Basics: Why Active low signals used?

Are you working on digital circuits? Then you must be using control signals in your design and must be hearing about terminologies like ACTIVE LOW, ACTIVE HIGH. We sometimes just follow the design recommendations and never think of the need to do it. Let us analyse the need for Active Low in our designs.

What is active low?

A general criteria while using any control signals (like Enable) is to make it HIGH from LOW state. But in some cases, like the chip select and reset it is in reverse, the signal will be initially HIGH and when it is turned LOW, it means that particular signal is asserted (when asserted, it changes the connected chip functionality as desired, can say connected chip changes state). Control signals which are used in this manner are termed Active Low. The terminology itself tells that signal is termed active when it is LOW.

How is Active LOW signal differentiated from Active HIGH signal?

The representation of the signal (as per the Active HIGH or Active LOW ) in the datasheet of that particular chip is different. Active LOW is represented by having a dash on top of the signal name. Chip Select if active low is represented as (CS).

Reasons for using Active LOW only:

• Let us assume that you want to choose a device between n-channel MOSFET and p-channel MOSFET for your application. One would always prefer going for n-channel as electrons are the majority carriers in this type of device and have the highest mobility then holes. In this case, when the transistor is ON, the output is driven low. So, a normal condition is that output is high and when transistor in on output is low. This is one of the factor which makes implementation of active low state easy. Same mobility case applies to BJT also where NPN is preferred compared to PNP.
• Active LOW always helps eliminate indeterminate states due to improper supply voltages. 
• A genuine reason is that it is easier to pull down a signal than pulling it up. 
• Under a Active low condition, it is always easy to use wired-or condition and apply common reset to several chips. So, fanout can be increased. Control signal count can be reduced.
• For critical signals like Reset, it is always important that a state is maintained properly, if they are made active high, any noisy transitions may cause improper reset in the circuit. So, a state of high initially and then making it low to reset the chip can be the best option. 
• Consider the sourcing capability for example of a micro controller which doesn't go beyond 10 mA. If you want to source more you may have to use an external driver. In these cases, it is always preferable that we use Active LOW. For digital circuits, sink current capability is more than source current.