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16.3: Memory

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  • Another important aspect is memory. Note that the AVR contains several different kinds of memory including flash program memory, static data memory or SRAM (Static Random Access Memory) and EEPROM (Electrically Erasable Programmable Read-Only Memory). Typical embedded applications run a single program repeatedly. This program must run from the moment the device is turned on until it is turned off. In many applications, this program is never updated (a microwave oven, perhaps). In other applications, it may be updated but only by qualified technicians, not by the consumer (for example, an automotive engine management system). Consequently, the program memory needs to be non-volatile, that is, it must be able to survive without power. Historically, this was done via ROM (Read-Only Memory) or PROM (Programmable Read-Only Memory). In both of these, individual bits can be set or cleared through the use of fuse/anti-fuse links. The difference is that ROMs are programmed at the time of manufacture while PROMs are programmed after manufacture. Both are permanent and cannot be reprogrammed. ROM is less expensive for large production runs but more expensive in small quantities. EEPROM has the advantage of being erasable and thus is a form of non-volatile read/write storage. Flash RAM is similar but is much less expensive. It has the downside that it must be programmed in blocks whereas EEPROM is byte-programmable. Therefore, Flash RAM (also called NVRAM) is ideal as a storage medium for the main program and EEPROM is useful as a medium for storing data that must survive a power cycle. A possible example use of EEPROM would involve saving user preference settings for a digital camera. When power is turned off and back on, the user expects the device to be “as they left it”, not reverting back to a default state. Typically, special instructions or procedures are needed to write to or read from EEPROM.

    In contrast to Flash RAM and EEPROM, SRAM is volatile, just like the common DRAM (Dynamic RAM) variants found in personal computers. Static RAM typically is made of six transistors arranged as a flip-flop for each bit of storage. DRAM, by comparison, typically consists of a single transistor-capacitor combination per bit. SRAM is less dense and more expensive per bit than DRAM but it is very fast and does not need to refreshed (the charge in a DRAM cell will leak over time requiring the cell to be refreshed at regular intervals). As a result, SRAM is used for general purpose registers and special purpose/IO registers. The ATmega 328P used in the Arduino Uno development board features 32k bytes of Flash RAM, 1k of EEPROM and 2k of SRAM. The Uno is detailed in the following chapter. By comparison, the ATmega 48A consists of 4k Flash, 256 bytes of EEPROM and 512 bytes of SRAM. Clearly, these are very modest memory footprints when compared to personal computers yet they are sufficient to power a wide range of applications. The 328P is by no means at the top of the heap, so to speak. Other controllers boast megabytes of addressable memory. There is, however, no need to over- specify a controller for a given embedded application. Extra memory that goes unused will not make the application faster; it will merely lead to more expensive hardware.

    Continuing on, all of the registers in the AVR core are memory mapped, that is, they are tied to specific addresses in the SRAM. For example, the high (ADCH) and low (ADCL) bytes of the 328P’s analog to digital converter output are found at addresses 0x79 and 0x78. Similarly, the output of port B (PORTB) is found at address 0x25 while the data direction register of port B (DDRB) is found at 0x24. A detailed table of all registers is found in the Atmel 2014 documentation under “Register Summary”. A portion of these are available in the Appendix.

    Finally, a series of other blocks such as the interrupt unit and the IO modules complete the design. We shall take a much closer look at these blocks in the upcoming chapters.