MuntsOS Embedded Linux

This framework supports Linux on several single board microcomputers. The goal of the MuntsOS project is to deliver a turnkey, RAM resident Linux operating system for very low cost single board microcomputers. With MuntsOS installed, such microcomputers can treated as components, as Linux microcontrollers, and integrated into other projects just like traditional single chip microcontrollers.

Quick Setup Instructions for the Impatient

Instructions for installing the MuntsOS cross-toolchain development environment onto a host computer are found in Application Note #1 and Application Note #2.

Instructions for installing MuntsOS to a target computer are found in Application Note #3 and Application Note #15.

Documentation

The documentation for MuntsOS (mostly application notes) is available online at:

http://git.munts.com/muntsos/doc

Embedded Linux Distribution in a Kernel

MuntsOS is a stripped down Linux distribution that includes a small compressed root file system within the kernel image binary itself. At boot time the root file system is unpacked into RAM and thereafter the system runs entirely in RAM.

Each kernel release tarball contains a kernel image file (.img), which may be common to several different microcomputer boards, and one or more device tree files (.dtb) that are specific to particular microcomputer boards. Some kernel release tarballs also contain one or more device tree overlay files (.dtbo) that can make small changes to the device tree at boot time.

Prebuilt MuntsOS kernel release tarballs are available at:

http://repo.munts.com/muntsos/kernels

Extensions

The MuntsOS root file system can be extended at boot time using any of three mechanisms:

First, if /boot/tarballs exists, any gzip tarball files (.tgz) in it will be extracted on top of the root file system. Typically you would use this mechanism for customized /etc/passwd, .ssh/authorized_keys, and similiar system configuration files.

Second, if /boot/packages/${BOARDBASE} exists, any Debian package files (.deb) in it will be installed into the root file system. Note that packages from the Debian project will probably not work with MuntsOS. Packages should be built specifically for MuntsOS. (The .deb package file format is simply convenient to use, as it is supported by BusyBox.)

The GPIO Server extension package demonstrates how to build a Debian package that adds application specific software to MuntsOS.

Thirdly, the system startup script /etc/rc can be configured via a kernel command line option to search for a subdirectory called autoexec.d in various places, such as SD card, USB flash drive, USB CD-ROM or NFS mount. If an autoexec.d subdirectory is found, each executable program or script in it will be executed when the system boots.

The idea is to build a MuntsOS kernel (which takes a long time) once and install it to the target platform. Then application specific software can be built after the fact and installed as one or more tarball files in /boot/tarballs, Debian package files in /boot/packages/${BOARDBASE}, or extension programs in /boot/autoexec.d.

Prebuilt MuntsOS extension packages and programs are available at:

http://repo.munts.com/muntsos/extensions

Thin Servers

Boot Files + Kernel Files + Extensions = Thin Server

The Thin Server is a system design pattern that is little more than a network interface for a single I/O device. Ideally, a Thin Server will be built from a cheap and ubiquitous network microcomputer like the Raspberry Pi. The software must be easy to install from a user's PC or Mac without requiring any special programming tools. It must be able to run headless, administered via the network. It must be able to survive without orderly shutdowns, and must not write much to flash media. It must provide a network based API (Application Programming Interface) using HTTP as a lowest common denominator.

MuntsOS, with its operating system running entirely from RAM, serves well for the Thin Server, and the two concepts have evolved together over the past few years. The simplest way to use MuntsOS is to download one of the prebuilt Thin Server .zip files and extract it to a freshly formatted FAT32 SD card. You can then modify autoexec.d/00-wlan-init on the SD card to pre-configure it for your wireless network environment, if desired, before inserting it in the target board. After booting MuntsOS, log in from the console or via SSH (user "root", password "default") and run sysconfig to perform more system configuration.

Note: BeagleBone boards require the boot flag to be set on the FAT32 boot partition on the SD card or on-board eMMC. The ROM boot loader in the CPU will ignore any partitions that are not marked as bootable.

MuntsOS Application Notes 3 and 15 contain more detailed instructions about how to install a MuntsOS Thin Server.

Prebuilt MuntsOS Thin Servers are at available at:

http://repo.munts.com/muntsos/thinservers

Boards

BeagleBone

The BeagleBone was one of the first low cost Linux microcomputers. It originally sold for USD $89 at its launch in October 2011.

