Circle

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Overview

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 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.

Release 44.5

This intermediate release offers a revised DWHCI USB low-level driver for the Raspberry Pi 1-3 and Zero. With the system option USE_USB_FIQ one can use the FIQ (Fast Interrupt Request) for this driver, which results in a more accurate timing on the USB. This may improve the compatibility with some USB devices and may help to prevent data loss, especially when receiving MIDI data from some USB MIDI controllers, which do have only small data buffers. Because there is only one FIQ source supported in the system, the FIQ cannot be used for other purpose than the USB with this system option. The xHCI USB driver for the Raspberry Pi 4 does not support this system option and remains unchanged.

To prevent data loss from USB MIDI controllers on the Raspberry Pi 1-3 and Zero, there is also the new option usbboost=true for the file cmdline.txt now. It speeds up the USB MIDI handling, but may generate more system load on the other hand.

The system option USE_EMBEDDED_MMC_CM4 has been renamed to USE_EMBEDDED_MMC_CM and is tested to support embedded MMC memory on the Compute Module 3+ and 4.

The CI2SSoundBaseDevice class driver for I2S sound devices supports the WM8960 DAC.

There is I2C support in the HD44780 LCD display driver now.

Bug fixes:

The Stereo channels were swapped in the CHDMISoundBaseDevice class before.
There seem to be USB devices, which send more data than it is expected. This fix should prevent a system crash by faking a frame overrun error, which should be handled by the upper layers.
Building the WLAN support with the NDEBUG option was not possible.

Don't forget to update the used firmware to the one downloadable in boot/!

The 44th Step

This release comes with new features, improvements and bug fixes. There is a new HDMI sound driver class CHDMISoundBaseDevice, which allows to generate HDMI sound without VCHIQ driver, which can be easier to integrate in an application. This is shown by the sample/29-miniorgan and sample/34-sounddevices. On the Raspberry Pi 4 only the connector HDMI0 is supported. The class CI2SSoundBaseDevice now supports the PCM5122 DAC.

A new class C2DGraphics has been added to the base library, which provides 2D drawing routines, which work without flickering or screen tearing. This is demonstrated in the sample/41-screenanimations.

The scheduler library has been improved and provides the new classes CMutex and CSemaphore. Multiple tasks can wait for a CSynchronzationEvent to be set now.

There is a new serial bootloader and flash tool (Flashy), which improves the download speed and reliability. Please see the second part of the file doc/bootloader.txt for more information! You can interrupt the download process with Ctrl-C now and start again, without resetting your Raspberry Pi. You should update your bootloader kernel image(s) on the SD card in any case. The old flash tool is still available.

Circle comes with a configure script now, which can be used to create the configuration file Config.mk easier. Please enter configure -h for a description of its options.

The C++ support has been improved. Now placement new operators and static objects inside of a function can be used. Furthermore the C++17 standard is optionally supported and can be enabled with the option --c++17 of configure, if you have a toolchain version, which supports it.

Further improvements:

There is a new system option NO_BUSY_WAIT. With this option enabled, the EMMC, SDHOST and USB drivers will not busy wait for the completion of synchronous transfers any more. This should improve system throughput and network latency, but requires the scheduler in the system.
The embedded MMC memory of the Compute Module 4 can be accessed, when the system option USE_EMBEDDED_MMC_CM4 has been defined.
The class CTFTPFatFsFileServer was added to addon/tftpfileserver to support TFTP access with the FatFs filesystem module.
The class CDS18x20 in addon/OneWire has been improved and is now part of the library, not of the sample as before. It determines the used power mode of the sensor automatically.
Functions for atomic memory access have been added to <circle/atomic.h>.

Bug fixes:

System timer IRQ handling may have stopped working after a while on the Raspberry Pi 1 and Zero before.
xHCI USB controller did not work on some Raspberry Pi 4 models.
Starting secondary cores 1-3 was not reliable.
Access to USB mass-storage devices was not reliable on Raspberry Pi Model A+, 3A+ and Zero before.
Add workaround for non-compliant low-speed USB devices with bulk endpoints.
Suppress concurrent split IN/OUT requests on Raspberry Pi 1-3 and Zero in USB serial drivers.
Enable serial FIFO in polling mode too.
The screen size select-able in cmdline.txt was limited to 1920x1080 before.
Semaphore implementation in addon/linux was not IRQ safe, but used from IRQ handler in VCHIQ driver.
Allow received text segment in TCP state SYN-RECEIVED.

Don't forget to update the used firmware to the one downloadable in boot/!

Features

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)
	SMI master (experimental)
	I2S sound output and input
	HDMI sound output (without VCHIQ)
	Hardware random number generator
	Watchdog device
	Official Raspberry Pi touch screen
	VCHIQ interface and audio service drivers
	BCM54213PE Gigabit Ethernet NIC of Raspberry Pi 4
	Wireless LAN access

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
	Touchscreen driver (digitizer mode)
	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)
	LVGL (by LVGL Kft)
	2D graphics class in base library

Not supported	Bluetooth
	Camera
	USB device (gadget) mode
	USB isochronous transfers and audio

Building

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 10.3-2021.07 (gcc-arm-10.3-2021.07-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 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.

AArch64

Circle supports building 64-bit applications, which can be run on the Raspberry Pi 3 or 4. 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 10.3-2021.07 (gcc-arm-10.3-2021.07-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 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-

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 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.

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. 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.

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

C2DGraphics: Software graphics library with VSync and hardware-accelerated double buffering
CBcmWatchdog: Driver for the BCM2835 watchdog device
CDMASoundBuffers: Concatenated DMA buffers to be used by sound device drivers
CGenericLock: Locks a resource with or without scheduler
CHDMISoundBaseDevice: Low level access to the HDMI sound device (without VCHIQ)
CMPHIDevice: A driver, which uses the MPHI device to generate an IRQ
CPtrListFIQ: Container class. List of pointers, usable from FIQ_LEVEL
CSMIMaster: Driver for the Second Memory Interface

USB library

CDWHCICompletionQueue: Queues USB requests ready for completion (with USE_USB_FIQ enabled)
CUSBTouchScreenDevice: Driver for USB HID-class touchscreens

Input library

CRPiTouchScreen: Driver for the official Raspberry Pi touch screen
CTouchScreenDevice: Generic touch screen interface device

Scheduler library

CMutex: Provides a method to provide mutual exclusion (critical sections) across tasks
CSemaphore: Implements a semaphore synchronization class

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 Trading.

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.