Memory and Programmable Logic Chapter 7 Introduction RAM

Memory and Programmable Logic Chapter 7

Introduction RAM: Random Access Memory ROM: Read Only Memory Write operation: Storing info into memory Read operation: Transferring info out of the memory RAM can perform both Write and Read operations ROM is a Programmable Logic Device (PLD) that can be written once and can only be read afterwards • PLA: Programmable Logic Array • PAL: Programmable Array Logic • FPGA: Field Programmable Gate Array • • •

Conventional and array logic diagrams

Random Access Memory unit that can be written or read Memory is composed of “words” Word is a group of bits Byte is a group of 8 bits (Denoted B) Words can have one or more bytes: a word of 32 bits has 4 bytes • Memory size is normally measured in bytes, e. g. , 1024 bytes = 1 KB • • •

Random Access Memory • When word size is known memory size can be given in words, e. g. , 1 word = 64 bits = 8 bytes. • 1 K requires 10 bits address, i. e. , 1024 = 210 • 1 M = 220 • 1 G = 230 • 4 G = 232 • Each word has an id. number called an address (0 to 2 k`-1, where k is the # of address lines)

Random Access Memory

Random Access Memory Contents of a 1 K x 16 memory

Write and read operations • Read and write take the same time, regardless of location; contrast with magnetic tape; read may take different time from write operation • Write: transfer-in operation • Read: transfer-out operation

Write and read operations • Steps to store a new word into memory 1. Apply binary address of desired word to address lines (k) 2. Apply data bits that must be stored in memory to data input lines (nin) 3. Activate the write input • Memory unit will take bits from input data lines and store them in word specified by address lines

Write and read operations • Steps to read a word out of memory 1. Apply desired binary address of desired word to address lines 2. Activate the read input • Memory unit will take bits from word specified by address lines and apply them to output data lines

Write and read operations Control inputs in typical commercial memory chips

Timing waveforms Cycle time: time required to finish a write operation

Timing waveforms access time: time required to finish a read operation

Timing waveforms Cycle Time (i. e. Write Time) T = 1/f. CLK CPUCLK ADDRESS VALID MEMORY SELECT DATA VALID FOR WRITE DATA VALID FOR READ

Types of memories • Static RAM or SRAM: consists of internal latches that store binary information • Dynamic RAM or DRAM: stores information in form of electric charge on capacitors provided by MOS transistors inside the chip, thus requires refresh every few ms • Volatile memory lose stored information when powered off. Magnetic memory is nonvolatile. ROM is another type of nonvolatile memory

Types of memories • DRAM refreshing requires cycling through words every few ms to restore decaying charge • DRAM offers reduced power consumption and larger storage capacity in a single chip • SRAM is easier to use Bonus Assignment: find out what Flash Memories are and how they are used

Memory Decoding

Coincident decoding LSB MSB • Decoder with k inputs and 2 k outputs requires 2 k AND gates • A 1 K memory requires a 10 x 1024 decoder • Use instead a 2 -D selection patter and 1 K requires two 5 x 32 decoders! • Word is selected by coincidence of one X and one Y line

Address multiplexing • SRAMs contain 6 transistors per cell • DRAMs contain one MOS transistor and one capacitor per cell • DRAMs achieve higher storage capacity per unit area – four times that of a SRAM - and lower power consumption • DRAM typical word size = 1 bit • DRAM is preferred for large memories for PCs • DRAM available from 64 K to 256 M bits • To reduce number of pins, thus chip size, use address multiplexing

Address multiplexing • Capacity = 256 x 256 = 28 x 28 =64 K • RAS = Row Address Strobe • CAS = Column Address Strobe

Error Detection and Correction • Dynamic physical interaction of electrical signals may cause occasional errors • Memories can use two types of codes: – Error detection codes, parity (chapter 3) – Error correction codes

Error Detection and Correction • Hamming code – Error correction code – Uses several parity bits per word – Can detect and correct 1 -bit errors

Hamming code Bit position 1 2 3 4 5 6 7 8 9 10 11 12 P 1 P 2 1 P 4 1 0 0 P 8 0 1 0 0 Parity generation equations Check bit equations (Syndrome)

Hamming code Bit 1 2 3 4 5 6 7 8 9 10 11 12 No error 0 0 1 1 1 0 0 Bit 1 error 1 0 1 1 1 0 0 Bit 5 error 0 0 1 1 0 0 0 1 0 0

Hamming code Syndrome Position of error C 8 C 4 C 2 C 1 No error 0 0 1 0 0 0 1 2 0 0 1 0 3 0 0 1 1 4 0 1 0 0 5 0 1 6 0 1 1 0 7 0 1 1 1 8 1 0 0 0 9 1 0 0 1 10 1 0 11 1 0 1 1 12 1 0 0 0

Single-error correction, double-error detection

Read-Only Memory

Read-Only Memory ROM programming according to table 7. 3

Combinational circuit implementation • ROM uses a decoder for address and decoder gives minterms • Outputs use OR gates thus ROM can be seen as: – A storage device – Combinational circuit implementing Boolean functions

Combinational circuit implementation Example 7. 1: ROM-based circuit that accepts a three-bit number and produces a binary number equal to the square of the input number.

Combinational circuit implementation Example

Types of ROM • Mask programming: done by semicondutor company during last fabrication process of unit • PROM (Programmable ROM): Programming by blowing fuses by applying a high voltage; blown fuse outputs a 0 • EPROM (Erasable PROM): Erase using special ultraviolet light • EEPROM or E 2 PROM (Electrically Erasable PROM)