The BeagleBone has a Texas Instruments Sitara AM3359 processor running at 720 MHz and 256 MB of RAM. It has two USB port sockets: One type A host port and one type mini-B device port. Unlike any of its successors, the original BeagleBone has its USB device port connected to a USB hub instead of directly to the AM3359. Three distinct USB devices are visible to the host on the device port socket: The AM3359 device port, a USB JTAG device, and a USB serial port device connected to the AM3359 console serial port. The BeagleBone also has two PRU (Programmable Realtime Unit) I/O processors on board that are capable of very fast I/O operations.

MuntsOS includes device tree overlays than can be enabled by sysconfig that allow configuring any of the expansion header GPIO pins with config-pin. The system startup script /etc/rc will initialize GPIO pin modes according to /etc/pinmux.conf. By default, the following devices are are enabled on the two 46-pin expansion headers:

• I²C bus controller device /dev/i2c-2
• Serial port device /dev/ttyS1
• Serial port device /dev/ttyS2
• Serial port device /dev/ttyS4
• Serial port device /dev/ttyS5
• SPI slave device /dev/spidev2.0
• SPI slave device /dev/spidev2.1

Newly manufactured BeagleBone boards assembled with a 1 GHz AM3358 processor are apparently still available from Special Computing.

BeagleBone Black

The BeagleBone Black is a cost reduced version of the BeagleBone. It currently sells for about USD $55. The BeagleBone Black originally sold for USD $45 at its launch in April 2013, which would have been an impressive feat except that the Raspberry Pi had already arrived on the market a few months earlier at USD $35. Although the BeagleBone Black was more capable thttp://git.munts.com/muntsos/doc/han the first couple of Raspberry Pi generations, it has been overshadowed by the Raspberry Pi Model 2 and 3, which sport quad-core processors. The great strength of the BeagleBone Black and its kin compared to the Raspberry Pi family is the sheer number of GPIO pins and peripheral ports available on its two 46-pin expansion headers. Even after eMMC, I²C, SPI, and UART pins have been allocated, there are 42 GPIO pins available.

The BeagleBone Black has a Texas Instruments Sitara AM3358 processor running at 1 GHz, 512 MB of RAM and 4 GB eMMC flash on board. It uses the same kernel as the BeagleBone, with a different device tree.

Unlike the original BeagleBone (above) and the BeagleBone Green (below), the BeagleBone Black has an HDMI video output (though with a pesky micro HDMI connecteor). The HDMI interface consumes a large number of GPIO pins when it is enabled. This MuntsOS port does not enable the HDMI interface.

BeagleBone Black Wireless

The BeagleBone Black Wireless is a variant of the BeagleBone Black that has replaced the wired Ethernet interface with a built-in Wifi radio. It also has replaced the mini-B slave USB receptacle with a more modern micro-B receptacle. It is otherwise highly compatible with the BeagleBone Black. It sells for about USD $70, considerably more than any of the other boards supported by MuntsOS, and also considerably more than a BeagleBone Green plus a USB WiFi adapter.

The BeagleBone Black Wireless uses the same kernel as the BeagleBone, with a different device tree.

MuntsOS does not currently support the on-board Bluetooth radio.

BeagleBone Green

The BeagleBone Green is a cost reduced version of the BeagleBone Black, from Chinese manufacturer Seeed Studio, that sells for about USD $44. Changes from the BeagleBone Black design are:

• Removed coaxial power jack . (+5V can be supplied via the slave USB port or P9 expansion header instead.)
• Removed HDMI receptacle and support circuitry.
• Changed the slave USB receptacle from mini-B to micro-B.
• Added two Grove System connectors, one carrying 3.3V I²C signals and one carrying 3.3V logic level serial port signals.

The BeagleBone Green uses the same kernel as the BeagleBone, with a different device tree.

The BeagleBone Green http://git.munts.com/muntsos/doc/is cost competitive with the Raspberry Pi, costing only a little more but including on board eMMC and a USB cable. It has only a single core processor, compared to the quad-core Raspberry Pi 3, but provides many more GPIO pins on its two 46-pin expansion headers. It also has separate dedicated host and slave USB ports as well as the two Grove sockets.

The BeagleBone Green plus a USB WiFi adapter is about USD $20 cheaper than a BeagleBone Black Wireless, while retaining the possibility for wired Ethernet.

