Page 339 - Embedded Microprocessor Systems Real World Design

AND Gate OR Gate
A B Output A B output

False False False False False False
False True False False True True
True False False True False True
True True True True True True

Multiplexers
In microprocessor designs, multiplexers are normally used for address decoding. As
shown in Figure C.l, a multiplexer has multiple outputs, but only one at a time is active.
Multiplexers normally have low-true outputs like the example in the figure, and they usually
have an enable line that makes all the outputs false. The multiplexer in Figure C.1 has four
outputs, selected with two inputs (A and B). Multiplexers are also available with eight outputs
and three select inputs. Of course, if a multiplexer function is implemented in a program-
mable logic device (PLD) or other configurable logic device, it may have any number of
outputs with any polarity (even mixed) and the enable may not be required.

SetcReset Flip-Flop

These are stmu@ devices. A flipflop remembers its state. A typical flip-flop will have two
inputs, set and reset, and an output, Q. When set goes low, Q goes high. Q then stays high
regardless of which state the set input goes to. Q does not go low until the reset input goes
low. Q then stays low until set goes low again. Flip-flops can be constructed with high/true
or low/true inputs and inverted or noninverted outputs.
Figure C.2 shows the logic symbol and timing diagram for a set/reset flipflop. As
indicated, this type of flipflop can be built using a pair of NAND gates. Only one output is
shown in the figure, but the output of the other NAND gate also can be used and will be an
inverting output. A pair of NOR gates wired the same way as Figure C.2 also will function as
a flip-flop, but the inputs will be high/true instead of low/true.
So what happens if both inputs to a NAND flip-flop go low at the same time? If you look
at the logic, both NAND gates have one input low, so both outputs will go high. However,
this condition is not latched, and when one input goes back high, the corresponding output
will go back low. If both of the inputs go high at the same time, the final state of the output
will be indeterminate. A similar result occurs in a NOR flip-flop; if both inputs are taken
high, both outputs go low.

Registers and Latches

Microprocessor circuits invariably require some kind of registered logic. This often is
embedded in the peripheral ICs connected to the processor. However, often some form of

320 Appendix C