Nuts and Volts - July 2010

#24

AVR Memory Part 2: EEPROM

Follow along with this series! Joe’s book & kits are available at www.nutsvolts.com

by Joe Pardue

Recap More on Computer Memory

Last month, we introduced computer memory and learned a little about how the C programming language uses pointers for addresses to data variables. We also learned to use Pelles C to test C programming concepts on a PC. This month, we will get back to looking at examples for the AVR memory as applied to reading the EEPROM.

Pointer Review

52 July 2010

Yes, we belabored this last month but I’ve never felt comfortable explaining pointers because I still tend to mess them up. (I remember how many false starts I had trying to learn them.) On the surface, they are simple: A pointer is a memory location containing the address of another memory location. In C, a pointer is a data type for a variable intended to hold the address of another variable. You tell C that a data type is a pointer by marking it with an asterisk ‘*’. So, when you define char *myCharPointer, you are telling the C compiler that myCharPointer is the address of a character. Then, if you want to set this variable to the address of a char, you use the ‘&’ operator to extract it. So, if you define char myChar = ‘a’, you can get the address of myChar by stating myCharPointer = &myChar. This may sound simple, but implementation can be the killer.

Cliff Lawson — the number one poster on AVRFreaks — manages large software projects with 50+ programmers and he says that 50% of the bugs come from pointers. And his guys know what they are doing. So, expect to have to approach learning about pointers many times and from many directions. The best thing I can suggest to help you learn to safely use pointers is to write small pieces of code and thoroughly test them before including them in larger pieces of code.

When you get experienced enough that pointers seem second nature, that’s when you will start to get into real trouble and you’ll get bugs that drive you, well … buggy. That’s part of the price of admission to C programming of microcontrollers. (Hey, what a nice title for a book — and coincidentally, a book that you can get from Nuts & Volts, along with an excellent hardware projects kit to give yourself a leg up on this C stuff.)

An ideal computer as imagined by John von Neumann would have a single memory that holds the program and data. In the real world, however, memory speed and cost play a role, and few real computers are entirely von Neumann in their architecture. Computers that use different kinds of storage for programs and data are referred to as having a Harvard Architecture, and most contemporary microcontrollers use modifications of this concept.

When a computer is running, it must read the program memory as rapidly as possible. Based on the instructions contained in program memory, it reads and writes data memory. The main difference in the program and data memory areas is that the computer only needs to read program memory, but it must both read and write to data memory. It turns out that from a hardware perspective, it is much cheaper to make memory that is mostly read from, than memory that is both read and written to. So, to fit these different needs and economies, two types of memory were developed. For data, we use RAM (Random Access Memory) that can be easily written to and read from. RAM can be volatile meaning that it doesn’t matter if it loses its data when the power is off. However, for the stored program that is mostly read we use ROM (Read Only Memory). ROM must be non-volatile so that it can retain the program when the power is off. ROM is much cheaper to manufacture than RAM. You often see computers with plentiful, cheap ROM, but with much smaller amounts of the more expensive RAM.

The terminology isn’t 100% clear, of course, because if ROM is read only, how do we store (write) the program on it in the first place? The answer is that it’s really a ‘read mostly’ design that can be written to — usually very slowly and only with special programming equipment. But, as we will see in a minute, one of the innovations of the AVR was that it uses a special type of ROM (Flash EEPROM) for the program memory that allows it to be programmed by ISP (In System Programming).