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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). It 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.
This release adds Wireless LAN access support in addon/wlan to Circle. Please read the README file of the sample program for details! The WLAN support in Circle is still experimental.
To allow parallel access to WLAN and SD card, a new SDHOST driver for SD card access on Raspberry Pi 1-3 and Zero has been added. You can return to the previous EMMC interface in case of problems (e.g. if using QEMU) or for real-time applications by adding DEFINE += -DNO_SDHOST to Config.mk. WLAN access is not possible then. On Raspberry Pi 4 the EMMC2 interface is used for SD card access now.
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 | |
| 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 | |
| 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) | |
| 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) | |
| LittlevGL GUI library (by Gabor Kiss-Vamosi) |
For building 64-bit applications (AArch64) see the next section.
Building is normally done on PC Linux. 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 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, (CM3) | Cortex-A7 |
| 3 | kernel8-32.img | 3, (CM3) | Cortex-A53 |
| 4 | kernel7l.img | 4 | 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
./makeall
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.
Building Circle from a non-Linux host is possible too. Maybe you have to adapt the shell scripts in this case. You need a cross compiler targetting (for example) arm-none-eabi. OSDev.org has an excellent document on the subject that you can follow if you have no idea of what a cross compiler is.
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.
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 or 4 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-
Then go to the build root of Circle and do:
./makeall clean
./makeall
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 config.txt file, provided in the boot/ directory, is only needed to enable 64-bit mode and has to be copied on the SD card in this case. It must not be on the SD card otherwise!
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: Some tools for using Circle more comfortable (e.g. a serial bootloader).
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
- Serial bootloader support
- Multi-core support
- Debugging support
- QEMU support
- Eclipse IDE support
- About real-time applications
- cmdline.txt options
- Screen escape sequences
- Keyboard escape sequences
- Memory layout
- Known issues
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.
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.
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