1. Bob Reese Micro II ECE, MSU
DRAM
Bob Reese
Micro II
ECE, MSU
5/10/2019

2. SRAM - Static RAM Fast ( cycle time = access time < 10 ns)
In PCs, used for motherboard cache
Basic cell is 6 transistors
Expensive per bit
5/10/2019

3. DRAM - Dynamic RAM Basic Cell is one transistor + 1 capacitor
contents will leak off if not accessed periodically
Dense, cheap per bit
In PC’s used for main memory
5/10/2019

4. 5/10/2019

5. DRAM Architecture
Address bus is multiplexed between ROW and COLUMN address
16 Mb would normally need 24 address lines, uses 12 address pins on DRAM
Row Address Strobe (RAS), Column Address Strobe (CAS) used to latch addresses.
Each new DRAM generation adds another address pin => double the total bits of previous generation
4Kx1 16Kx1 64Kx1 256Kx1 1Mx1 4Mx1 16Mx1 64Mx1 256Mx1
5/10/2019

6. Read Cycle
5/10/2019

7. Read Mode Comments Access time is NOT EQUAL to cycle time
Need a recovery time (used for precharge of column bit lines) after a memory cycle if next cycle is to a different row.
Tcyc = Tras + Trp = 95 ns
If subsequent accesses are within the same row, then can use a fast page mode
Does not require recovery time
cycle time = 35 ns
DRAM control logic has to detect if next address is to same row or not and use the appropriate timing.
5/10/2019

8. 5/10/2019

9. EDO DRAM EDO => Extended Data Out
This is a version of page mode in which the output drivers are turned off at the falling edge of the CAS for the next cycle instead of the rising edge of the current cycle.
Data is available for longer, allows for tighter timing.
Improvement is on order of 10% - 15%
May not seem like much, but in hyper-competitive DRAM market, any edge can mean big dollars.
5/10/2019

10. 5/10/2019

11. DRAM Refresh
A cell will loose its contents if not accessed periodically
Asserting the row select will refresh all cells in that row.
IBM 16M DRAM has 4096 rows, requires 4096 cycles. Refresh rate is 64 ms for 5V part, 256 ms for 3.3V part
Row address be externally provided or generated from an internal counter (the refresh counter in hidden refresh or self refresh modes)
5/10/2019

12. 5/10/2019

13. Looks like long read cycle to CPU
5/10/2019

14. Some DRAMs support a self refresh mode
Some DRAMs support a self refresh mode. One use is in low power DRAMs where DRAM refreshes itself during sleep mode so the DRAM controller can be sleeping as well to conserve power.
5/10/2019

15. Speeding up DRAMs
Even a fast page mode cycle time of 35 ns (~ 30Mhz) is slow compared to SRAMs (< 10ns)
Latest DRAM innovation is the SDRAM - Synchronous DRAM
Adds a clock to the interface - all signals latched on clock edge
Optimized for burst transfers - supply an address, then succeeding clock cycles access sequential bits
IBM 16M SDRAM has clock speeds of 83, 100 Mhz!!!
All motherboards will use SDRAMs by next year.
5/10/2019

16. Burst Transfer Optimization
Why should burst transfers be optimized for DRAMs?
CPU random accesses all go to the cache (SRAM)
If the data is not in the cache, the cache controller will transfer the needed block of data from DRAM to the cache, and write the old block back to the DRAM.
Block sizes vary, usually 32 to 256 bytes
DRAM is never accessed randomly - all reads/writes are block reads/writes.
5/10/2019

17. IBM 4Mx4
SDRAM
5/10/2019

18. Advantages of 2 banks
SDRAM has two internal memory arranged in two banks.
Row accesses can be ‘interleaved’ between the two banks such that while a row in bank 0 is being accessed, bank 1 is being precharged.
In many cases, can eliminate the precharge penalty for accessing two different rows!!!
5/10/2019

19. Other SDRAM Features
In addition to the clock pin, added a new pin called CS (command select)
When asserted, the RAS, CAS, WE, and address pins are used collectively as a COMMAND.
The command specifies one of the many new transfer methods for the SDRAM
Also used to load the new internal mode register.
5/10/2019

20. 5/10/2019

21. For burst mode, address only provided for first data, internal counter provides subsequent addresses.
5/10/2019

22. 5/10/2019

23. DRAM Packaging
Need to get as much DRAM as possible into as small as space as possible on PC motherboards
SIMM (single inline memory module) and DIMM (dual inline memory module) are small circuit boards with multiple DRAM chips
SIMM gives a 32-bit memory path, DIMM a 64-bit memory path (needed when 64-bit CPU busses became common, eg. the Pentium!).
Current largest available DIMM is a 128MB
5/10/2019

24. The Future: SLDRAM? Facts: SLDRAM => SyncLinkDRAM
DRAM is going to get denser, CPUs faster, even SDRAMs will not be able to keep up with CPU speeds.
SLDRAM => SyncLinkDRAM
Differential signaling on data, command pins
aiming for 400 Mb/s pin rates in burst mode
Still in planning stages, backed by consortium of major semiconductor memory players.
5/10/2019