 (gcc-arm-9.2-2019.12-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 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 or 4 for AARCH = 64. Alternatively you can create a 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
PREFIX64 = aarch64-none-elf-

Then go to the build root of Circle and do:

./makeall clean

By default only the latest sample (with the highest number) is build. The ready build kernel8.img or kernel8-rpi4.img file should be in its subdirectory of sample/. If you want to build another sample 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. Put the SD(HC) card into the Raspberry Pi.

The config32.txt file, provided in the boot/ directory, is needed to enable FIQ use in 32-bit mode on the Raspberry Pi 4 and has to be copied to the SD card in this case (rename it to config.txt). Furthermore the additional file armstub7-rpi4.bin is required on the SD card then. Please see boot/README for information on how to build this file.

The config64.txt file, provided in the boot/ directory, is needed to enable 64-bit mode and has to be copied to the SD card in this case (rename it to config.txt). FIQ support for AArch64 on the Raspberry Pi 4 requires an additional file armstub8-rpi4.bin on the SD card. Please see boot/README for information on how to build this file.


  • 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.
  • tools: Tools for building Circle and for using Circle more comfortable (e.g. a serial bootloader).


The following C++ classes were added to Circle:

USB library

  • CUSBSerialDevice: Base class and interface for USB serial device drivers
  • CUSBSerialCDCDevice: Driver for USB CDC serial devices (e.g. micro:bit)
  • CUSBSerialCH341Device: Driver for CH341 based USB serial devices
  • CUSBSerialCP2102Device: Driver for CP2102 based USB serial devices
  • CUSBSerialFT231XDevice: Driver for FTDI based USB serial devices
  • CUSBSerialPL2303Device: Driver for PL2303 based USB serial devices

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:


At the moment there are only a few classes described in detail for Doxygen.

Additional Topics


Raspberry Pi is a trademark of the Raspberry Pi Foundation.

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.

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