Hardware Reference
In-Depth Information
edge of the clock and the falling edge, doubling the data rate. Thus, an 8-bit-wide
DDR chip running at 200 MHz outputs two 8-bit values 200 million times a second
(for a short interval, of course), giving a theoretical burst rate of 3.2 Gbps. The
DDR2 and DDR3 memory interfaces provide additional performance over DDR by
increasing the memory-bus speeds to 533 MHz and 1067 MHz, respectively. At
the time this topic went to press, the fastest DDR3 chips could output data at
17.067 GB/sec.
Nonvolatile Memory Chips
RAMs are not the only kind of memory chips. In many applications, such as
toys, appliances, and cars, the program and some of the data must remain stored
even when the power is turned off. Furthermore, once installed, neither the pro-
gram nor the data are ever changed. These requirements have led to the develop-
ment of ROM s (Read-Only Memories), which cannot be changed or erased, inten-
tionally or otherwise. The data in a ROM are inserted during its manufacture, es-
sentially by exposing a photosensitive material through a mask containing the de-
sired bit pattern and then etching away the exposed (or unexposed) surface. The
only way to change the program in a ROM is to replace the entire chip.
ROMs are much cheaper than RAMs when ordered in large enough volumes to
defray the cost of making the mask. However, they are inflexible, because they
cannot be changed after manufacture, and the turnaround time between placing an
order and receiving the ROMs may be weeks. To make it easier for companies to
develop new ROM-based products, the PROM (Programmable ROM) was invent-
ed. A PROM is like a ROM, except that it can be programmed (once) in the field,
eliminating the turnaround time. Many PROMs contain an array of tiny fuses in-
side. A specific fuse can be blown out by selecting its row and column and then
applying a high voltage to a special pin on the chip.
The next development in this line was the EPROM (Erasable PROM), which
can be not only field-programmed but also field-erased. When the quartz window
in an EPROM is exposed to a strong ultraviolet light for 15 minutes, all the bits are
set to 1. If many changes are expected during the design cycle, EPROMs are far
more economical than PROMs because they can be reused. EPROMs usually have
the same organization as static RAMs. The 4-Mbit 27C040 EPROM, for example,
uses the organization of Fig. 3-31(a), which is typical of a static RAM. What is in-
teresting is that ancient chips like this one do not die off. They just become cheap-
er and find their way into lower-end products that are highly cost sensitive. A
27C040 can now be bought retail for under $3 and much less in large volumes.
Even better than the EPROM is the EEPROM which can be erased by apply-
ing pulses to it instead of putting it in a special chamber for exposure to ultraviolet
light. In addition, an EEPROM can be reprogrammed in place, whereas an
EPROM has to be inserted in a special EPROM programming device to be pro-
grammed. On the minus side, the biggest EEPROMs are typically only 1/64 as
 
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