User guide

20
DRAM Device Overview
DRAM devices are volatile memories offering a lower cost per bit than SRAM devices. A compact memory
cell consisting of a capacitor and a transistor makes this possible over the six-transistor cell used in SRAM.
However, the capacitor will discharge, causing the memory cell to lose its state, which means that DRAM
memory needs to be refreshed periodically. Generally, you would choose SRAM devices for applications
where latency or low interface complexity is important. You would choose DRAM where cost per bit is
important. Special types of DRAM challenge this norm by offering improved random access latency, as well
as a lower cost per bit.
Altera provides complete system solutions to help memory designers successfully interface Altera
®
FPGAs
to a variety of DRAM devices.
DDR SDRAM
Double data rate (DDR) SDRAM is an evolution
of single data rate (SDR) SDRAM. It offers
higher performance through increased bus
speeds using a lower I/O voltage (2.5 V), and
most importantly, data transfer on both clock
edges, doubling the raw bandwidth. DDR
SDRAM is a widely established memory
technology. It offers the lowest cost per bit, due
in part to its broad acceptance in almost any
marketplace.
DDR3
DDR3 SDRAM is an improvement over its
predecessor, DDR2 SDRAM. The primary
benefit of DDR3 is the ability to transfer at twice
the data rate of DDR2, thus enabling higher bus
rates and higher peak rates than earlier memory
technologies. In addition, the DDR3 standard
allows for greater chip capacities. The
advantages of DDR3 architecture over DDR2
architecture are summarized as follows:
• Data rate speed ranges from 800 to
1,600Mbps
• Device capacity ranges from 512Mb to 8Gb
• Power is lower than DDR3 SDRAM due to
reduced I/O and core voltage
DDR2 SDRAM
DDR2 SDRAM is an evolution of DDR SDRAM.
It operates using a lower voltage (1.8V). DDR2
offers increased densities and even higher
performance through higher bus speeds and an
optimized interface. The advantages of DDR2
architecture over DDR architecture are
summarized as follows:
• Data rate speed ranges from 400 to
667Mbps
• Power is lower than DDR SDRAM due to
reduced I/O and core voltage
• Smaller footprint for FBGA packages
DDR
DDR SDRAM is an evolution of SDR SDRAM. It
offers higher performance through increased bus
speeds using a lower I/O voltage (2.5V), and
most importantly, data transfer on both clock
edges, doubling the raw bandwidth. DDR
SDRAM is a widely established memory
technology. It offers the lowest cost per bit, due
in part to its broad acceptance in almost any
marketplace.