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A C++ bare metal environment for Raspberry Pi with USB (32 and 64 bit)

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Quick Overview

Circle is a C++ bare metal programming environment for the Raspberry Pi. It provides a foundation for developing operating systems, firmware, and low-level applications directly on Raspberry Pi hardware without relying on an existing operating system.

Pros

  • Offers direct hardware access and control for Raspberry Pi devices
  • Supports multiple Raspberry Pi models, including Pi 1-4, Zero, and Compute Modules
  • Provides a comprehensive set of libraries for various hardware components and protocols
  • Includes sample programs and tutorials for learning bare metal programming

Cons

  • Requires advanced programming skills and understanding of hardware architecture
  • Limited compared to full-fledged operating systems in terms of features and abstractions
  • May have a steeper learning curve for developers accustomed to higher-level programming environments
  • Documentation could be more extensive for some advanced topics

Code Examples

  1. Blinking an LED:
#include <circle/actled.h>
#include <circle/timer.h>

CActLED led;

void Kernel::Run(void)
{
    while (1)
    {
        led.On();
        CTimer::SimpleMsDelay(500);
        led.Off();
        CTimer::SimpleMsDelay(500);
    }
}
  1. Reading GPIO input:
#include <circle/gpiopin.h>

CGPIOPin button(17, GPIOModeInput);

void Kernel::Run(void)
{
    while (1)
    {
        if (button.Read() == LOW)
        {
            // Button pressed
        }
    }
}
  1. Using UART for serial communication:
#include <circle/serial.h>

CSerialDevice serial;

void Kernel::Run(void)
{
    serial.Write("Hello, World!\n", 14);
    
    char buffer[100];
    int bytesRead = serial.Read(buffer, sizeof(buffer));
    // Process received data
}

Getting Started

  1. Clone the Circle repository:

    git clone https://github.com/rsta2/circle.git

  2. Install the required toolchain (ARM cross-compiler) for your system.

  3. Build the Circle libraries:

    cd circle
./configure
make

  4. Create a new project in the sample directory or modify an existing sample.

  5. Build your project:

    cd sample/yourproject
make

  6. Copy the generated kernel.img to your Raspberry Pi's SD card and boot.

Competitor Comparisons

raspberrypi logo

pico-sdk

3,917

Pros of pico-sdk

  • Officially supported by Raspberry Pi, ensuring compatibility and regular updates
  • Extensive documentation and examples for easier development
  • Broader community support and ecosystem

Cons of pico-sdk

  • Limited to Raspberry Pi Pico and RP2040-based boards
  • Steeper learning curve for beginners compared to Circle's simplicity

Code Comparison

pico-sdk:

#include "pico/stdlib.h"

int main() {
    gpio_init(PICO_DEFAULT_LED_PIN);
    gpio_set_dir(PICO_DEFAULT_LED_PIN, GPIO_OUT);
    while (true) {
        gpio_put(PICO_DEFAULT_LED_PIN, 1);
        sleep_ms(500);
        gpio_put(PICO_DEFAULT_LED_PIN, 0);
        sleep_ms(500);
    }
}

Circle:

#include <circle/actled.h>
#include <circle/scheduler.h>

class CKernel : public CApplication {
public:
    virtual void Run(void) {
        while (1) {
            m_ActLED.On();
            CScheduler::Get()->MsSleep(500);
            m_ActLED.Off();
            CScheduler::Get()->MsSleep(500);
        }
    }
};

Both examples demonstrate LED blinking, showcasing the different approaches and syntax used by each framework.

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RF24

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OSI Layer 2 driver for nRF24L01 on Arduino & Raspberry Pi/Linux Devices

Pros of RF24

  • Focused library for nRF24L01 and nRF24L01+ 2.4GHz wireless modules
  • Extensive documentation and examples for various platforms
  • Active community and regular updates

Cons of RF24

  • Limited to specific wireless modules, not a general-purpose OS
  • Requires additional hardware components for full functionality
  • May have steeper learning curve for beginners in embedded systems

Code Comparison

RF24:

RF24 radio(7, 8);  // CE, CSN pins
radio.begin();
radio.openWritingPipe(address);
radio.write(&data, sizeof(data));

Circle:

#include <circle/startup.h>
CKernel Kernel;
CScreenDevice Screen;
Kernel.Initialize();
Screen.Write("Hello World", 11);

Key Differences

  • RF24 is a specialized library for wireless communication, while Circle is a bare metal programming environment for Raspberry Pi
  • Circle provides a broader set of features for OS-like functionality, whereas RF24 focuses on a specific wireless protocol
  • RF24 is cross-platform and can be used with various microcontrollers, while Circle is specific to Raspberry Pi hardware
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arduino-esp32

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Pros of arduino-esp32

  • Broader hardware support: Designed specifically for ESP32 microcontrollers
  • Extensive library ecosystem: Leverages Arduino's vast collection of libraries
  • Easier integration with existing Arduino projects

Cons of arduino-esp32

  • Less low-level control: Abstracts some hardware-specific features
  • Potentially higher resource usage due to Arduino framework overhead
  • Limited to ESP32 family, while Circle supports multiple Raspberry Pi models

