Transcript SRAM pres.
COEN 180 SRAM SRAM High-speed Low capacity Expensive Large chip area. Continuous power use to maintain storage Technology used for making MM caches SRAM Single cell stores single bit. 4T+2R design (old) 6T design SRAM 4T+2R SRAM Word line Asserted: connects to complementary bit lines. SRAM Resistor-Transistor pair divide voltage between Vcc and GND T2 high resistance: A close to VCC T2 low resistance A close to Gnd. A SRAM T2 high impedance: A close to VCC T3 enabled T3 low impedance B close to Gnd T2 low impedance A close to Gnd. T3 disabled T3 high impedance B close to VCC B A SRAM Two stable states. Asserted word line sends complimentary values to the two bit lines. This is the stored bit. Bitline 0 contains bit Bitline 1 contains inverse of bit SRAM There is always a current through one of the transistor-resistor pairs. Use transistors instead of resistors to save energy. However, transistors can use up more space. SRAM Cell consists of two lines of transistors, dividing the voltage between VCC and GND Cross-coupled. T2 in high impedance T5 in low impedance T2 in low impedance T5 in high impedance SRAM Assume T2 high impedance, T5 low impedance. Point A ~ VCC T3 in low impedance and T6 in high impedance Point B ~ GND A T2 in high impedance, T5 low impedance. B Stable State SRAM Assume T2 low impedance, T5 high impedance. Point A ~ GND T3 in high impedance and T6 in low impedance Point B ~ GND A T2 in low impedance, T5 high impedance. B Stable State SRAM 6T cell is in two stable states. If the word line is asserted, complementary values are placed on the two bit lines. SRAM Bit cells are arranged in a large memory array. Address is divided into row address and column address. SRAM Data access Split address into row address (N bits) and column address (M bits). Row address activates one of 2N word lines leading into the array. This puts the contents of all 2M bit cells in that row onto the 2M column lines. Each column line consists of two bit complementary bit lines. Use a sense amplifier in order to remove any signal loss (because of capacitance of bit line). Column decoder selects one of these bit lines and gates them into the I/O buffer. SRAM SRAM Write access Everything as before. However: Value in Read / Write Circuit overwhelms contents in the two bitlines. This switches the state of the one selected bit cell. SRAM Typical SRAM array allows access to more than a single bit in parallel. SRAM SRAM Faster: Faster chip technology Tighter chip technology Use different materials (GaAs) Increase voltages Cool circuit Change pinout to cut input / output noise. SRAM Faster Can use input buffer to latch data. Access parameters: Read-access time Propagation delay from the time when the address is presented at the chip to the time data is available at the output. Cycle time Minimum time between initiation of a read operation and the initiation of another operation.