The Raspberry Pi +

Model B - Rev 1. 1Q 2012	Model B - Rev 2. 3Q 2012	Model B+ 3Q 2014
Pi 2 - Model B - Rev 1.1 1Q 2015	Pi 3 - Model B - Rev 1. 1Q 2016

The Raspberry Pi (R-Pi)

The Raspberry Pi was developed in the UK by the charitable Raspberry Pi Foundation. The goal was to produce a low cost, single board, computer that could be used by children inside or outside of school to develop computer programming skills that they could take forward into further education.

The performance of this credit card sized computer is described on the Raspberry Pi Frequently Asked Questions page as ".... graphics capabilities are roughly equivalent to Xbox 1 level of performance. Overall real world performance is something like a 300MHz Pentium 2, only with much, much swankier graphics."

The first boards were slated to become available in 1Q 2012 and very quickly sold out, at one point in March, demand was reported as running at 700 units per second, with demand fuelled by a range of customers including hobbyist and programmers as well as education. At the time of writing, demand is still high and availability is limited, but I managed to get hold of two before they became generally available - 1 each from Farnell (Element 14) and RS-Components.

When first released, the only way to meet the target $25 and $35 price points for the Model A and Model B was to have the Raspberry Pi manufactured in China. Since summer 2012, the Raspberry Pi has also been produced in the UK by Sony at their factory in Pencoed, South Wales. As of November 2012, they were producing around 16,000 Raspberry Pis a week. Element 14/Premier Farnell have made the decision to move the bulk of their Raspberry Pi manufacture to South Wales. This development has resulted in 30 new jobs being created at the Sony factory. (You can read the full story in this article on the Raspberry Pi Blog). The Rev 2. boards made by Sony have "Made in the UK" printed next to the power connector - click on the photos above to see the difference between the boards in more detail.

The Raspberry Pi website is home for the Pi and the User Forum contains a wealth of information from users and developers, including details of some of the many projects that have sprung up around the Pi.

The Raspberry Pi Hub on the elinux.org wiki has lots of useful information about R-Pi hardware and software.

In a significant upgrade to the Model B specification with no increase in price, the Model B+ was announced on 14th July 2014, "This isn’t a “Raspberry Pi 2″, but rather the final evolution of the original Raspberry Pi." The key features of the new model, extracted from the Raspberry Pi blog page are :-

"The Model B+ uses the same BCM2835 application processor as the Model B. It runs the same software, and still has 512MB RAM" with following key improvements:

• More GPIO. The GPIO header has grown to 40 pins, while retaining the same pin-out for the first 26 pins as the Model B.

• More USB. We now have 4 USB 2.0 ports, compared to 2 on the Model B, and better hot-plug and over-current behaviour.

• Micro SD. The old friction-fit SD card socket has been replaced with a much nicer push-push micro SD version.

• Lower power consumption. By replacing linear regulators with switching ones we’ve reduced power consumption by between 0.5W and 1W.

• Better audio. The audio circuit incorporates a dedicated low-noise power supply.

• Neater form factor. We’ve aligned the USB connectors with the board edge, moved composite video onto the 3.5mm jack, and added four squarely-placed mounting holes.

Raspberry Pi Model A

The model A has a single USB port and no built-in LAN connector. This is the board expected to be taken by schools who are not expected to require the LAN connection.

Although some minor changes have also been made to the Model A, it is no longer recommended by the RPi Foundation for schools use. "We recommend the Raspberry Pi 2 Model B for use in schools: it offers more flexibility for learners than the leaner (Pi 1) Model A+, which is more useful for embedded projects and projects which require very low power."

As the Model A/A+ is not really "mainstream" any more, if it ever was, I do not intend to document any Model A developments on this page, you can find details of the Model A+, released in November 2014, on the Raspberry PI Foundation Model A+ page.

Over-clocking (Model B)

When the Raspberry Pi was released, it was possible to over-clock the processor from the default 700Mhz by limited amount by editing the config.txt file that was read when the system booted. It was also possible to run the processor are even higher frequencies by "over-volting" the processor but this set a bit in the processor that recorded the fact that the chip had been over-volted and voided the warranty.

Further testing by the Foundation has shown that it is possible to over-volt the chip without damaging it by controlling the over clocking and over-volting to a temperature limit. The Pi can now be over-clocked to 1GHz without voiding the warranty, leading to a significant performance increase. This is dynamically controlled to ensure that the processor temperature does not exceed 85 degrees C - the full story is on the Raspberry Pi Blog here.

You can read a good set of posts about Over-Volting on the OverClockers Forum here.

Raspberry Pi Boot Process (From elinux.org)

When power is applied to the RPi, it is the GPU that is initially active, and boots the system. At power-up, the CPU is offline, and a small RISC core on the GPU is responsible for booting the SoC, therefore most of the boot components are actually run on the GPU code, not the CPU.

In the most recent Raspbian distributions for RPi, The boot order and components are as follows:

• First stage bootloader - This is used to mount the FAT32 boot partition on the SD card so that the second stage bootloader can be accessed. It is programmed into the SoC itself during manufacture of the RPi and cannot be reprogrammed by a user.    
• Second stage bootloader (bootcode.bin) - This is used to retrieve the GPU firmware from the SD card, program the firmware, then start the GPU.
• GPU firmware (start.elf) - Once loaded, this allows the GPU to start up the CPU. An additional file, fixup.dat, is used to configure the SDRAM partition between the GPU and the CPU (the default is 64MB for the GPU, with the balance used by the CPU). At this point, the CPU is released from reset and execution is transferred over
• User code - This can be one of any number of binaries. By default, it is the Linux kernel (usually named kernel.img), but it can also be another bootloader (e.g. U-Boot), or a bare-bones application.

 The boot partition also contains config.txt which can be used to set various configuration options such as video parameters, the ARM CPU clock speed etc.

⁺ Raspberry Pi is a trademark of the Raspberry Pi Foundation, this website is in no way affiliated to the Raspberry Pi foundation.