A C++ bare metal environment for Raspberry Pi with USB
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Latest commit 39d4030 Nov 15, 2018



Raspberry Pi is a trademark of the Raspberry Pi Foundation.

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Circle is a C++ bare metal programming environment for the Raspberry Pi. It should be useable on all existing models (tested on model A+, B, B+, on Raspberry Pi 2 and 3 and on Raspberry Pi Zero). It provides several ready-tested C++ classes which can be used to control different hardware features of the Raspberry Pi. Together with Circle there are delivered some samples which demonstrate the use of its classes.

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.

The 36th Step

In this step the class CUserTimer has been added to Circle, which implements a fine grained user programmable interrupt timer. It can generate up to 500000 interrupts per second (if used with FIQ).

CUserTimer is used in the new sample/36-softpwm to implement Software PWM (Pulse Width Modulation), which can be used to control the brightness of a LED or a servo. See the README file in this directory for details.

This release of Circle comes with an updated version of the FatFs module (in addon/fatfs). Furthermore there have been fixes to the touchscreen driver and the bootloader tools. Finally there are the new system options SAVE_VFP_REGS_ON_IRQ and SAVE_VFP_REGS_ON_FIQ in include/circle/sysconfig.h, which can be enabled if your application uses floating point registers in interrupt handlers.

You may be interested in the Alpha GDB server by Farjump, which can be used to source-level debug single-core Circle applications with the GNU debugger (GDB).

The options to be used for cmdline.txt are described in doc/cmdline.txt.


Circle supports the following features:

Group Features
C++ build environment 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 and 3)
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
QEMU support
Legacy devices GPIO pins (with interrupt, Act LED) and clocks
Frame buffer (screen driver with escape sequences)
UART (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 and slave
SPI0 master (Polling and DMA driver)
SPI1 auxiliary master (Polling)
I2S sound output
Hardware random number generator
Official Raspberry Pi touch screen
VCHIQ interface and audio service drivers
USB Host controller interface (HCI) driver
Standard hub driver
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 (reduced 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
GUI uGUI (by Achim Doebler)
Bluetooth Device inquiry support only
USB BR/EDR dongle driver
Internal controller of Raspberry Pi 3 B


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 you need a toolchain with Cortex-A7/-A53 support. A toolchain, which works for all of these, can be downloaded here. Circle has been tested with the version 7-2018-q2-update from this website.

First edit the file Rules.mk and set the Raspberry Pi version (RASPPI, 1, 2 or 3) 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):

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) ARMv7-A
3 kernel8-32.img 3, (CM3) Cortex-A53

For a binary distribution you should do one build with RASPPI = 1 and one with RASPPI = 2 and include the created files kernel.img and kernel7.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 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 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, or how to make one.


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.


  • 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 following C++ classes were added to Circle:

Base library

  • CUserTimer: Fine grained user programmable interrupt timer (based on ARM_IRQ_TIMER1)

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.