Architecture of Raspberry Pi

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Architecture of Raspberry Pi

December 5, 2015 | Article | No Comments

Raspberry pi is a credit-card-sized single-board computer developed by Raspberry Pi Foundation with the intention of promoting the teaching of basic computer science in schools.

Raspberry pi has a Broadcom BCM2853 System on Chip (SoC) which includes ARM1176JF-S 700 MHz processor. The firmware includes number of “Turbo” modes so that user can attempt overclocking up to 1 GHz without affecting the warranty. Pi also implements VideoCore IV GPU and originally shipped with 256 MB of RAM, later upgraded to 512 MB. It does not include a built-in hard disk or solid-state drive as used on PC / notebook, but uses and SD car for booting and long-term storage.

Raspberry Pi foundation offer two versions of Pi, priced at US$25 and US35.

For the Operating System, The Foundation provides Debian and Arch Linux ARM distribution for download and also planned tools for supporting Python as the main programming language with support for BBC Basic (via RISC OS image or the “Brandy Basic” clone for Linux), C, and Perl.

This article will discuss about Raspberry Pi’s architecture, for model A ($25) and model B ($35).

General Specification

Model A has one USB port and no Ethernet controller with less cost than the Model B with two USB ports and a 10/100 Ethernet controller. Eventhough the Model A doesn’t have an RJ45 Ethernet port, it can connect to a network by using a user-supplied USB Ethernet or Wi-Fi adapter. In reality there is no difference between a model A with an external Ethernet adapter and a model B with one built in, because the Ethernet port of the model B is actually a built-in USB Ethernet adapter. As is typical of modern computers, generic USB keyboards and mice are compatible with the Raspberry Pi.

The Raspberry Pi does not come with a real-time clock,so an OS must use a network time server, or ask the user for time information at boot time to get access to time and date for file time and date stamping. However, a real-time clock (such as the DS1307) with battery backup can be added via the I²C interface.

Hardware accelerated video supported default by Pi is H.264. At the same time The Raspberry Pi Foundation has release two additional codecs that can be bought separately, MPEG-2 and Microsoft’s VC-1. Also Pi will support CEC, enabling it to be controlled with the television’s remote control.

The Processing Unit

Both model A and model B has Broadcom BCM2835 (CPU, GPU, DSP, SDRAM). The CPU use ARM1176JFZ-S core (ARM11 family) and run on 700MHz clock. The SDRAM is shared between CPU and GPU but in different amount on two model. Model A has 256 MB of memory while model B has 512 MB of memory.

On the older beta model B boards, 128 MB was allocated by default to the GPU, leaving 128 MB for the CPU. On the first 256 MB release model B (and Model A), three different splits were possible. The default split was 192 MB (CPU RAM), which should be sufficient for standalone 1080p video decoding, or for simple 3D, but probably not for both together. 224 MB was for Linux only, with just a 1080p framebuffer, and was likely to fail for any video or 3D. 128 MB was for heavy 3D, possibly also with video decoding (e.g. XBMC).  For the new model B with 512MB RAM initially there were new standard memory split files released( arm256_start.elf, arm384_start.elf, arm496_start.elf) for 256MB, 384MB and 496MB CPU RAM (thus leaves 256MB, 128MB and 16MB video RAM). But a week or so later the RPF released a new version of start.elf that could read a new entry in config.txt (gpu_mem=xx) and could dynamically assign an amount of RAM (from 16 to 256MB in 8MB steps) to the GPU, so the older method of memory splits became obsolete, and a single start.elf worked the same for 256 and 512 MB Pis.

The Input Output

For video output aspberry use Composite RCA (PAL and NTSC), HDMI (rev 1.3 & 1.4), and raw LCD Panels via DSI 14 HDMI from 640×350 to 1920×1200 plus various PAL and NTSC standards for both model.

For audio output both model utilize 3.5 mm jack, HDMI, and also I²S audio.

For a low-level purpose, Raspberry Pi utilize 8 x GPIO, UART, I²C bus, SPI bus with two chip selects, I²S audio +3.3 v, +5V, ground.

Power Rating and Source

To run Raspberry Pi a minimal 5 volt must be supplied to the board. While the model A need 300mA (rating 1.5 W), the model B use 700 mA (rating 3.5 W). The power can be delivered to MicroUSB or GPIO header.


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