EEPROM - Arduino Sketches: Tools and Techniques for Programming Wizardry - page 102

volatile than you might at i rst think; it needs to be refreshed frequently to keep
the data in place. This might sound like poor engineering, but the truth is that
Dynamic RAM (DRAM) is extremely fast, dense, and relatively cheap, allowing
for inexpensive memory chips that work very well.
Volatile memory is used to store variables and data. The actual program is
placed in nonvolatile memory and uses volatile memory to operate. Your alarm
clock might have this function. You can set an alarm, but if the power is cut,
you have to reprogram the alarm clock; otherwise, you won't wake up on time.
Nonvolatile memory is memory that retains data when power is removed. The
i rst implementation of nonvolatile memory was an implementation of volatile
memory with a small cell battery. When that battery ran out, the data would be
lost. One solution to this was EPROM memory, as shown in Figure 6-1.
Figure 6-1: EPROM memory chip
Electrically Programmable Read Only Memory (EPROM) is a special memory
that retains its data even when power has been removed. Early versions of
EPROM required specialized equipment to be programmed. True ROM chips
existed well before the arrival of EPROM, but EPROM added something that
ROM chips did not have; they could be erased and reprogrammed.
Reprogramming the i rst EPROM chips was not something particularly easy
to accomplish; these devices had a quartz “window” on the top of the chip. By
placing the chip under ultraviolet light, the device could be erased within 20
minutes. When fully erased, the device could be reprogrammed.
Although such devices did work well, they were not always practical. They
could store programs or nonvolatile variables, but devices became more intelligent