why active-low? specifically on control signals.

Many control signals like, memory read/write, reset, etc are active low. Is there a specific reason for having active low signals. I designed a memory, and by default the read and write signals are active high. Just to implement this, I need to use an inverter at the pad. So, I would like to know, any advantage of using active low signals.

Thanks in advance.

Active-low logic decreases leakage power (static power) compared to active-high, and most designs are such that it takes fewer transistors to make an active-low design rather than active-high. It's probably not enough difference to matter these days, but it's still most common because of historical inertia and perception.

If you added inverters to make your controls active-high, you've definitely increased leakage and dynamic power. Why bother adding an inverter? Logic designers and synthesis tools don't care if it's active low or active high -- why force them to have extra inverters they don't need?

For control signals it is largely academic. Main use is for i/o, as many cpu's either float or go high on the pins during reset, it is not a great idea to have peripherals briefly activate during this time, so active low is the preferred option. On the input side it seems that historically active low was the preferred option. This of course would allow a pull down to a common logic ground from different votage sources. Also if it is in relation to cpu's, usually you would have internal pull ups you could enable, this will save a few components, so is a little simpler and cheaper to manufacture. May also be better for noise consideration. Another reason is that many older processors could sink a lot more than they could source, useful for output, but not so relevent these days.

somu.atluri said:

Is there a specific reason for having active low signals.

Click to expand...

Both NMOS and TTL feature pull-down drivers that are much stronger than their pull-up drivers. Even today, many CMOS devices have stronger pull-downs than pull-ups. In many cases, there would be no practical difference between using pull-up or pull-down drivers, provided the originator and consumer of the signal were expecting the same polarity. One slight advantage to using active-low is that adding pull-up resistors to pins that should be inactive when floated will tend to balance the performers of the weaker pull-up transistors and stronger pull-down transistors.