Microcomputer Circuits and Processes

CHAPTER 4
MEMORIES, INTERFACES,
AND APPLICATIONS
This chapter contains some short notes about some specific parts of a
microcomputer system, such as memory technologies, and also some
applications information suitable for a small physics or engineering
laboratory.
MEMORY
TYPES
Read only memory (ROM)
Read only memory (ROM) is the simplest type to understand. ROM
cells can be read only by the application of RD signals; the user of the
computer cannot store data or a program in them. They are manufactured
containing the program - each cell is set to 1 or 0 by the
manufacturer according to the customers' wishes. It is not a very cheap
system.
Figure 4.1 shows how you could make ROM using diodes, wires,
and a soldering iron. There are eight memory locations, each of four
bits, shown.
The locations are shown selected by a simple switch, but you could
design some address decoding logic yourself, using eight 3-input AND
gates and three inverters.
You do not find many ROMs being used in small machines these
days, because of cost. A much better type of read only memory is the
programmable read only memory (PROM). Here the user 'burns in' his
own program into a PROM chip, which arrives with all cells set to logic
1. In the cells where the user wants a 0, he must burn open the links to
those cells using a 'PROM-programmer'. Of course, once the program
is loaded, it cannot be changed.
The next development .came with the eraseable programmable read
only memory (EPROM). In order to appreciate the construction of this
type of memory, one needs a detailed understanding of solid-state
physics. But with EPROM, the user can insert a program not by
burning open links, but as stored charges. To change the program, these
stored charges, the Is and Os,may be removed by exposing the memory
to ultra-violet light, making the memory components temporarily
conductive, allowing the charges to escape. Depending on the strength
of the light, the erasing time may be up to 30 minutes.
The latest development in read only memory has been the electrically
eraseable programmable read only memory (EEPROM or
E 2 pROM) where the erasing is done electrically, and takes 20ms per
chip (2Kbytes). This relies on a quantum-mechanical process called
'tunnelling' to make the chip temporarily conducting, to allow the
stored charges to run out.
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