Memory Flashcards
Define ROM (read only memory)
-Memory that cannot be written, it holds data.
-ROM holds the instructions for a computer to start once the power is on.
Define non-volatile in terms of memory
Retains data even when power is off.
Define access time
the time it takes for the correct data to become available after an address is presented to the device
How has memory performance changed over time?
Manufacturers have made access time as short as possible
from 500ns to 5ns or less
How can memory devices be categorised?
-programmed by the user
-programmed by the manufacturer
Define ROM
Read only memory.
-Data cannot be overwritten after manufacturer has built the program into the chip.
What are the benefits of ROM
-Low cost
-High volume
-stable program
What are the most common user-programmable devices?
-EEPROM (electronically erasable) - written byte to byte
-Flash Memories - written in a block
What are some pros of EEPROM over Flash memory?
-EEPROM can be reporgrammed multiple times
-non-volatile
-faster than flash memory in write and erase operations
Define EPROM
Erasable programmable read only memory
-has a glass window where strong UV light erases the data
-or can erased via electrical pulses
-EEPROM/UVEPROM
Explain the importance of access time
If the access time is longer than required by the processor, errors occur.
If the access time is shorter than required, the device will be more expensive than necessary
What are the main differences between Flash and E2PROM?
-E2PROM: individual locations can be erased
-FLASH: erasing circuits are common to a block of memory cells
-Flash is more cost effective
Why is the number of write/erase cycles important?
Certain software can wear out memory if written incorrectly
Define RAM
Random access memory- volatile memory that stores information from the microprocessor or other digital devices
Define SRAM
Static RAM
-uses transistors and latches to store data
-volatile
Define DRAM
Dynamic RAM
-based on capacitors and stores each bit of data into a cell
Define NVRAM
Non-volatile RAM
A type of flash memory that contains an internal battery to maintain data when the power supply is off
Define FRAM
-Ferroelectric RAM
A low power non-volatile memory with fast RAM
-Has the best parts of Flash/EEPROM and RAM
What is the difference between FRAM and other memory devices?
-FRAM stores data in ferroelectric material
-Write and read access times are 2-3 digit nanoseconds
-Is non-volatile but has very fast access
Give an example of an FRAM device
Ramtron FM1608
Compare SRAM and DRAM
SRAM: Uses transistors
Very fast
more expensive
DRAM: Uses capacitors
slower
less expensive
-cannot retain data as long
Compare SRAM to EEPROM
SRAM has infinite write/erase cycles
What is considered in terms of memory when designing a microcontroller circuit
-what will happen when the power supply is off when a location is being written
-microprocessor supervisory circuits help prevent critical data being lost
What is the smallest unit of data?
Byte
Explain the write operation
-Address decoder selects specific location in memory
-memory gets write command
-data is bussed and stored in memory address
Explain the read operation
Address decoder receives a code from address bus
-specific location in memory is selected
-memory gets read command and data is copied and outputted
What is the difference between ROM and PROM?
ROM:
-permanently programmed by manufacturer
PROM:
-works the same but is programmed by user
Why is unlimited read/write cycles good?
Every time data is erased and written, it doesn’t affect the lifespan of the memory. Better endurance
Compare non-volatile memory to volatile memory in terms of endurance, cost and access time
Volatile (RAM):
-very fast access times
-(almost) unlimited read and write cycles
-generally cheaper
Non-volatile:
-Generally slower, access time depends on type of memory
-Limited write cycles which affects lifespan
-typically more expensive
