Simple prototype operating system for the Raspberry Pi
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README.md

PiOS

Simple prototype operating system for the Raspberry Pi. It aims to be a simple operating system for learning and teaching the ins and outs of operating system development in general and the architecture of a real world System on a Chip (BCM2835) in particular.

Goals

The main goal of the project is to learn loads of interesting facts about operating system development. Maybe it will be possible to use the operating system in OS-classes for teaching and experimenting.

The operating system will probably not become stable enough to use in real-life. You’ve been warned.

Maybe this can become a set of drivers and features which can be used as a static library by other embedded programming projects on the Raspberry Pi. They probably all exist already somewhere else, like in the RaspberryPi-Repository of dwelch67.

“Current” state

The code is currently being updated and rewritten in C. The following drivers are tested and work well:

  • GPIO
  • UART (AUX)
  • Postman requesting a framebuffer from VideoCore IV-GPU (only)

The following device-drivers might not work as expected:

  • I2C-interface
  • Interface to an LCD character display

I’m thinking about an interface for adding functions to specific interrupt service routines and also about a generic driver interface.

Building

First you have to decide what target you want to build for. See the subdirectory boards. For example, you may want to build for a QEMU virtual machine, then go into the folder boards/qemu/ and take every file (board.mk and pios_port_config.h) and put it into the root-folder of the repository.

Then go ahead and:

  • git submodule init && git submodule update for fetching the newlib-sources
  • make newlibfor building newlib
  • make for building the libpios-Library and the operating system kernel

If you only want to build libpios.a then you just need to call make lib.

Make sure to always build with a consistent board.mk and pios_port_config.h-definitions! Changing or swapping these half way through might result in unexpected behaviour, but that is to be expected then.

JTAG debugging

PiOS supports JTAG-debugging. You need to initialize the correct Pins of the Raspberry Pi with pios_jtag_init, these are the following pins:

BCM Pin Number Pin-number on the header JTAG-Pin Use ALT-value
25 22 tck ALT4
4 7 tdi ALT5
24 18 tdo ALT4
27 13 tms ALT4
22 15 trst ALT4

I personally use the FT232H mini module and use the openocd-configuration files included (ftdi.cfg). I took the wiring from RaspberryPi-Repository of dwelch67, who proposes the following wiring (if you need the long form, read there):

JTAG FT board Raspberry Pi
TCK CN2-7 P1-22
TDI CN2-10 P1-7
TDO CN2-9 P1-18
TMS CN2-12 P1-13
UART:
TXRX CN3-26 P1-10
RXTX CN3-25 P1-8
Ground:
GND CN2-2 P1-6
Raspi to Raspi:
TRST P1-1 P1-15
FTDI board to FTDI board:
VCCIO CN2-1 CN2-11
VCC CN3-1 CN3-3

OpenOCD debugger opens up port 3333 for arm-none-eabi-gdb to connect to. Please refer to the gdb-manual on how to do that and how to debug a program.