Page 109 - Digital Electronics by harish
P. 109
4.4 Dynamic RAM
Dynamic RAM stores the data as a charge on the capacitor. Figure shows the concept
of dynamic RAM.
Figure : Concept of dynamic RAM
When ROW (Control) and COLUMN (Sense) lines go HIGH, the MOSFET conducts
and charges the capacitor. When the ROW and COLUMN lines go low, the MOSFET opens
and the capacitor retains the charge. The transistor acts like a switch only. In this way, it
stores 1 bit. Since only a single MOSFET and capacitor are needed, the dynamic RAM
occupies very less space when compared with static RAM.
The charge stored in the capacitor will discharge slowly and hence the data stored in
the cell will lost in the course of time even when the power is present. This is the main
disadvantage of dynamic RAM. Extra hardware is used to retain the charge in the capacitor.
The extra hardware is called „Refresh logic‟ and the operation is called „Refreshing‟.
The Write, Read and Refresh operations are illustrated in figure. Three buffers are
used for Write, Read and Refresh operations.
Figure : Dynamic RAM Cell
109
