Memory Flashcards
Name the three Embedded system’s functionality aspects.
P rocessing
• processors
• transformation of data
Storage
• memory
• retention of data
Communication
• buses
• transfer of data
In memory what dose m x n refer to?
m x n: m words of n bits each
• word: group of bits handled as a unit by processor
• k = log2(m) address input signals (m = 2^k words)
What should an ideal Memory have as characteristics?
Ideal Menory should be written from inside the system, FAST, have a unlimited rewritability and store words for ever.
Write ability: Manner and speed a memory can
be written
Storage permanence: ability of memory to hold
stored bits after they are written
What is ROM and what are the characteristics?
“Read-Only” Memory
• Nonvolatile memory
• Can be read from but not always written to,
by a processor in embedded system
• Traditionally written to (programmed) before
inserting to embedded system
• Access times are often slower than RAM:
roughly 1.5 times that of DRAM
What dose a Decoder do?
Converts its binary input into a one-hot output
• One-hot output: Exactly one of output lines is given a 1 at a time
▪ Decoder with n outputs and log2 n inputs is called a
log2 n x n decoder
What is Mask-programmed ROM? What are advantges/disadvantege? what are they used for?
Connections “programmed” at fabrication
• set of masks
Lowest write ability, only once
Highest storage permanence, bits never change unless damaged
Typically used for final design of high-volume systems
• spread out NRE cost for a low unit cost
What is OTP ROM? What are advantges and what are they used for?
One-time programmable ROM
Connections “programmed” after manufacture by user with special device, ROM programmer blows fuses where connections should not exist
Very low write ability, written only once and requires ROM programmer device
Very high storage permanence, bits don’t change unless reconnected to programmer and more fuses blown
Commonly used in final products
• cheaper, harder to inadvertently modify
What is EPROM? What are advantges and what are they used for?
Erasable programmable ROM
• Transistor has “floating” gate surrounded by an insulator
• Programmed using higher voltage
• EPROM is erased in their entirety
• Ultraviolet light (UV) is used for erasing (takes 5 to 30 minutes)
Better write ability
• can be erased and reprogrammed thousands of times
▪ Reading is much faster than writing
▪ Reduced storage permanence
• Data lasts about 10 years but is susceptible
to radiation and electric noise
Typically used during development,
limited usage in productions parts
(window covered)
What is EEPROM? What are advantges and what are they used for?
Electrically EPROM, Electrically erasable programmable ROM
can program and erase individual words
▪ Better write ability
• in-system programmable with built-in memory controller
• writes very slow due to erasing and programming
Similar storage permanence to EP ROM (≈ 10 years)
• Far more convenient than EP ROMs, but more expensive
What is Flash Memory? What are advantges and what are they used for?
Extension of EEPROM
Same floating gate principle
• Same write ability and storage permanence
• Block operation only, typically several kilo bytes
Fast erase
Writes to single words may be slower than EEP ROM
Most microcontrollers use Flash as program memory
▪ Embedded systems: Storage for large data items in nonvolatile memory
What is SRAM? What are advantges?
Static Random-access”/read-write memory
Memory cell uses flip-flop to store bit
Holds data as long as power supplied, no refresh required
What is DRAM? What are advantges?
Dynamic Random-access”/read-write memory
Memory cell uses MOS transistor and capacitor to store bit
• “Refresh” required due to capacitor leak
• typical refresh rate 15.625 µs
• Slower to access than SRAM, but cheaper than SRAM
What is NVRAM? What are advantges?
Nonvolatile “Random-access”/read-write memory
Holds data after external power removed
Battery-backed RAM
• SRAM with own permanently connected battery
• Write cycles as fast as read cycles (nanoseconds)
• No limit on number of writes unlike nonvolatile ROM-based memory
SRAM with EEP ROM or flash of same size
• Stores complete RAM contents on EEPROM or flash before power turned off
fast but Expensive
unlimited erase cycles
What types of Composing memory exist and what is the use?
- Connect side-by-side to increase width of words
- Connect top to bottom to increase number of words
- Combine techniques to increase number and width of words
