This preview shows page 1. Sign up to view the full content.
Unformatted text preview: l, and once it fell it would never return to the vertical position. Due to its bistable nature, an SRAM memory cell will retain its value indeﬁnitely, as long as it is kept powered. Even when a disturbance, such as electrical noise, perturbs the voltages, the circuit will return to the stable value when the disturbance is removed. . 6.1. STORAGE TECHNOLOGIES 277 Dynamic RAM
DRAM stores each bit as charge on a capacitor. This capacitor is very small—typically around 30 femtofarads, that is, ¿¼ ¢ ½¼ ½ farads. Recall, however, that a farad is a very large unit of measure. DRAM storage can be made very dense—each cell consists of a capacitor and a single-access transistor. Unlike SRAM, however, a DRAM memory cell is very sensitive to any disturbance. When the capacitor voltage is disturbed, it will never recover. Exposure to light rays will cause the capacitor voltages to change. In fact, the sensors in digital cameras and camcorders are essentially arrays of DRAM cells. Various sources of leakage current cause a DRA...
View Full Document