Active Low or Active High

Hi all,

We all know that microprocessors/microcontrollers have active low &
active high signals.Is there any specific advantage for active low
signals?Can't it be all active high or active low?

 

Most digital logic is edge-triggered or edge-sensitive,
meaning one edge is the trigger and the other isn't. The
important edge must be the fast one, and some technologies,
such as TTL, pull faster edges in one direction than the other.

As a result, TTL tends to use active-low, since it's the high
to low transition that's the fast one.

Clifford Heath.

 

For microcontroller outputs that drive things in the real world, one
thing you might want to look at is any internal pullup / pulldown
resistors. It can be an advantage if you design your outputs so they
are automatically pulled to the "off" state during reset.

 

Broadly yes.
In the details it can matter :
If you are driving a LED, the LED polarity dictates your active level.
Ditto for if you are driving a NFET.
If you want to "OR Wire" a BUS, that usually points to Active-Low,
Open-Collector topology in the simplest form.
NFETs drive more than PFETs on a same-area basis, so if you want highest
drive, into relays, or Solenoids, that's usually NFET active low.

-jg

 

Yes, the Select lines, Reset lines and many others are active low.
Especially the control signals are active low.
This is because, most logic families can sink more current than they
can source.
That is, because of this, the logic gate output will be high to drive
a number of inputs of other logic gates of the same type.

Karthik Balaguru

 

Read the datasheet and figure out whether the pins are high, low,
or floating during and right after reset. Then design so that is
the safe condition (motors off, etc.).

 

It is the deep historical and religious question. It has to do with the
fact that for the ancient 74xx series the outputs normaly act as drains,
and the inputs act as sources.


Vladimir Vassilevsky
DSP and Mixed Signal Design Consultant
http://www.abvolt.com

 

I always thought that it went way back, in relay circuits. I have an
idea that it was so you could not easily short the supply to ground
and pop fuses.


Martin

 

Back in the day, TTL inputs floated high. Therefore interface
signals were often made active low so that they would float to
the inactive state.

These days, CMOS logic inputs don't typically float high, so
there's no overriding reason to use active low. Active high is
more "common sense" so it tends to get used more on new
designs, but there are still alot of interfaces that were
spec'ed in the TTL days.

So, you'll see both active high and active low.

 

There were other causes, too.

The noise margin of a TTL high is much
larger than the noise margin of a low.

Selecting low active minimized the risk
of data corruption in inactive parts
by noise.

Also, before three-state TTL chips, a
bus was built using open-collector
elements, which made passive high
a necessary selection.

Been there - done that (about 40 years ago).

 

Mustn't forget other open-collector families such as DTL...

Regards,

Michael

 

The active low was very common on old bipolar logic families, such as
RTL, DTL or TTL.

Since in those days it was not practical to make both NPN and PNP
transistors on the same chip, so only NPN transistors were used. The
output stage was a Common emitter NPN stage and it was easy to drive
current into the base of the NPN, putting the transistor into
saturation, with a large current flowing from collector to emitter
with a low voltage drop, thus the output is low, when current flows
into the base of the output transistor.

It would be very hard to make an active high output stage, since an
emitter follower transistor with the collector at Vcc and the output
at the emitter. You would actually need an additional supply voltage
higher than Vcc in order to be able to drive sufficient current into
the base of the NPN transistor in order to get the output voltage even
close to Vcc.

Of course ECL used NPN emitter followers with negative Vee at -5.2 V,
but that was an oddity.

Paul

 

The original relay circuits were mostly used in positive ground
telephone circuits, so it is the high condition that is less likely
to short out and blow fuses.

It's an interesting story as to how it came to be that telephones
are mostly positive ground and modern cars are mostly negative
ground. When Michael Faraday did his pioneering work on electrolysis
he determined that what we now call the positive electrode (anode)
lost material and what we now call the negative electrode (cathode)
gained material. So he called the electrode that was putting material
into the liquid positive and the electrode that was taking material
out of the liquid negative. He had no way of knowing that there were
electrons moving from negative to positive and ions moving from
positive to negative.

Because whichever part of the system is negative gains metal and
whichever part of the system is positive loses metal, the designers
of the telephone system made the ground positive, because in a
telephone system, ground stakes are a lot easier to replace than
wires are. In automobiles, body and frame parts are harder to
replace than wires are, so the automotive designers eventually
settled on negative ground.

Telegraphs had a different system: a battery on each end. This
resulted in negative ground at one end and positive ground at
the other end, and ground halfway between positive and negative
somewhere in between.

 

Actually the immediate predecessor (to NPN DTL gates) were PNP DTL
gates, which swang between a negative voltage and ground. However,
the ground level was still the power direction. They went out
about 50 years ago, but reappeared in early PMOS designs.