Code Comparison

arduino-esp32:

#include <WiFi.h>

void setup() {
  WiFi.begin("SSID", "PASSWORD");
  while (WiFi.status() != WL_CONNECTED) {
    delay(1000);
  }
}

Circle:

#include <circle/net/netsubsystem.h>

CNetSubSystem NetSubSystem;

void Initialize() {
  NetSubSystem.Initialize();
  NetSubSystem.GetConfig()->SetDHCP(TRUE);
}

The arduino-esp32 code demonstrates easier Wi-Fi setup, while Circle shows more low-level network initialization. arduino-esp32 benefits from simpler syntax and built-in Wi-Fi support, whereas Circle offers more granular control over system components.

arduino logo

ArduinoCore-avr

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The Official Arduino AVR core

Pros of ArduinoCore-avr

  • Extensive library support and ecosystem for Arduino boards
  • Well-documented and beginner-friendly API
  • Large community and widespread adoption

Cons of ArduinoCore-avr

  • Limited to AVR microcontrollers, less flexible for other platforms
  • Higher-level abstraction may result in less efficient code
  • Slower development cycle for core updates

Code Comparison

ArduinoCore-avr:

void setup() {
  pinMode(LED_BUILTIN, OUTPUT);
}

void loop() {
  digitalWrite(LED_BUILTIN, HIGH);
  delay(1000);
  digitalWrite(LED_BUILTIN, LOW);
  delay(1000);
}

Circle:

#include <circle/actled.h>
#include <circle/scheduler.h>

void CActLED::On(void)
{
    m_nEnableCount++;
    Write(TRUE);
}

Summary

ArduinoCore-avr is ideal for beginners and rapid prototyping on AVR-based Arduino boards, offering a vast ecosystem and easy-to-use API. Circle, on the other hand, provides a bare-metal environment for Raspberry Pi, allowing for more low-level control and potentially better performance. The choice between the two depends on the target platform and the developer's needs for abstraction versus control.

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README

Circle

Overview

Circle is a C++ bare metal programming environment for the Raspberry Pi. It should be usable on most existing models (tested on model A+, B, B+, on Raspberry Pi 2, 3, 4, 400, 5 and on Raspberry Pi Zero (2)), except on the Raspberry Pi Pico, which is not supported. It is not known, if it works on the Raspberry Pi 500 and on Compute Module 5. 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.

The 49th Step

This release comes with an improved dot-matrix display management. All driver classes for dot-matrix displays should be derived from the class CDisplay now. The old character display support for ST7789- and SSD1306-based displays is still available, but will be deprecated in a future version. Instead there is the new class CTerminalDevice, which implements a scrolling character terminal display for any display driver, which is derived from CDisplay. This class is also used to implement the class CScreenDevice now for the known terminal display on a firmware-driven frame buffer device. The following displays are currently supported by CDisplay-derived driver classes:

  • Firmware-driven frame buffer (CBcmFrameBuffer)
  • ST7789 SPI display (CST7789Display)
  • SSD1306 I2C display (CSSD1306Display)
  • ILI9341 SPI display (CILI9341Display)

Beside the terminal support also the 2D graphics (C2DGraphics) and LVGL (CLVGL) support have been updated to work with all these displays. The 2D graphics support works with logical colors (T2DColor) now. There is a new class C2DImage, which manages the color conversion from logical to physical colors for 2D sprite images.

Classes, which support the displaying of text on dot-matrix displays, allow the selection of the used font now. The default system font can by defined with system option DEFAULT_FONT.

There is a new class CWindowDisplay, which allows to use multiple non-overlapping windows on a dot-matrix display. This is demonstrated in the multi-core program sample/43-multiwindow.

sample/08-usbkeyboard, sample/41-screenanimations and addon/lvgl/sample have been updated for the new display management and support SPI and I2C displays too.

There are a number of API breaking changes for the new display support, which are listed in this article.

More news:

  • The external PCIe bus of the Raspberry Pi 5 can be accessed using the class CBcmPCIeHostBridge now. See the test/pcie-external for details. Interrupts from the external PCIe bus are available via the INTA pin at the IRQ number ARM_IRQ_PCIE_EXT_HOST_INTA.
  • A driver for XPT2046-based touchscreens has been added. See test/xpt2046-touchscreen for details.
  • The FatFs support has been updated to R0.15a + patch1.
  • The LVGL support has been updated to v9.2.2.
  • DMA channels are usable in different modes now. Before a DMA channel, which has been used for an asynchronous transfer, could not be used for synchronous transfers afterwards without re-initialization. The completion routine has to be set prior to each asynchronous transfer now.

