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Semiconductor Memory Design. Organization of Memory Systems Driven only from outside Data flow in and out A cell is accessed for reading by selecting.

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Presentation on theme: "Semiconductor Memory Design. Organization of Memory Systems Driven only from outside Data flow in and out A cell is accessed for reading by selecting."— Presentation transcript:

1 Semiconductor Memory Design

2 Organization of Memory Systems Driven only from outside Data flow in and out A cell is accessed for reading by selecting its row and column. Memories may simultaneously select 4, 8, 16 …columns.

3 Overall Architecture of Memory Design n=m= =25,536 bits= 2Kb

4 RAM Read-write random access memories (RAM) – Store data in active circuits; information is lost if the power supply is interrupted Common Types – Static RAM (SRAM) – Dynamic RAM (DRAM)

5 SRAM – Store value in flip-flop circuits as long as power is on – High speed memories with clock cycles in the range of 5 to 50 ns

6 DRAM – Store values on capacitors – Prone to noise and leakage problems – Slower than SRAM, clocking at 50 ns to 200 ns. – More dense than SRAM

7 RAM Timing Parameters Write signal is active low t AC (read access time): presentation of address Until data is out t AC =(0.5 to 0.8)T cycle T cycle : minimum time needed in order to complete successive read and write operation

8 Organization of Memory Systems

9 AND and NOR Decoders Take an n-bit address. Produce 2 n outputs, One of which is activated. Problem: n=6 implies 1.64 NAND inverters It is difficult to implement NAND6 in standard CMOS

10 Predecoder Configurations Use a 2 stage design to implement NAND6 Use logical effort to determine the best design

11 Structure of Two-Level Decoder Wire 1: from A 0, A 1 Wire 2: from A 2, A 3 Wire 3: from A 4, A 5 Need 12 precoders since n=6 Each precoder Will drive 2 4 final decoders

12 Static RAM Cell Design Static Memory Operation

13 Basic SRAM and VTC A wordline is used to select the cell Bitlines are used to perform read and write operations on the cell

14 Cross Coupled Configuration The cell can only flip its internal state when one of its internal cross V S. During a read op, we must not disturb its current state. During a write op, we must force the internal voltage to swing past V S to change a state.

15 6T SRAM Cell Can be replaced by undoped polysilicon to minimize area. Use high threshold transistors to reduce leakage current.

16 Wordline and Double Bitline Configuration One wordline is enabled. The decoder must drive: (2 gate cap + wire cap) x # of cells in a row

17 Design of Transistor Size for Read Operation Assume: q=0 and qb=1 Initially: b=VDD, bb=VDD C bit is discharged through M 1. b begins to drop. bb remains high. Vbb and Vb is added to a sense amplifier and stored on a data buffer. Upon completion of the read, wl returns to 0 Cbig precharged To VDD.

18 Bitline capacitance Bitline capacitance: (S/D cap+ wire cap+S/D contact cap) X # of cells in a column

19 Sizing of M3 and M1 Icell could charge the gate capacitance of M2, thus lowering qb. Solution: Adjust the sizing of M3 and M1 to minimize changes in q. W 3 /W 1 can be determined.

20 Discharge time is controlled by sizing of M3 and M1 Icell should be large enough to Discharge bitline capacitance within 20% to 30% of the cycle time. I cell =C bit (dV/dT)

21 Write Operation

22 Transistor Sizing V QB =0.4=V TN

23 SRAM Cell Layout

24 Optional materials

25 Column Pull-Up Configurations

26 Address Transition Detection Circuit

27 Column Decoding and Multiplexing

28 Column Selection

29 4-bit Column Address

30 Write Driver Circuit

31 Basic Read Circuitry

32 Differential Voltage Sense Amplifier

33 Detecting “0” and “1”

34 Latch-based Sense Amplifier

35 Replica Circuit for Sense Amplifier Clock Enable

36 Replica Cell Design

37 Basic Memory Architecture

38 Divided Wordline Strategy to Reduce Power and Delay

39 Bitline Partition to Reduce Delay

40 Peripheral Circuits Decoders Sense Amplifiers Column Precharge Data Buffers


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