In This Chapter

Understanding the evolution of the Raspberry Pi

Exploring the options of the general-purpose input/output (GPIO) port

Discovering the secrets of the revision number held in your Pi

Finding the impact of your Raspberry Pi model on projects in this book

In the beginning was the Raspberry Pi, and all was good. Then the Pi begat the revision 2 with more memory, which begat the Model A, which begat … well, you get the idea. With all this begating, we want to introduce you to what the Raspberry family looks like and how what you have may affect the projects you want to do. In this chapter, we show you how to understand the differences in the versions of Raspberry Pi that have been produced. By looking at the memory, GPIO, and video and processor variations that have been produced, you can see how the Raspberry Pi has improved over the years.

In the Beginning: Looking at the Evolution of the Raspberry Pi

The original Raspberry Pi, like so many firsts, did not need a qualifier to its name, but it had one. It was known as the Raspberry Pi Model B. The second version was known as the Model A. It’s impossible for end-users to complete their own upgrades, so the two versions are separate models.

The Model A has no Ethernet socket, only one USB connector, and typically less memory; as a consequence, it’s about $10 cheaper than the Model B. Also, the Model A consumes about one-third less power than the Model B, which can be important for battery-powered standalone applications.

The Raspberry Pi Foundation, the creator of the Raspberry Pi, is a registered charity with the aim of promoting computer literacy. They worked for many years developing prototypes before they released anything. The original Raspberry Pi 1 Model B set the benchmark for what a Raspberry Pi was, and it’s great credit to the developers that each new variant on the Raspberry Pi had improvements but did not break the compatibility with the previous models.

One confusing aspect of the Raspberry Pi is that changes and improvements to the Pi were not always accompanied by a change in name, so users are never quite sure what they’re buying. We want to try to clear that confusion: There have only been three basic named versions of the Raspberry Pi to date — the original (or 1), the plus (+), and the 2. Given that there is a Model A and a Model B variant for each model, that means there are six named types of Raspberry Pi (see Table 4-1).

Deciding Which Raspberry Pi to Buy

Only immediately after the launch of a new version of the Raspberry Pi do you have any choice in what to buy. With backward compatibility and no change in price, there is little demand for older models. So your only real choice is whether to get a Model A or a Model B. In most cases, this is a no-brainer — the Model B is the obvious choice. You should only buy a Model A if you have some specific reason for doing so (for example, the cost, smaller form factor, or lower power consumption).

Memory

The memory chip used on every Raspberry Pi 1 was in an unusual form factor in the field of computers. It was the so-called chip-on-chip system — the processor chip had contacts on both sides (one set on the base to connect to the printed circuit board, and another set on the top for the memory chip). The memory chip was then soldered directly onto the processor chip to give a very compact design, only 1mm thick. This technique is more usually found in smartphones where space is at a premium. With the Raspberry Pi, it had the unfortunate effect of limiting the amount of memory that could be fitted, because this sort of memory chip was only available in a few memory sizes, with 512MB being the biggest.

The latest Raspberry Pi 2 versions have discontinued that trend and allowed a larger memory chip to be fitted. The Raspberry Pi 1 Model B+ and the Raspberry Pi 2 Model B+ look almost identical from the top, but on the underside you can clearly see the separate memory (the large black block), as shown in Figure 4-1 and Figure 4-2.

General-purpose input/output (GPIO)

The plus models have 40-pin dual-row header pins to allow access to the general-purpose input/output (GPIO) port pins. Some of these pins are needed to turn the processor into a computer system by accessing the SD card and Ethernet and USB peripherals; others of the spare pins are brought out for the user. The GPIO pins carry logic signals that the computer can control or read and are how you can get the Raspberry Pi to switch outputs and sense inputs. Not all these pins are GPIO pins; there are several instances of 5V, 3V3, and ground. The pre-plus models had only a 26-pin connector and fewer pins were brought out. Figure 4-3 shows the pin out of this connector.

With the exception of the original Issue 1 of the Raspberry Pi, the GPIO pins have remained on the same physical pin of the connector through subsequent revisions.

Each GPIO pin can be switched inside the processor to perform a different specialist function. This has led to some people designating these pins to these functions, and then labeling the remaining pins as GPIO from 0 upward. In our opinion, this is stupid because it adds an extra, pointless, totally artificial layer of obfuscation that you have to wade through. Another form of confusion arises when some people label these pins with the physical pin number on the connector. So, for example, they refer to pin 3 in the software and that gets translated internally to GPIO 2. That’s a little more understandable, but it’s no substitute for referring to GPIO pins as what they are in the processor.

