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Sdram

Computer memory, where do I start. It should be with the physics of the devices I suppose, My semiconductor book doesn't mention RAM at all and Sze had some thing to do with the development of solid state memory. Memory isn't a device its an IC. Ignoring the physics of the IC. There is static ram(SRAM), Basically SRAM allows you to put data into an address and read the data from that address later, later being before you remove the power. The SRAM cell is an array of flip flops( four FETs ?).

The basics of RAM and CPU operation are shown in figure 1. the CPU is connected by data and address buses to the RAM. There are some additional connections, CE which is chip enable would be set to access a particular RAM chip, WE which is write enable this would be set according to whether you wanted to read or write to the chip. So when the CPU wants some thing from an address, oh lets say 0xh, it makes the chip select high for that particular chip, the write enable goes high(signalling a read) and the address 0xh is set high on the address bus. A short time later the RAM chip puts the contents of 0xh on the data bus. Simple, well I have ignored bundles of details.

 


Figure 1 Simplified schematic of CPU and memory.

Figure 2 Clock cycles, when data is requested and when it's returned.

The time taken from strobing the chip select and the write enable to the time the data is available to the CPU is the address access time, so an access time of 80 ns means 12.5 MHz(slooow). As I said SRAM memory cells are basically four FETs Well along came the DRAM cell which is a FET and a capacitor, so DRAMs are much more compact. This results in memory with greater memory-density and speed, but Drams don't hold their data for long so must be accessed or refreshed every so often (I think this is in the order of 10s of ms).

SDRAM is synchronous dynamic random access memory. Well synchronous means at the same time? The chipset and the RAM run on a common clock. Figure 2 is a timing diagram for a CAS 2 SDRAM, PC100 running at 100 MHz, receiving a 100 MHz signal to a pin on the dimm in time with the 100 MHz signal to the chipset. So when does the CPU ask for and get the data. The request is made before the rising edge of the clock, all signals obviously persist and in this case the "envelope" must encompass the rising edge as in figure 2. The data is then put on the bus but again the timing is important, the data must be on the bus synchronously with the clock and in figure 2 it is 2 rising edges after the request. This is what Column Address Strobe(CAS) latency is all about, data out two rising edges after the request comes in is CAS 2. As there is CAS there must also be RAS(row address strobe). The CAS latency rating can be played with i.e. if you want to run your pc100 at 133 MHz and it proves unstable, run it at CAS 3 and it will be OK(probably). ECC: I always forget what acronyms mean, Error correction ? Well I can remember what happens so here goes. Parity checking was used with some memory, this meant slapping another bit on a byte and if the byte was even sticking a 0 in the parity check bit, or if the byte was odd sticking a 1 in it; then checks were made that the 9 bit byte was even(there was also odd parity checking) if not an error had occurred. ECC uses code to add a byte of error checking and when an error occurs(shouldn't happen all that often) corrects it. ECC must therefore stand for error correction code, I had to work out some error probability stuff in statistics years ago all I remember was they don't happen a lot and I wouldn't worry if my SDRAMs weren't ECC. Unbuffered well I will write that later this page is getting a bit long.

       

 

 
     
  Cobbled together on the 14 July 1999
by
Richard

{last modified 4th Dec 2000}