Page 261 - Mechanical Engineers' Handbook (Volume 2)

250 Digital Integrated Circuits: A Practical Application

are best suited to prototyping and to low-volume applications. Programming a PROM is
accomplished by the user who provides a file indicating the desired ROM contents. A tool
called a ROM programmer then configures each programmable connection according to the
user-specified file. A basic PROM uses a fuse for each programmable connection. The ROM
programmer blows the selected fuses in the array by passing a large current at high voltage
through them. However, once a fuse is blown, the connection can never be reestablished.
This type of programming cannot be undone. For this reason, basic PROM is often referred
to as one-time-programmable device, or OTP. As a result of this ability, OTP devices are
frequently used in military and space applications for their robustness in the presence of
radiation. 3
Programmable ROMs do come in erasable versions. One type of erasable PROM, called
EPROM, utilizes a floating-gate complementary metal–oxide–semiconductor (CMOS) device
4
as the programmable component, as shown in Fig. 12. Typically, a higher voltage (12–25
V) is applied to the control gate and the drain. This causes avalanche injection whereby
electrons tunnel into the floating gate. Once the voltage is removed, the charges are trapped
in the floating gate and programming is complete. To erase the programming, the electrons
must be excited enough to escape from the gate. One source of energy used is ultraviolet
light, or UV. The exposure time to UV is typically 5–30 minutes through a small quartz
window on top of the chip. In an effort to eliminate the time spent erasing the programming,
the development of an electronically erasable device, or EEPROM, was developed. These
devices are more expensive but are also more convenient to use. One type of EEPROM is
FLASH memory, which has the advantage of being able to reprogram certain regions of the
memory rather than the whole memory at once.
Random-access memory, or RAM, has the advantage of being able to read from and
write to. In contrast to ROM, RAM is not programmed. It is empty when placed in the
embedded system and is written to or read from during its execution. There are two basic
types of RAM: static and dynamic. Static RAM, or SRAM, is the faster version and uses a
memory cell consisting of a FF to store a bit. Six transistors are required to hold this bit.
The term static refers to the fact that the RAM will hold its data only as long as power is
applied. SRAM is typically used for high-end applications such as cache memory.
Dynamic RAM (DRAM) is more compact than SRAM because it has one MOS tran-
sistor and a capacitor to store each bit. The disadvantage is that the capacitor cannot store
the information indefinitely because it will leak charge and eventually lose the data. This
forces the requirement to recharge or refresh this memory. Refresh rates are typically around
15.625 s. Refreshing can be done by reading the contents of the cell, which forces the data
to be stored in a buffer and then rewritten to the memory cell. Refreshing the memory cells
tends to make DRAM slower than SRAM.

20 V
Control
gate
Floating
gate
20 V

n+ n+ n+ n+
Source Drain Source Drain
Figure 12 Floating gate transistor programming for erasable PROMs. EPROMs, EEPROMS, and Flash
use different methods for controlling the charge of the floating gate.