Figuring Out Which Raspberry Pi You Have

You may not know exactly which Raspberry Pi you have. You can easily see if you have a 26-way or a 40-way GPIO connector, but how can your program tell which pins are available?

In Linux, everything is a file, so there is a way of finding out exactly what you have. Just go to a command-line prompt and type the following:

cat /proc/cpuinfo

On a Raspberry Pi 1 system, you see the following:

Processor: 0
model name : ARMv6-compatible processor rev 7 (v6l)
Features : swp half thumb fastmult vfp edsp java tls
CPU implementer : 0x41
CPU architecture : 7
CPU variant : 0x0
CPU part : 0xb76
CPU revision : 7

Hardware : BCM2708
Revision : 0010
Serial : 00000000a5fb87e8

The interesting thing is the revision number, which tells you a lot about your Raspberry Pi. Table 4-2 tells you what the revision number means. The words in parentheses are the factories where the Raspberry Pi boards were made.

You may notice that revision 0011 is the Computer Module, a Raspberry Pi with a very different form factor. This is a Pi built on a small printed circuit board (PCB) with all the connections brought out on PCB edge connectors. This then plugs into a base board of your own design and is meant for industrial or commercial applications. There is also a blank breakout board to do your development on.

If you look at the CPU info on the Raspberry Pi 2 system, you see four processor blocks in much the same format as the Raspberry Pi 1 — one for each core. The last three entries look like this:

Hardware : BCM2709
Revision : a01041
Serial: 000000001664c635

The revision number is in an entirely different format with different groups of bits having specific meanings. A collection of bytes is known as a word, and the processor in the Raspberry Pi uses words that are four bytes or 32 bits long. Bit 23 in the revision word tells you if the revision number is in the old format or the new. For a new format revision number, this bit will be a 1. The interpretation of the revision number under the new format is given by splitting the word up into bytes in this manner:

SRRR MMMM PPPP TTTT TTTT VVVV

Where

• S is one bit giving the revision scheme (0 = old, 1 = new).
• RRR is three bits indicating the RAM size (0 = 256MB, 1 = 512MB, 2 = 1,024MB)
• MMMM is four bits giving the manufacturer (0 = Sony, 1 = Egoman, 2 = Embest, 3 = Unknown, 4 = Embest).
• PPPP is four bits giving the processor (0 = 2835, 1 = 2836).
• TTTT TTTT is eight bits giving the type (0 = A, 1 = B, 2 = A+, 3 = B+, 4 = Pi 2 B, 5 = Alpha, 6 = Computer Module).
• VVVV is the four-bit version or revision (0–15).

So, the hex number a01041 in the preceding readout gives the following bit pattern when you write it out in binary:

1010 0000 0001 0000 0100 0001
SRRR MMMM PPPP TTTT TTTT VVVV

For a quick hexadecimal-to-binary converter and a conversion table, check out www.binaryhexconverter.com/hex-to-binary-converter or search the web for “convert hexadecimal to binary.”

So, breaking this down:

• S is 1, which means it’s the new revision scheme.
• R is 010, which is 2 in binary, and that means the RAM is 1,024MB.
• M is 0000, which is 0 in binary, and that means the manufacturer is Sony.
• P is 0001, which is 1 in binary, which means the processor is 2836.
• T is 0000 0100, which is 4 in binary, which means it’s a Raspberry Pi 2 Model B.
• V is 0001, which is 1 in binary, which means it’s version 1.

For information on how to read binary, check out www.dummies.com/how-to/content/digital-electronics-binary-basics.html.

There is a warranty bit that gets set if you attempt to overclock the system. This is to prevent people from overclocking the Raspberry Pi, breaking it, and then claiming it was faulty. This warranty bit is bit 24, but when the Raspberry Pi 2 was introduced, the faster clock and higher voltage required for the processor’s core was automatically setting this bit, so the warranty bit has been changed to bit 25 for the Raspberry Pi 2. If you’re buying a Raspberry Pi secondhand, check this bit before parting with your cash.

For all the projects in this book, you can use any of the versions of the Raspberry Pi. Newer models will run faster, but this shouldn’t have much of an impact because physical computing is heavily involved with human interaction. If you find that something runs too fast, a simple small sleep command will throttle back the speed.

On the upside, faster speed is useful if you’re using graphics programs like Gimp to prepare graphics for your programs. And a quad-core machine holds out the possibility of running several programs simultaneously at full speed.