Common Memory Types

Read Only Memory (ROM)

This is a nonvolatile memory. It can only be read from but not written to, by a processor in an embedded system. Traditionally written to, “programmed”, before inserting to embedded system.

Uses
– Store software program for general-purpose processor.
• program instructions can be one or more ROM words.
– Store constant data needed by system.
– Implement combinational circuit.



Example

The figure shows the structure of a ROM. Horizontal lines represents the words. The vertical lines give out data. These lines are connected only at circles. If address input is 010 the decoder sets 2nd word line to 1. The data lines Q3 and Q1 are set to 1 because there is a “programmed” connection with word 2’s line. The word 2 is not connected with data lines Q2 and Q0. Thus the output is 1010.



Implementation of Combinatorial Functions

Any combinational circuit of n functions of same k variables can be done with 2k x n ROM. The inputs of the combinatorial circuit are the address of the ROM locations. The output is the word stored at that location.



Mask-programmed ROM

The connections “programmed” at fabrication. They are a set of masks. It can be written only once (in the factory). But it stores data for ever. Thus it has the highest storage permanence. The bits never change unless damaged. These are typically used for final design of high-volume systems.

OTP ROM: One-time programmable ROM

The Connections “programmed” after manufacture by user. The user provides file of desired contents of ROM. The file input to machine called ROM programmer. Each programmable connection is a fuse. The ROM programmer blows fuses where connections should not exist.
• Very low write ability: typically 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.

EPROM: Erasable programmable ROM

This is known as erasable programmable read only memory. The programmable component is a MOS transistor. This transistor has a “floating” gate surrounded by an insulator. The Negative charges form a channel between source and drain storing a logic 1. The Large positive voltage at gate causes negative charges to move out of channel and get trapped in floating gate storing a logic 0. The (Erase) Shining UV rays on surface of floating-gate causes negative charges to return to channel from floating gate restoring the logic 1. An EPROM package showing quartz window through which UV light can pass.

The EPROM has:
• Better write ability
– can be erased and reprogrammed thousands of times
• Reduced storage permanence
– program lasts about 10 years but is susceptible to radiation and electric noise
• Typically used during design development



EEPROM

EEPROM is otherwise known as Electrically Erasable and Programmable Read Only Memory. It is erased typically by using higher than normal voltage. It can program and erase individual words unlike the EPROMs where exposure to the UV light erases everything.

The EEPROM has:
• Better write ability.
– can be in-system programmable with built-in circuit to provide higher than normal voltage.
• built-in memory controller commonly used to hide details from memory user.
– writes very slow due to erasing and programming.
• “busy” pin indicates to processor EEPROM still writing.
– can be erased and programmed tens of thousands of times.
• Similar storage permanence to EPROM (about 10 years).
• Far more convenient than EPROMs, but more expensive.

information shared by www.irvs.info