Memory Cells and DRAM

Memory is made up of bits arranged in a two-dimensional grid.

In this figure, red cells represent 1s and white cells represent 0s. In the animation, a column is selected and then rows are charged to write data into the specific column.

Memory cells are etched onto a silicon wafer in an array of columns (bitlines) and rows (wordlines). The intersection of a bitline and wordline constitutes the address of the memory cell.

DRAM works by sending a charge through the appropriate column (CAS) to activate the transistor at each bit in the column. When writing, the row lines contain the state the capacitor should take on. When reading, the sense-amplifier determines the level of charge in the capacitor. If it is more than 50 percent, it reads it as a 1; otherwise it reads it as a 0. The counter tracks the refresh sequence based on which rows have been accessed in what order. The length of time necessary to do all this is so short that it is expressed in nanoseconds (billionths of a second). A memory chip rating of 70ns means that it takes 70 nanoseconds to completely read and recharge each cell.

Memory cells alone would be worthless without some way to get information in and out of them. So the memory cells have a whole support infrastructure of other specialized circuits. These circuits perform functions such as:

  • Identifying each row and column (row address select and column address select)
  • Keeping track of the refresh sequence (counter)
  • Reading and restoring the signal from a cell (sense amplifier)
  • Telling a cell whether it should take a charge or not (write enable)

Other functions of the memory controller include a series of tasks that include identifying the type, speed and amount of memory and checking for errors.

Static RAM works differently from DRAM. We'll look at how in the next section.