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:
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)
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
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
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
Basic SRAM and VTC A wordline is used to select the cell Bitlines are used to perform read and write operations on the cell
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.
6T SRAM Cell Can be replaced by undoped polysilicon to minimize area. Use high threshold transistors to reduce leakage current.
Wordline and Double Bitline Configuration One wordline is enabled. The decoder must drive: (2 gate cap + wire cap) x # of cells in a row
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.
Bitline capacitance Bitline capacitance: (S/D cap+ wire cap+S/D contact cap) X # of cells in a column
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.
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)