D IGITAL ELECTRONICS Prepared By - Patel Aniket ( )

Concept: Is read-only memory. Do not lose data when power is lost. ROM memory is used to produce chips with integrated CMOS BIOS program Characteristics of the ROM BIOS BIOS ROM containing the software configuration and system diagnostics, and routine input / output low level that DOS uses. These programs are encoded in ROM and is called firmware. An important feature of the ROM BIOS is detecting new hardware in the computer and reconfigure the operating system as device driver.

PROM (Programmable Read Only Memory) Type of ROM that information is only installed once. The CD can be called PROM. EPROM (Erasable Programmable ROM): Type of ROM that can erase and rewrite it. "CD-Erasable" can be called EPROM. EEPROM ( Electronic Erasable Programmable ROM ): It form enhence of EPROM, a difference compared to the EPROM is able to write and remove the information again and again by software rather than hardware. Example: "CD-Rewritable". Application specific EEPROM is "flash BIOS". ROM is the type of information can install or upgrade software

A read-only memory, or ROM, is a special kind of memory whose contents cannot be easily modified. The WR and DATA inputs that we saw in RAMs are not needed. Data is stored onto a ROM chip using special hardware tools. ROMs are useful for holding data that never changes. Arithmetic circuits might use tables to speed up computations of logarithms or divisions. Many computers use a ROM to store important programs that should not be modified, such as the system BIOS. PDAs, game machines, cell phones, vending machines and other electronic devices may also contain non-modifiable programs.

R EAD O NLY M EMORY (ROM) A DVANTAGES /L IMITATIONS Advantages: Can implement any function (all the minterms are available) Program is derived directly from the truth table (uses the canonical form) Disadvantages: Becomes complex for a large number of inputs n (# of ANDs = 2 n, each n-input wide) Does not support multi-level circuits (no outputs brought back as inputs)

Combinational Circuits: ROMs can be used to implement combinational circuits from their truth tables (i.e. SOm form, without the need for minimization to SOP) Sequential Circuits: Use ROMs to design the combinational part of the sequential circuit

Example 1: Implement the following two combinational functions using a ROM F1 (X,Y) = ∑ m (1,2,3) F2 (X,Y) = ∑ m (0,2) Solution: Specifying the ROM required: ROM has n = 2 inputs (  2 2 = 4 locations) and m = 2 outputs (  Each location has 2 bits) … 4 x 2 bit ROM Specifying the ROM data content (to be programmed into the ROM): Directly from the truth table of the two functions

ROM- BASED D ESIGNS : S EQUENTIAL C IRCUITS Conventional Design ROM-Register Based Design

There are some important differences between ROM and RAM. ROMs are “non-volatile”—data is preserved even without power. On the other hand, RAM contents disappear once power is lost. ROMs require special (and slower) techniques for writing, so they’re considered to be “read-only” devices. Some newer types of ROMs do allow for easier writing, although the speeds still don’t compare with regular RAMs. MP3 players, digital cameras and other toys use CompactFlash, Secure Digital, or MemoryStick cards for non-volatile storage. Many devices allow you to upgrade programs stored in “flash ROM.”

ROMs are actually combinational devices, not sequential ones! You can’t store arbitrary data into a ROM, so the same address will always contain the same data. You can think of a ROM as a combinational circuit that takes an address as input, and produces some data as the output. A ROM table is basically just a truth table. The table shows what data is stored at each ROM address. You can generate that data combinationally, using the address as the input.