# Memory and Programmable Logic

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Memory and Programmable Logic
Ch. 7 Memory and Programmable Logic

Memory and Programmable Logic
Random-Access Memory Memory Decoding Error Detection and Correction Read-Only Memory Programmable Logic Array Programmable Array Logic Sequential Programmable Devices

Memory Memory Type of memory Write operation Read operation
A device to which binary information is transferred for storage. Type of memory random access memory , RAM read-only memory, ROM Write operation Storing new information into memory Read operation Transferring the stored information out of memory

RAM RAM Storage unit–byte Capacity of memory–total number of bytes
The time it takes to transfer information to or from any desired random location is always the same Storage unit–byte byte：8 bits Length of a word：multiple of 8 bits word：represent a number, an instruction, alphanumeric character Capacity of memory–total number of bytes

Block diagram of memory unit
k address lines：select one particular word read, write：specify the direction of transfer n data input line：provide the information to be stored in memory n data output line：supplying the information coming out of memory

Capacity of memory Range of in memory size bytes Memory 1K x 16
210~232 words bytes K=210、 M=220、 G=230 。 64K=216 、2M=221 、4G=232 。 Memory 1K x 16 10 bits address，16 bits in each word Determine the no. of bits for address k: no. of address bits m: total number of words

Control inputs to memory chip

Memory cycle timing waveforms
access time the time required to select a word and read it cycle time the time required to complete a write cycle access time 、 cycle time equal to a fixed number of CPU clock See Fig. 7-4

Types of memory The mode of access of a memory RAM-volatile
Static RAM(SRAM) internal latch easier to used and shorter read and write time Dynamic RAM(DRAM) electric charges on capacitor less power consumption larger storage capacity ROM-nonvolatile Read/write time depend on the distance between the magnetic reader/writer and the data

Memory Decoding Decoder
select the memory word specified by the input address 2-dimensional coincident decoding is a more efficient decoding scheme for large memories

Memory cell One bit memory cell

4X4 RAM

Coincident Decoding － two-dimensional selection scheme
Decoder with k input and 2k output requires 2k AND gates with k input k input decoder can be implemented by two k/2 input decoders with one for column and another for row e.g., 10×1024 decoder can be implemented by two 5×32 decoders

Example for two-dimensional decoder

Read-Only Memory ROM：permanent binary information is stored
k input, n output ROM

ROM No data input Integrated circuit ROM have one or more enable input
Sometimes come with three-state outputs to facilitate the construction of large arrays of ROM

Internal logic of 32X8 ROM

ROM truth table Table ×8 ROM truth table

Programmomg the ROM according to Taable 7-3
× denote a connection in place of a dot used for permanent connection

Example 7-1 Design a combinational circuit with 3-input using a ROM.
Output = square(input)

ROM implementation of Example 7-1

Types of ROMs The required path in a ROM may be programmed in four different ways. mask programming (mask ROM) Mask is done by Fab. company during the last fabrication Customer must fill out the truth table High cost programmable read-only memory(PROM) allows users to program in Lab. the program is irreversible

Types of ROMs Erasable PROM(EPROM)
by ultraviolet light electrically-erasable PROM(EEPROM or E²PROM)， by electrical signal can be erased without removing it from tis socket

Types of PLD (Programmable Logic Device)

Programmable Logic Array (PLA)
similar to PROM does not provide full decoding and does not generate all the minterms decoder is replaced by an array of AND gate

PLA with 3 inputs, 4 product terms, and two outputs

PLA Programming Table PLA Programming Table consists of three sections
1st, list the product terms numerically 2nd, specify the required path between inputs and AND gates 3rd, specifies the paths between the AND and OR gates

Example 7-2 Implement the following two Boolean functions with a PLA:
Simplified by K-map：

Solution of Example 7-2