This Entry will look at what's inside the Raspi and, more importantly, what it all does. The Peripherals Entry will then look at what other equipment you'll need to run your Raspi.
There will initially be two models of the Raspi, Model A and Model B:
| Model A | Model B | |
| Intended Price | £16 | £22 |
| CPU/GPU/RAM | Broadcom BCM2835 'system-on-a-chip' with 700MHz ARM11 CPU | |
| RAM capacity | 128 MB | 256 MB |
| USB ports | 1 | 2 |
| Storage | Memory Card reader | |
| Audio/Video outputs | HDMI (audio/video) Composite cable (video) 3.5mm jack (audio) | |
| Network Card | None | 10/100 Ethernet |
| Power Supply | 5V Micro USB connector | |
| Power Ratings | 500mA | 700mA |
| Low-Level Peripherals | GPIO, SPI, UART, I2C, I2S | |
System On A Chip
The brain of the Raspi is the Broadcom BCM2835 'system-on-a-chip', which includes the main components needed for a computer system. These are:
- The Central Processing Unit (CPU, or 'processor'), which handles the main workload.
- The Graphics Processing Unit (GPU), which accelerates the process of producing the complicated graphics you see on your screen.
- The Random Access Memory (RAM) which acts as somewhere for the CPU to keep the information that it is working on (as opposed to your hard drive or memory card, where things are stored more permanently).
The Raspi doesn't use an x86 processor like the ones found in many Microsoft Windows PCs. Instead, it is powered by a 700 MHz ARM11 processor. This is a processor with some history behind it:
The first ARMs, then known as Acorn RISC Machines, were made by Acorn, the company that made the BBC Micro. Designed for the BBC Computer Literacy Project in the 1980s, the Micro had the BASIC programming language built in, allowing schools to teach children about real Computer Science. The BBC Micro was also used by Acorn to design the Acorn Archimedes, the first home computer powered by an ARM processor.
Acorn closed in 1998, but the licence for the ARM processor was sold off as 'ARM Holdings', which still exists today as ARM PLC. The rest of the company was eventually purchased by Broadcom. This means ARM processors are still used today – in fact, you'll find an ARM11 processor inside the Nintendo DS, the Apple iPad, TomTom sat-navs, and many smartphones.
USB Port
The Universal Serial Bus (USB) replaced all the old methods of attaching peripherals (mice, keyboards, printers etc) to your computer. For those who have only ever known USB: yes, there used to be a different metal slot for each peripheral you wanted to plug in. The cable ends that plugged into them were big lumps of metal and plastic, some of which actually had screws that you tightened to hold them in place. With the advent of USB, all it takes is one little USB port and you can attach almost anything to your computer (or games console)1. Each model of the Raspberry Pi has at least one USB port.
Memory Card Reader
A lot of computers still make use of a Hard Disk Drive (HDD). A stack of discs called platters are coated in a thin layer of magnetic material, mounted on a spindle and spun at speeds of up to 15,000rpm. Each platter is divided into millions of tiny regions, each of which is a magnetic dipole which represents either a 1 or a 0 depending on its orientation. Read-and-write heads mounted over each disc can detect (and change) the orientation of each dipole.
A tidier alternative is Solid State Memory, so called because there are no moving parts. Solid State devices work by storing electrons in between pairs of tiny components called transistors. Each pair stores a '0' if there are electrons blocking the way between the transistors, or a '1' if the way between the two transistors is clear. Many thousands of transistor pairs can fit inside a little plastic card – a Secure Digital (SD) card is hardly bigger than a postage stamp but will hold up to 32 gigabytes of information. The latest generation of SD cards will soon be available, and will be able to hold up to 2 terabytes (2,048 gigabytes). An HDD with the same capacity as an equivalent SD card will cost more and generate more noise and heat, and so the Raspi is fitted with a memory card reader. You can therefore run your Raspi's operating system off an SD card and store your files there.
Audio and Video Outputs
The Raspi includes a High-Definition Multimedia Interface (HDMI) output. HDMI cables are pretty cheap (don't be fooled by the promise of better output from expensive cables) and produce High Definition audio and video output. HDMI can be easily converted to Digital Visual Interface (DVI), but not to Visual Graphics Array (VGA) – see the Peripherals entry.
In addition, there are outputs for a composite cable and a 3.5mm jack. The 3.5mm jack will be familiar to most readers as the headphone socket in computers and MP3 players, and most headphones and speaker sets will be compatible. The composite output is also common elsewhere, as it uses the yellow cable from those red/white/yellow composite cables used by modern consoles. The red and white cables carry audio, and are also seen in some stereo sets. It's possible to use a composite output and a 3.5mm jack to produce the red/white/yellow output that will plug into your TV, in some cases using a SCART adapter – this is covered in the Peripherals entry. It's also possible to convert from composite to VGA, but this generally requires some form of powered adaptor to convert the signal.
Network Card and Alternatives
Model B includes a 10/100 Ethernet card, allowing it to be connected by cable to a modem, router or another computer. The RJ45 cable this uses has shiny metal 'teeth' on one side and a plastic clip that has to be pressed down on the other side. 10/100 refers to the speed at which data can be transferred over the cable: 100 megabits per second is the speed usually used and equates to 12.5 MB per second, which is faster than the average broadband Internet connection in most countries as of 2012. Both models of Raspberry Pi could also be connected to a wireless adapter using the USB port – the current 802.11n wireless standard allows for speeds of up to 300 megabits per second, so this would also be a reasonable option.
Power Supply
The Micro USB adaptor on the Raspi is used on many tablets and mobiles to connect to a power supply. This requires a 5V Micro USB power supply capable of providing at least 500mA for model A, and at least 700mA for a model B. This isn't hard to find, as most Micro USB mobile phone chargers provide more than this.
Low-Level Peripherals
The Raspi also has basic pins that allow knowledgeable users to attach other computer boards, send signals to motors to drive robots, and power the computer without using the Micro USB slot. These are listed in the table above for reference only, as they take time to explain. The Raspberry Pi Foundation have produced the Gertboard, which can connect to the low-level peripheral pins and will light up LEDs, drive motors and read from sensors. Note that the Gertboard connects directly to the low-level peripheral pins, which may be separate and need soldering onto the Raspberry Pi.
1 A USB cable contains a pair of wires for data transfer and another pair allowing the peripheral to draw power from your computer, which is why a wired mouse doesn't need its own power supply. It also means you can power a reading light or a tiny vacuum cleaner using a USB port, if you want.
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