BeagleBone Green Wireless

The BeagleBone Green Wireless is a variant of the BeagleBone Green that has replaced the wired Ethernet interface with a built-in Wifi radio. It is otherwise highly compatible with the BeagleBone Green. It sells for about USD $53.

The BeagleBone Green http://git.munts.com/muntsos/doc/Wireless uses the same kernel as the BeagleBone, with a different device tree.

MuntsOS does not currently support the on-board Bluetooth radio.

The BeagleBone Green Wireless is a mixed blessing: It has 4 USB ports and on-board WiFi, but commandeers quite a few of the expansion header GPIO pins for the on-board radios. Among other things, this seems to prohibit using SPI1. Also, the physical layout prevents using the BeagleBone Click Shield, which has some advantages over the newer mikroBus Cape.

PocketBeagle

The PocketBeagle is a cost and size reduced version of the BeagleBone Black. It currently sells for about USD $25 and is intended for the same market niche as the Rasperry Pi Zero. Although considerably more expensive than either version of the Raspberry Pi Zero, the PocketBeagle has many more I/O devices directly accessible from its expansion headers.

The PocketBeagle uses the same kernel as the BeagleBone, with a different device tree. The PocketBeagle device tree enables the following devices on its two 36-pin expansion headers:

• USB host port
• I²C bus controller device /dev/i2c-1
• I²C bus controller device /dev/i2c-2
• PWM output 0:0
• PWM output 2:0
• Serial port device /dev/ttyS0
• Serial port device /dev/ttyS4
• SPI slave device /dev/spidev1.0
• SPI slave device /dev/spidev2.1

The expansion headers are cleverly arranged such that the two inner rows match the MikroElektronika mikroBUS specification. If female sockets are installed on the top of the PocketBeagle, two Click Boards can be plugged directly into the expansion headers.

Like the Raspberry Pi Zero, the PocketBeagle comes without on-board eMMC, USB cable, micro-SD card, or expansion headers.

Unlike the Raspberry Pi Zero, the PocketBeagle expansion headers do not match its progenitors, so BeagleBone capes cannot be used on it.

Raspberry Pi

The Raspberry Pi is a family of low cost Linux microcomputers selling for USD $5 to $75 (depending on model). There have been four generations of Raspberry Pi microcomputers, each using a successively more sophisticated Broadcom ARM core CPU.

Some Raspberry Pi models have an on-board Bluetooth radio that uses the serial port signals that are brought out to the expansion header. By default, MuntsOS port disables the on-board Bluetooth radio, in favor of the serial port on the expansion header.

Raspberry Pi 1

Raspberry Pi 1 models have a BCM2708 ARMv6 single-core CPU running at 700 to 1000 MHz and come with with 256 MB to 512 MB of RAM. They have 10/100 Ethernet, 1 to 4 USB ports, HDMI, RCA composite video and a stereo headphone or three-pole A/V jack. They also have several miniature connectors for camera and LCD display modules as well as a single 26 or 40 pin 2.54 mm pitch GPIO expansion connector.

All Raspberry Pi 1 models use the same 32-bit ARMv6 kernel and toolchains, with different device trees.

With the advent of the Raspberry Pi Zero 2, all Raspberry Pi 1 models are now obsolete. MuntsOS Embedded Linux will continue to support all Raspberry Pi 1 models.

Raspberry Pi 2 and 3

The Rasbperry Pi 2 Model B has a 900 MHz BCM2709 ARMv7 Cortex-A7 (900 MHz BCM2710 ARMv8 Cortex-A53 on later production boards) quad-core CPU and comes with 1 GB of RAM. It is mechanically compatible with the Raspberry Pi 1 Model B+, with 10/100 Ethernet, 4 USB ports, 3.5 mm A/V jack, and a 40-pin GPIO expansion header.

The Rasbperry Pi 3 Model B has a 1200 MHz BCM2710 ARMv8 Cortex-A53 quad-core CPU and has 1 GB of RAM along with on-board Bluetooth and WiFi radios.

The Raspberry Pi 3 Model A+ has the same form factor as the Raspberry Pi 1 Model A+, with only one USB host port and no wired Ethernet. It has a 1400 MHz BCM2710 ARMv8 Cortex-A53 quad-core CPU and has 512 MB of RAM along with on-board Bluetooth and WiFi radios.

