index.md (2108B)
1 +++ 2 title = 'Memory types' 3 +++ 4 # Memory types 5 ## Static memories (SRAM) 6 circuits that can retain state as long as power is applied 7 8 fast, but cells require several transistors 9 10 one cell has two inverters that are cross-connected — a latch 11 12  13 14 Read operation 15 16 - word line is activated, closes switches T1 and T2. 17 - If cell in state 1, signal on b is high and signal on line b’ is low. Vice versa. 18 19 Write operation 20 21 - sense/write circuit drives bit lines b and b’ 22 - places appropriate value on line b and its complement on b’, activates word line 23 - forces cell into corresponding state, retains when word line is deactivated 24 25 ## Dynamic memories (DRAM) 26 do not retain state for a long period unless accessed frequently 27 28 info is stored in form of charge on a capacitor (only for tens of milliseconds) 29 30 contents are periodically refreshed when they are accessed/written to 31 32 example of single transistor-capacitor DRAM cell: 33 34  35 36 A full 32M x 8 chip: 37 38  39 40 Refresh (and read) operation: 41 - transistor in selected cell is turned on 42 - sense amplifier on bit line checks if charge in capacitor is above threshold value 43 - if above, sense amplifier drives bit line to full voltage (1) 44 - otherwise, pulls bit line to ground level 45 46 Fast page mode: 47 - each sense amplifier is also used as latch 48 - so when a row address is applied, contents of all cells in the row are loaded into latches 49 - so all bytes in the row can be transferred sequentially, increasing block transfer speed. 50 51 ## Synchronous DRAMs 52 operation is synced with a clock signal 53 54 built-in refresh circuitry with a refresh counter to refresh specific rows 55 56 ## Double-Data-Rate SDRAM 57 58 large number of bits are accessed at the same time when a row address is applied 59 60 data are transferred both on rising and falling edges of clock 61 62 ## Rambus Memory 63 proprietary 64 65 uses fewer wires with a higher clock speed 66 67 makes use of differential-signaling technique to transfer data 68 69 signals are transmitted using small voltage swings of ±0.1V around reference value