Data Storage

An m word memory can store m x n: m words of n bits each. One word is located at one address therefore to address m words we need.

k = Log2(m) address input signals
or k number address lines can address m = 2k words

Example 4,096 x 8 memory:
• 32,768 bits
• 12 address input signals
• 8 input/output data signals



Memory access

The memory location can be accessed by placing the address on the address lines. The control lines read/write selects read or write. Some memory devices are multi-port i.e. multiple accesses to different locations simultaneously.



Memory Specifications

The specification of a typical memory is as follows:

The storage capacity: The number of bits/bytes or words it can store .

The memory access time (read access and write access): How long the memory takes to load the data on to its data lines after it has been addressed or how fast it can store the data upon supplied through its data lines. This reciprocal of the memory access time is known as Memory.

Bandwidth

The Power Consumption and Voltage Levels: The power consumption is a major factor in embedded systems. The lesser is the power consumption the more is packing density.
Size: Size is directly related to the power consumption and data storage capacity.





There are two important specifications for the Memory as far as Real Time Embedded Systems are concerned.
– Write Ability.
– Storage Performance.

Write ability

It is the manner and speed that a particular memory can be written.
• Ranges of write ability
– High end
• processor writes to memory simply and quickly e.g., RAM
– Middle range
• processor writes to memory, but slower e.g., FLASH, EEPROM (Electrically Erasable and Programmable Read Only Memory)
– Lower range
• special equipment, “programmer”, must be used to write to memory e.g., EPROM, OTP ROM (One Time Programmable Read Only Memory)
– Low end
• bits stored only during fabrication e.g., Mask-programmed ROM
• In-system programmable memory
– Can be written to by a processor in the embedded system using the memory
– Memories in high end and middle range of write ability

Storage permanence

It is the ability to hold the stored bits.

Range of storage permanence.
– High end
• essentially never loses bits.
• e.g., mask-programmed ROM.

– Middle range
• holds bits days, months, or years after memory’s power source turned off.
• e.g., NVRAM

– Lower range
• holds bits as long as power supplied to memory.
• e.g., SRAM

– Low end
• begins to lose bits almost immediately after written.
• e.g., DRAM

Nonvolatile memory
– Holds bits after power is no longer supplied.
– High end and middle range of storage permanence.


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