Synchronous Static Random Access Memory (SSRAM)

1. Synchronous Static Random Access Memory (SSRAM)

2. Internal Structure of a SSRAM AREG: Address Register CREG: Control Register INREG: Input Register OUTREG: Output Register ADS_L: Address Select (Low Voltage Assert) GW_L: Global Write (Low Voltage Assert)

3. Burst mode for consecutive access. (No ADDR needs to be sampled after Y) AREG serves as a counter Valid for 2 cycles INREG is sampled one clock tick after the AREG is loaded. Thus, ADS_L has to be inhibited (H in the next rising edge)

4. Flow-through output Data Output available with 1 cycle delay … But the output is valid for a whole cycle. Thus it may allow operation at higher clock freq.

5. SRAM Address Block

6. Linear vs. Interleaved Burst Modes Mode Pin is Low Mode Pin is High

9. Big Endian Little Endian 000 12 78 001 34 56 010 56 34 011 78 12

10. Conventional SSRAMs share the same pins for both input data and output data. • Based on the patterns of data-bus and SRAM-array use, there is a turn-around penalty – a clock period must be idle when a read is followed by a write • The penalty is eliminated in Zero-bus-turn-around (ZBT) SSRAMs

13. Why SSRAM? • An operation (R/W) set up before the rising edge of the clock is performed during the subsequent clock period (since the use of registers as buffers) • However, in higher frequency of clock rate, it takes less number of cycles to access memory.

14. When the clock rate reaches to 50 MHz or higher, async mode needs 2 cycles each for second thru fourth access. When the clock rate reaches to 75 MHz or higher, pipeline burst mode needs only 1 cycle for second thru fourth access.

15. DDR • Data transferred on both the rising and falling edges of the clock signal. • Also known as double pumped, dual-pumped, and double transition. 

16. DDR Write Cycle

17. DDR Read Cycle

18. How to fully utilize the data bus? • 2-word-burst mode occupies a single cycle of data bus • 4-word-burst mode occupies two cycles of data bus • What, then, will be the frequency of providing Address in each mode?

19. DDR2 • Like all SDRAM implementations, DDR2 stores data in memory cells that are activated with the use of a clock signal to synchronize their operation with an external data bus. • DDR2 I/O buffer transfers data both on the rising and falling edges of the clock signal • The key difference is that for DDR2 the memory cells are clocked at 1 quarter (rather than half) the rate of the bus. • This requires a 4-bit-deep prefetch queue, but, without changing the memory cells themselves, • DDR2 can effectively operate at twice the bus speed of DDR.

20. In 4-bit prefetch architecture, DDR2 SDRAM can read/write 4 times the amount of data as an external bus from/to the memory cell array for every clock, and can be operated 4 times faster than the internal bus operation frequency. • External clock frequency = 2 times of internal bus operation frequency • Double data rate output = 2 times of external clock frequency

21. Comparison between DDR2 SDRAM, DDR SDRAM, and SDR SDRAM