The Memory Chip

SimpleTech used their own 838S032T04A DDR SDRAM chip.

It appears to be a 256-Mbit chip with an access time of 4.0ns. That means that it was designed to run at 250MHz. Now wait a minute. Isn't this chip supposed to run at 500MHz?

Well, technically no. You see, these DDR chips actually run at 250MHz but they have been internally widened to deliver a single 2n-bits wide (where n is a number) data transfer per clock cycle. This is different from standard SDRAM which only deliver one n-bit wide data transfers per clock cycle.

At the I/O pins however, the single 2n-bit wide data transfer is split into two n-bit wide, half clock cycle data transfers. The two pieces of data are then transferred to the memory controller on both the rising and falling edges of the signal. This differs from normal SDRAM which only carries data on the rising edge of the signal.

In short, the internal DRAM core had been redesigned to allow twice as much data to reach the I/O pins in a single clock cycle. The data is then split up so that half of it is transmitted via the rising edge of the signal while the other half is carried by the falling edge of the signal.

Because twice as much data is being transferred per clock cycle, manufacturers and end-users alike continue to label 250MHz DDR SDRAM as 500MHz DDR SDRAM. This is like saying that AGP2X actually runs at 133MHz, which it does not.

Neither the memory chips nor the interface run at 500MHz. They are just twice as fast because they do twice as much work in a single clock cycle. But irrespective of what they call it, DDR SDRAM is definitely twice as fast as normal SDRAM. However, to avoid confusing everyone, I will continue to use the DDR misnomers to describe the speed of this DDR SDRAM chip.

Chip Specifications

Now, let's take a look at this memory chip's specifications :-