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Monostable Multivibrators


Monostable multivibrators or one-shots are digital logic devices that have only one stable state. By contrast, flip-flops and latches are bistable devices that can remain in either a set or reset state for an indefinite period of time. Monostable multivibrators remain in their stable state indefinitely, until an external input is applied. Once triggered, the output is temporarily switched to another state. Because they are timed devices, monostable multivibrators can hold their unstable output state for only a limited amount of time. With semiconductor monostable circuits, this timing function is often accomplished through the use of resistors and capacitors. In terms of a mechanical analogy, monostable multivibrators are like momentary contact pushbuttons that spring-return to a normal (stable) position when pressure is moved from the button actuator. Conversely, bistable devices such as flip-flops and latches are like standard wall or toggle switches that turn lights either on or off.

There are two types of monostable multivibrators: non-retriggerable and retriggerable. Non-retriggerable devices cannot produce pulses longer than the prescribed pulse width and cannot be triggered again when the output is in the unstable state. By contrast, retriggerable devices can be triggered again when the output is in the unstable state. In terms of triggering, there are two trigger types. Negative-edge triggers can change the output on only the negative edges of the clock. By contrast, positive-edge triggers can change the output on only the positive edges of the clock. Monostable multivibrators with Schmitt triggers include circuitry to counteract spurious behavior due to noise. Some monostable multivibrators include a master reset or complementary output. Others are hardened against radiation or provide protection against electrostatic discharge (ESD)





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