When E' is low, the device is
enabled and acts like a regular gate.
When E' is high, the device
is disabled and acts like an open circuit.
Thristate devices are used to
let mutliple devices (blocks of logic) share the same set of
wires. In the example above, any of the 5 devices can drive the
bus as long as the other devices are disabled. Thus, only one
device at a time may be active.
Only one device can be active
at any given time.
Open Collector/Open Drain
Truth Table
A
B
F
0
0
0
0
1
0
1
0
0
1
1
Z
Like most gates, there are two
output states. However, instead of a HIGH the ouput of the gate
is in a high impedance state for a logic 1.
The pull-up resistor is necessary
to provide a high output voltage at F when the gate is in a high
impedance state.
This device is called an open
collector/open drain device.
For the IEEE/ANSI standard,
the open collector/drain is shown by the diamond symbol near
the output of the gate with a horizontal bar at the bottom.
On Pg. 69, Figure. 3-23, the
book uses the wrong symbol (they put the horizontal bar at the
top).
There are also open emitter/open
source devices that have a high impedance state for a logical
output 0. These devices require a pull-down resistor that is
connected to ground. However, these devices are much more rare
and will not be discussed in detail in this class.
Wired AND
An advantage of open collector/drain
devices is that you can connect their output of multiple gates
together.
You can only connect the outputs
of open collector devices and tristate devices together. With
tristate devices you must make sure that not more than one device
is enabled at any given time.
The book describes wiring the
outputs together as being equivalent to an OR. It is actually
equivalent to an AND as shown by the dotted outline in the figure
above. This can be shown by a truth table proof.
Capacitors
This is the fifth analog device
we've talked about. The other devices were power supplies, resistors,
diodes, and transistors.
Up until now we have discussed
the function of devices from a static, or DC, perspective.
Starting now we will start to
introduce the element of time into the picture and discuss how
devices change over time.
Conceptually capacitors can
be thought of as two large parallel plates that are used to temporarily
store charge (electrons surplusses and deficits).
In steady state, capacitors
act like an open circuit and no current flows to or from them.
However, because it takes time
to change the amount of charge stored on a capacitor, they resist
changes in voltage.
Mathematically, this is expressed
as
I = C dV/dt
where C is the capacitance (in Farads), V is the voltage across
the capacitor, and I is the current.
Note from this equation that
in order to change the voltage quickly (a large value of dV/dt)
requires a large current. Theoretically an instantaneous change
in voltage would require an infinite current, which is impossible.
Capacitors in parallel add.
Capacitors in series act like
resistors in parallel, as illustrated above.
Capacitors are used in digital
logic to help keep voltages constant.
This is especially important
for the power supply voltages which can dip due to the large
current flows when all of the gates switch simultaneously.
Ideally, the larger the value
of C the more constant the voltage will be.
However, ideal capacitors do
not really exist. Like power supplies, each has non-ideal characteristics.
Consequently, various sizes
are used to keep the voltage constant for different clock rates.
Timing Diagrams
Timing diagrams are yet another
method of showing the function of a gate or circuit.
If the function of the gate
is known, you should be able to draw the output waveform for
a gate given the input waveforms.
You will generate timing diagrams
for the project to show that the circuits you design work.
