Non-Volatile Memory

Any processor based system requires instructions. These are typically written in a high level language such as C, which is compiled and eventually broken down into binary code representing the instructions. This binary code is used by the processor and is stored in program memory. Program memory does not change under normal operating conditions and so needs to maintain memory state when power is removed. Memory that maintains its state when power is removed is called non-volatile memory. The most common type of non-volatile memory used for program storage is Flash memory.

Volatile Memory

Processor based systems also require memory for the temporary storage of data. This memory changes very often and does not need to maintain its state when power is removed. It is referred to as volatile memory. RAM is used as volatile memory in all processor-based systems.

External vs. Internal memory

Small embedded systems often use microcontrollers. A microcontroller is a microprocessor system on a chip and can include both program and data memory. Although this limits the possibility of expansion, onboard microcontroller memory is faster than external memory and has the added benefit of more security. On the other hand, external memory allows for virtually unlimited expansion of memory.

The typical interface for main processor memory is a parallel interface, with individual address and data lines. For some types of memory expansion, a serial interface such as SPI or I²C can be used. This is often specialty memory that can tolerate slower speeds.

SRAM

SRAM (Static RAM) is RAM that can be programmed and read any number of times as long as power is applied. A single bit in an SRAM cell typically uses four, six, or eight transistors to control access to the cell, provide read and write abilities, and maintain the cell state of a 1 or 0. SRAM is easy to use both from a hardware and a programming standpoint. An SRAM memory chip just needs power and ground to keep its state. Separate address and data buses are used to read and write data, with one or more chip selects used as control signals.

Some specialty SRAM chips use an SPI interface. These are useful for modifying existing systems where additional SRAM is required, a pin-compatible replacement to upgrade the existing SRAM is design limited, and redesigning the circuit board is impractical.

DRAM

Another type of RAM is called DRAM, or Dynamic RAM. As compared to SRAM, a DRAM memory cell can be only one transistor with a capacitor. This has the advantage of making the memory matrix very dense, providing low-cost RAM with very high capacity. The chief disadvantage of DRAM is that if left alone, the memory state in the cell dissipates over time. For that reason, DRAM memory must be refreshed on a regular basis to prevent the memory cell state from dissipating. This refresh is accomplished by regularly reading the memory cells. In order to do this a separate circuit performs regular reads of the cells to keep them from losing their state. Like SRAM, DRAM loses its state when power is completely removed from the chip.

For modern desktop and laptop computers, the amount of DRAM is measured in Gigabytes. DRAM is inserted into computers in small circuit boards called DIMMs or SIMMs.

EEPROM

EEPROM is another form of non-volatile memory. It is similar to Flash except that the memory can be programmed in smaller blocks, or even one byte at a time. EEPROM is much slower than Flash memory and also uses more room than Flash, and so it used for data storage. EEPROM memory cells are also less dense than Flash and so are not as cost-effective for program memory storage as Flash. EEPROM is commonly added to systems by a serial interface, either SPI or I²C.

F-RAM

Ferroelectric RAM is a relatively new technology that has not yet seen widespread use. The transistor structure is similar to a DRAM cell, except that a ferroelectric material is used to store the charge. It uses less power and can be programmed much faster than Flash memory.

Cache is another type of SRAM and is used to help speed the CPU of processor based systems. Cache SRAM chips are designed to be extremely fast for read and write operations, and consequentially are more expensive than other types of non-volatile memory.

SD Cards

SD Cards are larger than microSD cards, about the size of a large postage stamp. These Flash cards are suitable for applications where the card may need to be frequently removed, such as cameras.

SSDs

Hard drives are being slowly replaced by Solid State Drives (SSDs) which are in a hard drive format but instead of magnetic platters, use updated forms of reliable Flash memory. While SSDs do not have the write endurance of hard drives, they are much faster and have a much higher memory density.