Hardware Reference
In-Depth Information
Read is simil ar t o write. The address decoding is exactly the same as for write.
But now the CS
RD line is low, so all the write gates are disabled and none of the
flip-flops is modified. Instead, the word select line that is chosen enables the AND
gates tied to the Q bits of the selected word. Thus, the selected word outputs its
data into the four-input OR gates at the bottom of the figure, while the other three
words output 0s. Consequently, the output of the OR gates is identical to the value
stored in the word selected. The three words not selected make no contribution to
the output.
Although we could have designed a circuit in which the three OR gates were
just fed into the three output data lines, doing so sometimes causes problems. In
particular, we have shown the data input lines and the data output lines as being
different, but in actual memories the same lines are used. If we had tied the OR
gates to the data output lines, the chip would try to output data, that is, force each
line to a specific value, even on writes, thus interfering with the input data. For
this reason, it is desirable to have a way to connect the OR gates to the data output
lines on reads but disconnect them completely on writes. What we need is an elec-
tronic switch that can make or break a connection in a fraction of a nanosecond.
Fortunately, such switches exist. Figure 3-29(a) shows the symbol for what is
called a noninverting buffer . It has a data input, a data output, and a control
input. When the control input is high, the buffer acts like a wire, as shown in
Fig. 3-29(b). When the control input is low, the buffer acts like an open circuit, as
shown in Fig. 3-29(c); it is as though someone detached the data output from the
rest of the circuit with a wirecutter. However, in contrast to the wirecutter analogy
the connection can be subsequently restored in a fraction of a nanosecond by just
making the control signal high again.
Data
in
Data
out
Control
(a)
(b)
(c)
(d)
Figure 3-29. (a) A noninverting buffer. (b) Effect of (a) when control is high.
(c) Effect of (a) when control is low. (d) An inverting buffer.
Figure 3-29(d) shows an inverting buffer , which acts like a normal inverter
when control is high and disconnects the output from the circuit when control is
low. Both kinds of buffers are tri-state devices , because they can output 0, 1, or
none of the above (open circuit). Buffers also amplify signals, so they can drive
many inputs simultaneously. They are sometimes used in circuits for this reason,
even when their switching properties are not needed.
 
Search WWH ::




Custom Search