Why do we prefer active low signals????

[rakesh_aadhimoolam]

active low signals are usually used

hello::::::::;;

i was studying microproc........ and i suddenly got struck with one point that

"why do designers go for active low pin configs"

somebody help me out.......

thanx

 

[maks10]

active low logic pull down

reset signals are usually active low because you
frequently need them to be well behaved during power-on and power-off
events, when the dc supplies for the circuit are not stable. While it may
be difficult (depending on the logic families used) to generate a good
low-active reset during a power transient, you can see that it would be
impossible to generate a good high-active reset when the power supplies were
below the high logic threshold

 

[starfish]

The Chip Enables , Chip Select signals are active low. Suppose u make them active high and randomly a glitch occurs for a very short time, then that chip would be selected due to that glitch and it may cause unpredicted values on data bus etc. Making Chip Enable signals high prevents it from this sort of problem. In order to select a particular chip u have to bring ur select signal from high to low and hence it is more secure than active high chip selects.

 

[Alan69]

hello::::::::;;

i was studying microproc........ and i suddenly got struck with one point that

"why do designers go for active low pin configs"

somebody help me out.......

thanx

NPN or N channel is favored, electrons (in the N channel) move easier than the 'holes' in P channel. N is the side that switches down to ground (N channel is connected to ground and takes a +V to turn on, P channel hooks to +V and takes a lower voltage to turn on), not up to supply, so for the same size transistor you can pull down better than up. This was really important way back when you could barely make transistors and ICs, it is why TTL is designed where the active output pulls down and up is only a pull-up. You could hook a P type up to the GND, but then you'd need a -V to turn it on.

So then the active pull down is cleaner than the passive pull up when the output is high. That made anything with a lot of TTL inputs to run work a lot better if low was the active signal.

Most controllers etc have a transistor to pull up as well as down, so it's not nearly as important in a lot of cases.


And sometimes the logic simply works out better. If you work it out with transistors, and all of your logic is active high, you often end up having to use extra transistors to control the other high signals.. Better to use complementary logic and less gates or transistors, which means some of your signals will be active low.