use of low active signals in microprocessors ???

i have seen so many signals given to microprocessors as low active ..can anyone tell me wat is it significance ??

Active low means that the "assertion" state (as opposed to the negation) state is a logic "0". For example, using the apostrophe to indicate active low:
~ Reset' means that a logic "0" is required to initiate the reset condition.
~ Reset means that a logic "1" is required to initiate the reset condition.
~
Active low is handyin that it allows several devices to initiate the same condition. For example multiple open drain devices can be connected to the same active low input, using a single pullup resistor. Turning on any one of the open drain devices will inititate the condition (the "reset" cpondition in the above example).
~
I hope this helps.
Regards,
Kral

Hi~~

I am not very sure, but, I think, some pins especially used to initialize a device might be better used if the pins are active low from the operational point of view. When power is on, some part of the circuit inside and outside the device are not setup well. Some nodes will be charged up to high and some nodes will remain at the low level. Then, if a pin used to initialize a device, say reset pin, is active low, then, it could eliminate some irrelavant actions of the circuit.

ysenthilece,

i have seen so many signals given to microprocessors as low active ..can anyone tell me wat is it significance ??

Click to expand...

Such microprocessors with signals as low active or active low has good reasons for doing so.

If you notice carefully, such active low signals have their pins specially used as passive output, not normal bidirectional or input/ouput port pins. In other words, they source currents. It is intended for external circuit like a Reset Circuit or Interrupt Circuit to trigger a logic low level or low level pulse with fixed duration, during which the pin will source some current and sink it down your pull-down resistor. This usually results a short RC delay and it is an intentional.

This is to prevent accidental clock pulses and crosstalks, stray signals or EMIs to trigger false logics which occur above VIH (voltage input logic high) that affects normal I/O pins.

If the pins are made to be active low, VIH doesn't affect them at all. Hence avoided accidental or false trigger.