are many mechanisms to refresh DRAM, including RAS only refresh, CAS before RAS (CBR) refresh,
and Hidden refresh. CBR, which involves driving CAS active before driving RAS active, is used most
often.
Figure 2. Representation of a write operation for FPM or EDO RAM
DRAM storage density and power consumption
The storage capacity (density) of DRAM is inversely proportional to the cell geometry. In other words,
storage density increases as cell geometry shrinks. Over the past few years, improvements in DRAM
storage density have increased capacity from almost 1 kilobit (Kb) per chip to 2 gigabit (Gb) per
chip. In the near future, it is expected that capacity will increase even further to 4 Gb per chip.
The industry-standard operating voltage for computer memory components was originally at 5 volts.
However, as cell geometries decreased, memory circuitry became smaller and more sensitive.
Likewise, the industry-standard operating voltage has decreased. Today, computer memory
components operate at 1.8 volts, which allows them to run faster and consume less power.
Memory access time
The length of time it takes for DRAM to produce the data, from the CAS signal until the data is
available on the data bus, is called the memory access time or CAS Latency. Memory access time is
measured in billionths of a second (nanoseconds, ns) for asynchronous DRAM. For synchronous
DRAM, the time is converted to number of memory bus clocks.
Chipsets and system bus timing
All computer components that execute instructions or transfer data are controlled by a system bus
clock. The system chipset controls the speed, or frequency, of the system bus clock and thus regulates
the traffic between the processor, main memory, PCI bus, and other peripheral buses.
The bus clock is an electronic signal that alternates between two voltages (designated as "0" and "1"
in Figure 3) at a specific frequency. The bus frequency is measured in millions of cycles per second,
or megahertz (MHz). During each clock cycle, the voltage signal transitions from "0" to "1" and back
to "0". A complete clock cycle is measured from one rising edge to the next rising edge. Data transfer
along the memory bus can be triggered on either the rising edge or falling edge of the clock signal.
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