11/29/2004EE 42 fall 2004 lecture 371 Lecture #37: Memory Last lecture: –Transmission line equations –Reflections and termination –High frequency measurements This lecture: –Static Ram –Dynamic Ram

11/29/2004EE 42 fall 2004 lecture 372 Memory types In addition to registers, there is usually a need in digital logic devices for a denser (and therefore cheaper) means of storing information in larger amounts. The main memory types are known as RAM (Random Access Memory) as contrasted with long shift registers from which information can only be extracted or added in sequence.

11/29/2004EE 42 fall 2004 lecture 373 RAM In a RAM, an address is provided to a block of logic, along with an address strobe and a write signal line. The memory block then either reads a block of bits and puts it onto a bus, or writes the data from the bus into the indicated area of memory.

11/29/2004EE 42 fall 2004 lecture 374 Types of memory SRAM (Static RAM) is a memory which is essentially an array of flip flops. It can be very fast, and is integrated with logic. DRAM (Dynamic RAM) is a dense memory which uses a single capacitor controlled by a switch to store a bit. Dram must be rewritten after a write, and must be refreshed periodically.

11/29/2004EE 42 fall 2004 lecture 375 Other types of memory Read only memory (ROM) is a memory whose content is fixed at manufacture, and thus can only be read. Content addressable memory (CRAM) is memory which can be addressed, at least in part by its contents, rather than an address. E 2 memory (electrically erasable) is memory which can be read and written, but whose content is nonvolatile. Writing is often much slower than reading

11/29/2004EE 42 fall 2004 lecture 376 RAM logical organization Note that the number row decoder lines and column lines goes like the square root of the memory size Column Decoder Memory array … D Q Row Decoder … Address lines Address strobe Read/Write

11/29/2004EE 42 fall 2004 lecture 377 SRAM Cache uses SRAM: Static Random Access Memory –No refresh –Size 6 transistors/bit –Fast, can be optimized for speed or area reduction –Compatible with dense logic, so can be integrated with microprocessors with no extra masks

11/29/2004EE 42 fall 2004 lecture 378 Static RAM (SRAM) Six transistors in cross connected fashion –Provides regular AND inverted outputs –Implemented in CMOS process Single Port 6-T SRAM Cell

11/29/2004EE 42 fall 2004 lecture 379 SRAM cell Each cell of an SRAM is a pair of cross connected small inverters As long as power is supplied, the latch will hold the value. When the row line is held high, the outputs are driven with the contents of the memory. The latch is written by using a more powerful driver on the column lines to overcome the smaller transistors in the latch which forces the desired state.

11/29/2004EE 42 fall 2004 lecture 3710 DRAM Main Memory is DRAM: Dynamic Random Access Memory –1 transistor –Dynamic since needs to be refreshed periodically (8 ms, 1% time) –Addresses divided into 2 halves (Memory as a 2D matrix): RAS or Row Access Strobe CAS or Column Access Strobe

11/29/2004EE 42 fall 2004 lecture 3711 SRAM cells exhibit high speed/poor density DRAM: simple transistor/capacitor pairs in high density form Dynamic RAM Word Line Bit Line C Sense Amp......

11/29/2004EE 42 fall 2004 lecture 3712 Basic DRAM Cell Planar Cell –Polysilicon-Diffusion Capacitance, Diffused Bitlines Problem: Uses a lot of area (< 1Mb) You can’t just ride the process curve to shrink C (discussed later)

11/29/2004EE 42 fall 2004 lecture 3713 DRAM Operations Write –Charge bitline HIGH or LOW and set wordline HIGH Read –Bit line is precharged to a voltage halfway between HIGH and LOW, and then the word line is set HIGH. –Depending on the charge in the cap, the precharged bitline is pulled slightly higher or lower. –Sense Amp Detects change The signal is decreased by the ratio of the storage capacitance to the bitline capacitance –Increase density => increase parasitic capacitance –As geometries shrink, still need large bit capacitance Word Line Bit Line C Sense Amp......

11/29/2004EE 42 fall 2004 lecture 3714 Advanced DRAM Cells Stacked cell (Expand UP)

11/29/2004EE 42 fall 2004 lecture 3715 Advanced DRAM Cells Trench Cell (Expand DOWN)

11/29/2004EE 42 fall 2004 lecture 3716 DRAM logical organization (4 Mbit) Square root of bits per RAS/CAS Column Decoder SenseAmps & I/O MemoryArray (2,048 x 2,048) A0…A10 … 11 D Q Word Line Storage Cell Row Decoder …

11/29/2004EE 42 fall 2004 lecture 3717 DRAM sense amp Bit line Both precharged to ½ V Data out +V

11/29/2004EE 42 fall 2004 lecture 3718 DRAM sense amplifier The reason that DRAM is slow, is that a very small charge is captured on the capacitor, and the small voltage change on the line must be sensed. time V Precharge→ Charge dumped to bit line Sense amp decides 0 or 1

11/29/2004EE 42 fall 2004 lecture 3719 DRAM/SRAM tradeoffs By it’s nature, DRAM isn’t built for speed –Response time dependent on capacitive circuit properties which get worse as density increases DRAM process isn’t easy to integrate into CMOS process –DRAM is off chip –Connectors, wires, etc introduce slowness –IRAM efforts looking to integrating the two Memory Architectures are designed to minimize impact of DRAM latency –Use dram for high density, store data which is used often in smaller, higher speed SRAM cache.

11/29/2004EE 42 fall 2004 lecture 3720 FLASH Memory Floating gate transistor –Presence of charge => “0” –Erase Electrically or UV (EPROM) Performance –Reads like DRAM (~ns) –Writes like DISK (~ms). Write is a complex operation

11/29/2004EE 42 fall 2004 lecture 3721 Tunneling Magnetic Junction RAM (TMJ-RAM): –Speed of SRAM, density of DRAM, non-volatile (no refresh) –New field called “Spintronics”: combination of quantum spin and electronics –Same technology used in high-density disk-drives More esoteric Storage Technologies?

11/29/2004EE 42 fall 2004 lecture 3722 Tunneling Magnetic Junction