Electronic – Why are things like RESET/MCLR active low on most ICs



Easier to implement?

Another reason?

Is there a reason things like MCLR or RESET on microcontrollers are active-low, that is, you have to pull them down to reset the IC, and pull them up to "run" the IC.

I'm just curious because this causes me some problems. If it were active high, I could avoid the capacitor on MCLR required in some instances and deal with just a pull-down resistor. It seems only to add to complexity.

Best Answer

Look at what happens during power-up: Vcc rises to a point where it's high enough to make everything work properly. However, that point isn't clearly defined and may vary from device to device. It makes sense not to use this voltage to reset the controller.
It's easy, however, to keep a level low regardless of Vcc. After all, Reset is already active the instant you switch power on, since at that moment everything is at a low level.

The graph below illustrates how the output voltage of the reset controller (i.c. an MC34064) remains low until Vcc is high enough to have the complete microcontroller stable.

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