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
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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
Random Access Memory • 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 = 1KB
Random Access Memory • When word size is known memory size can be given in words, e.g., 1 word = 64 bits = 8 bytes. • 1K requires 10 bits address, i.e., 1024 = 210 • 1M = 220 • 1G = 230 • 4G = 232 • Each word has an id. number called an address (0 to 2k`-1, where k is the # of address lines)
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 • Apply binary address of desired word to address lines (k) • Apply data bits that must be stored in memory to data input lines (nin) • 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 • Apply desired binary address of desired word to address lines • Activate the read input • Memory unit will take bits from word specified by address lines and apply them to output data lines
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/fCLK CPUCLK ADDRESS 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
Coincident decoding LSB • Decoder with k inputs and 2k outputs requires 2k AND gates • A 1K memory requires a 10x1024 decoder • Use instead a 2-D selection patter and 1K requires two 5x32 decoders! • Word is selected by coincidence of one X and one Y line MSB
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 64K to 256M bits • To reduce number of pins, thus chip size, use address multiplexing
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 Parity generation equations Check bit equations (Syndrome)
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 implementationExample 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.
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 E2PROM (Electrically Erasable PROM)
