Updated 2011-01-23 16:35:24 by RLE

Or Bistable Multivibrator [1]

by Theo Verelst

Only the most important electronical circuit for an essential one bit memory cell.

And electronicists actually call them the above name, for easy enough reasons, but not so easy historical reasons. A vibrator may be an electronical circuit, but maybe it is some relay circuit from early telephone exchange lingo. Maybe someone knows in some long standing dignified research lab. Stable is clear. Bi means two, of course.

Electronicists also discern monostable and astable multivibrators, which flip back or keep oscillating, respectively. The electronic state space is continuous for such example, and somewhat complex, though an overseeable case can be sketched with lots of simplifications. Not strickly simple for engineers even, though, just comparitively speaking.

The idea is doable, though. Its like having a circuit which feeds forward to itself, which we can force in one of two extremes. Like a circuit which amplifies a voltage, and has its output fed back to its input, without inverting. Any normal circuit will end up in either the highest or lowest output voltage that why, assuming it amplifies more than +1, and time to make the output swing is given.

When we force the output in the other extreme somehow, we can give the simple memory cell another bit value to remember: sustained low or sustained high, 0 or 1.

Logical circuits are like amplifiers with relatively small delay time and bi value optimized output and input, and an input circuit which can combine low and high valued inputs according to some logical function.

NAND circuits will happily act as the components of which a memory cell can be constructed. A nand can be modeled as a simple [function] with double bivalued input domain and bivalued co-domain.

Apart from the mathematical unusualness of feeding a function back to itself, not uncommon in differential equations, though, both mathematically as well as digital designwise as electronically the circuit shown on top of this page acts as a memory cell, with indeterminate starting state, forcable in either of two main states over two transition states for the practical implementations (with non-mathematical, finite function evaluation time or transition time).

Simply put, the nands are in stable state as 1-0 or 0-1, until one of the inputs shortly is made to carry a 0 instead of both 1, which forces either global state.

Interestingly, and I've seen millions consuming design systems which sin against some design by correctness idea of using networks of functions by not understanding networks of functions do not AT ALL guarantee overall functional behaviour, as is illustrated by this very basic and fundamental computer circuit example.

Also networks of functions do not give guaranteed state machines as model neatly, for instance, they could oscillate, giving no possible mathematica stable solution, or at least give a state machine with indetermined initial and intermedeate states.

In a simulator for general electronics circuits or specifically for digital circuits, various models with various accuracies of modeling the voltage changes and their timing are possible, which can all lead to perfectly good working models. They can do the combination of such circuits, too, including the pretty exact voltage change graphs for state changes.

The next step of building important and totally essential and fundamental computer circuits is called the 'Master Slave Flip Flop' maybe after the german education idea in certian practical work areas, or some designer wanting to make a point, I don't know, it's normal terminology in the area.

These are not my inventions, they are part of normal electrical engineering curriculum.

Scott Nichols: I studied electronics for my undergraduate degree. The classroom instructors called them JK flip flops. The J and the K letters stood for where the wires just happened to be plugged into the breadboard on the very first flip flop built in a lab.

A more recent electronic device with the word flip in it also is the flip chip. This is a type of silicon microchip that is soldered upside down directly to the printed circuit board.

TV Allways good to get feedback, though the emergence of this page is just because I changed the references to a server...

A JK flipflop traditionally is a master slave flipflop with special inputs J and K, which when set both to 1 make the circuit divide it's clock signal by two (in frequency). This circuit cannot do that, it is traditionally (and still, check www.ti.com or something for recent logic families) called a RS (set/reset) flip flop. This is a asynchronous circuit, the JK flip flop is synchronous (all changes happen a clock edge)!

More official nomenclature from electronics is 'bistable multi-vibrator', as opposed to monostable mv (monoflop in digital datasheets, e.g. 74HC123) and astable mv (a generator or oscilator), which respectively fall back to a fixed state after a while or oscillate (change state all the time).

a flip chip, oh my gawd... That might send you flop shopping