Microcomputer Circuits and Processes

measurements made from a microprocessor-controlled instrument,
then random access memory (RAM) is needed. Remember that, as
explained in Chapter 2, RAM is made up of a number of cells. Each
individual cell is selected by row select and column select signals,
generated from the address sent to the memory chip. This is shown
again in figure 4.2.
address
data in (0 or 1)--4f" __-:-:-:::-:==::I
data out (0 or 1)
RD WR
control lines
address
Figure 4.2
RAM cells shown with their support circuitry. Selection of each cell is determined by the
address sent to the row and column select logic. Input and output are done in
conjunction with the column select logic.
Here, we are interested in what actually makes up an individual
memory cell, able to store a binary 0 or 1. The simplest type of RAM to
use is called static RAM, where each cell is a simple flip-flop. This is
shown in figure 4.3.
(b)
(a)
R
R
B
B
0-- - - - to other
cells
data~ J
in
C
C
I
~data
Figure 4.3
Static RAM. (a) The switches C and R used to select the column and row of a cell are in
reality transistors. (b) The detail shows a single cell flip-flop shown as two cross-coupled
inverters.
The flip-flop is represented here as two cross-coupled
inverters - figure 4.3(b). In commercial chips, like the 2147, four
out
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