B
Bob Monsen
- Jan 1, 1970
- 0
I have a comp sci degree, and something that was never explained were flip
flops.
I never understood on a chip how switching occurs with no moving parts, no
this is not a troll but a genuine newbie to electronics question, which as I
get involved with am beginning to really enjoy, having just built a working
kit. The course lectures never went near the electronics side of things, the
closest we got were logic gates.
TIA,
It's caused by the fact that a double negative is a positive... no, really.
It is done with what is called feedback. The output of a circuit is used
to set its own input. This means that when the original input goes
away, the output is still keeping itself in the same state.
Changing the output generally involves overriding this 'feedback' input
with an outside input.
As an example, inverters are logic gates that, when given an input of
either LOW or HIGH, output the opposite, HIGH or LOW, respectively. Now,
imagine two inverter gates, A and B, which are connected so that the
output of A is the input of B, and the output of B is the input of A. Now,
assume that for whatever reason, the input of A is high. Then the output
of A will be LOW, causing the input to B to be LOW, causing the output of
B to be high, and thus making the input to A HIGH, like we assumed. A
double negative makes a positive. Thus, the circuit is in a stable state,
A LOW, B HIGH. Now, suppose we somehow force the input of A to suddenly be
LOW without changing anything else (perhaps by grounding the input with a
pushbutton). Then the output of A goes high, the output of B goes low, and
so it is again in a stable state, this time A HIGH, B LOW. Bill's
transistor flipflop is really just this circuit; the two transistors are
each configured as an inverter, where if the input at the base is a high
voltage, the collector of the transistor is a low voltage.
Digital flip-flops use this situation to 'remember' that things have
occured. There are lots of different kinds of flipflops, but they all use
something like this as their basis. The differences have to do with how
they get set to the different states. RS flipflops have two different
inputs, one which sets it one way, the other of which sets it the other
way. JK flipflops also do this, but use a clock signal to synchronize the
input; the state only changes when the clock changes. T flipflops change
state whenever a pulse is seen on the input. D flipflops are clocked, and
set up so the output follows whatever is on the input when the clock
transitions.
You can read this site to find out more:
http://www.play-hookey.com/digital/