Fixes:

  • The detection of WM8960-based I2S codecs did not work, when the I2C address was explicitly specified.
  • Commit ae00d9d8 in Step48 was leading to lost MIDI events with USB MIDI devices on the Raspberry Pi 1-3 and has been reverted. In the rare case that you are using an USB device, which has a MIDI interface and an other (e.g. HID) interface, and the device is directly connected to the root port without USB hub in-between (e.g. on Raspberry Pi Zero), you have to define the system option USE_NAK_USB_FIX now.
  • The MQTT client might have crashed after receiving a disconnect from peer before.
  • The check for the length, opcode and block number of incoming ACK packets in the TFTP daemon used an invalid logical operator. This could have caused receiving invalid files on read requests.
  • The HideLink THEC64 USB keyboard did not work before.
  • The initial LVGL mouse cursor was not centered on the Raspberry Pi 5.
  • The configure script and cFlashy can be used on macOS now. cFlashy caused a build error before on macOS.

The recommended firmware version has been updated. The option initial_turbo=0 has been added to the file config.txt, because newer firmware versions enable initial_turbo=60 by default now, which can disturb the Circle device initialization.

Features

Only the features with a "x" or other info are currently supported on the Raspberry Pi 5.

Circle supports the following features:

GroupFeaturesRaspberry Pi 5
C++ build environmentAArch32 and AArch64 supportAArch64 only
Basic library functions (e.g. new and delete)x
Enables all CPU caches using the MMUx
Interrupt support (IRQ and FIQ)IRQ only
Multi-core support (Raspberry Pi 2, 3 and 4)x
Cooperative non-preemtive schedulerx
CPU clock rate managementx
Clang/LLVM support (experimental)x
Debug supportKernel logging to screen, UART and/or syslog serverx
C-assertions with stack tracex
Hardware exception handler with stack tracex
GDB support using rpi_stub (Raspberry Pi 2 and 3)
Serial bootloader (by David Welch) includedx
Software profiling support (single-core)x
QEMU support
SoC devicesGPIO pins (with interrupt, Act LED) and clocksx
Frame buffer (screen driver with escape sequences)limited
UART(s) (Polling and interrupt driver)x
System timer (with kernel timers)x
Platform DMA controllerDMA40 only
RP1 platform DMA controller (Raspberry Pi 5 only)x
EMMC SD card interface driverx
SDHOST SD card interface driver (Raspberry Pi 1-3)
PWM output (2 channels)4 channels
PWM sound output (on headphone jack)with adapter
I2C master(s) and slavemasters only
SPI0 master (Polling and DMA driver)x
SPI1 auxiliary master (Polling)
SPI3-6 masters of Raspberry Pi 4 (Polling)SPI1-3 and 5
SMI master
I2S sound output and inputx
HDMI sound output (without VCHIQ)x
Hardware random number generatorx
Watchdog devicex
Official Raspberry Pi touch screen (v1 only)
VCHIQ interface and audio service drivers
BCM54213PE Gigabit Ethernet NIC of Raspberry Pi 4
MACB / GEM Gigabit Ethernet NIC of Raspberry Pi 5x
Wireless LAN accessx
Driver for XPT2046-based touch screensx
USBHost controller interface (HCI) driversx
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 CDC Ethernet devices (RTL815x, QEMU)
Driver for USB mass storage devices (bulk only)x
Driver for USB floppy disk devices (experimental)x
Driver for USB audio streaming devices (RPi 4 only)x
Drivers for different USB serial devicesx
Audio class MIDI input supportx
Touchscreen driver (digitizer mode)x
Printer driverx
MIDI gadget driver
Serial CDC gadget driver
Mass-storage device gadget driver
File systemsInternal FAT driver (limited function)x
FatFs driver (full function, by ChaN)x
TCP/IP networkingProtocols: ARP, IP, ICMP, UDP, TCPx
Clients: DHCP, DNS, NTP, HTTP, Syslog, MQTT, mDNSx
Servers: HTTP, TFTPx
BSD-like C++ socket APIx
GraphicsOpenGL 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 libraryx
Not supportedBluetooth

Building

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 13.2.Rel1 (arm-gnu-toolchain-13.2.rel1-x86_64-arm-none-eabi.tar.xz) from this website. This is the recommended toolchain for AArch32 builds.

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:

RASPPITargetModelsOptimized for
1kernel.imgA, B, A+, B+, Zero, (CM)ARM1176JZF-S
2kernel7.img2, 3, Zero 2, (CM3)Cortex-A7
3kernel8-32.img3, Zero 2, (CM3)Cortex-A53
4kernel7l.img4B, 400, CM4Cortex-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.

AArch64

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 13.2.Rel1 (arm-gnu-toolchain-13.2.rel1-x86_64-aarch64-none-elf.tar.xz) from this website. This is the recommended toolchain for AArch64 builds.

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.

Installation

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.

Directories

  • 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).

Classes

The following C++ classes were added to Circle:

Base library

  • CDisplay: Base class for dot-matrix display drivers
  • CTerminalDevice: Terminal support for dot-matrix displays
  • CWindowDisplay: Non-overlapping window on a display

USB library

  • CUSBFloppyDiskDevice: Driver for USB floppy disk devices (CBI transport)

Input library

  • CXPT2046TouchScreen: Driver for XPT2046-based touch screens

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.

Additional Topics

Trademarks

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.