Month: January 2011

Accessing AVR EEPROM memory in AVRGCC

AVR microcontrollers have some amount of EEPROM memory on-chip. For instance, Atmega328 has 1K of byte-addressable EEPROM. EEPROM memory can be used to store and read variables during program execution and is nonvolatile. It means that it retains values when the power supply is off. EEPROM memory comes in handy when we need to store calibration values, remember program state before powering off (or power failure) or store constants in EEPROM memory when you are short of program memory space, especially when using smaller AVRs. Think of a simple security system – EEPROM is the ideal place to store lock combinations, code sequences, and passwords. AVR Datasheets claim that EEPROM can withhold at least 100000 writes/erase cycles.

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Store constants and arrays in program memory

In many cases, our embedded projects have to deal with various constants or arrays that aren’t changed during program execution. So why put them in RAM if we can store constant data in flash memory and leave RAM untouched. RAM is precious in micro-controllers, so leave it for stack and heaps. For instance, you are using an LCD in your project and want to send “Hello World” to it, you simply grab one of common LCD libraries and use commands like: By using this innocent action we end up in storing this string in flash memory during compilation and loading it to RAM during program start-up routines in the microcontroller. When LCDstring() command is executed string is read from SRAM not from FLASH. So we end up with two copies of same string (flash and SRAM), and worse – we occupy SRAM with constants. AVR flash memory locations can be read by the program, so this feature can be used to read constants directly from flash without loading them to RAM. Before doing this we need to use one special…

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