The Raspberry Pi 3 Model B+ has a 1400 MHz BCM2710 ARMv8 Cortex-A53 quad-core CPU and has improved power management and networking components.

The Raspberry Pi Zero 2 has the same form factor as the Raspberry Pi Zero W, with a 1000 MHz BCM2710 ARMv8 Cortex-A53 quad core CPU and 512 MB of RAM along with on-board Bluetooth and WiFi radios.

All Raspberry Pi 2 and 3 models can use the same 32-bit ARMv7 kernel and toolchains, with different device trees. There are also 64-bit AArch64 toolchains and ARMv8 kernels available for the Raspberry Pi 3, though with 1 GB or less of RAM there is no particular advantage in running 64-bit.

Raspberry Pi 4

The Raspberry Pi 4 Model B has a 1500 MHz BCM2711 ARMv8 Cortex-A72 quad-core CPU and is available with 1 to 8 GB of RAM. It diverged significantly from the Raspberry Pi 1 B+ form factor, with the USB and Ethernet ports reversed, two micro-HDMI connectors instead of a single full size HDMI connector, and a USB-C power connector instead of micro-USB. Two of the USB ports are 3.0 and two are 2.0. A major improvement is a Gigabit Ethernet controller connected via PCI Express instead of the USB connected Ethernet used for all earlier models. The Raspberry Pi 4 Model B uses the same wireless chip set as the 3+.

The Raspberry Pi Model 4 B is much more powerful than any previous Raspberry Pi, and is completely usable as a general purpose desktop computer. It does require significantly more power and generates significantly more heat. It supports dual monitors, which is mostly irrelevant for embedded systems. I have some doubt about the long term physical robustness of the micro-HDMI connectors.

Raspberry Pi USB Gadget Kernels

MuntsOS also provides Raspberry Pi kernels with dedicated USB Gadget support enabled. These kernels run on Models 1 A, A+, CM1, Zero, Zero Wireless, 3 A+, CM3, Zero 2, 4 B, and CM4. You can supply power to and communicate with a compatible Raspberry Pi solely through the USB port. This kernel supports USB Network, Raw HID, and Serial Port gadgets, selected by bits in the OPTIONS word passed on the kernel command line. The USB Gadget Thin Servers have USB Network Gadget selected by default.

The absolute minimum possible usable Raspberry Pi kit consists of a Raspberry Pi Zero, Zero W, or Zero 2, a micro-USB cable, and a micro-SD card with one of the MuntsOS Raspberry Pi USB Gadget Thin Servers installed.

Cross-Toolchains

I build a custom Ada/C/C++/Fortran/Go cross-toolchain (using Crosstool-NG) for each MuntsOS platform family. Each cross-toolchain requires a number of additional software component libraries, which are packaged and distributed separately but installed into the same directory tree as the parent cross-toolchain.

I also build Free Pascal cross-compilers. These rely on the libraries in the Ada/C/C++/Fortran/Go cross-toolchain, which must be installed first.

Sometimes cross-toolchains can be shared among different platforms: For example, the Raspberry Pi 2 and Raspberry Pi 3 can use the same cross-toolchains (the 32-bit ARMv7 cross-toolchains nominally built for the Raspberry Pi 2), and the Raspberry Pi 3 and Raspberry Pi 4 can also use the same cross-toolchains (the 64-bit AArch64 cross-toolchains nominally built for the Raspberry Pi 3).

Cross-toolchain packages built for Debian Linux are available at:

http://repo.munts.com/debian11

Since they are statically linked, it may be possible to use these cross-toolchain packages on other Linux distributions (possibly with the help off a conversion utility like alien).

For the convenience of users of Linux distributions other than Debian, snapshot toolchain tarballs for each platform family are available at:

http://repo.munts.com/muntsos/toolchains

Git Repository

The source code for MuntsOS is available at:

https://github.com/pmunts/muntsos

Use the following command to clone it:

git clone https://github.com/pmunts/muntsos.git

File Repository

Prebuilt binaries for MuntsOS are available at:

http://repo.munts.com/muntsos

Make With Ada Projects

• 2017 Ada Embedded Linux Framework
• 2019 Modbus RTU Framework for Ada (Prize Winner!)

Questions or comments to Philip Munts [email protected]

I am available for custom system development (hardware and software) of products based on embedded Linux microcomputers or other processors.