Continue to Site

# What's the tri-state? Why to use the tri-state in design?

Status
Not open for further replies.

#### winking

##### Newbie level 3
In IC design field, I always hears the tri-state, but I donn't know the real meaning that the tri-state indicates. So I wanna know what is the tri-state, and why to use the tri-state in some design? Thanks a lot!

:|:|

Re: What's the tri-state? Why to use the tri-state in design

an important messiah in digital electronics...all lines used in digital devices have 1 (presence of voltage) or 0 (absence of voltage) ...but during normal or idle conditions line swill be free...that may be considered as bit '0'..an error operation..at that time your tristate comes to resue.. it can act as high impedance state (one the state other than high and low)so that nothing weill be assumed by the device ....

Amalinda

### Amalinda

Points: 2
Re: What's the tri-state? Why to use the tri-state in design

You have two system buses mutilexed to a single bushow you relay on the received bits if both the buses are giving either zeo or one .In order to isolate either one of the buses you need some sort of method to do this .This is acheived by the tristate drivers .Tristate drivers will offer high impedence to one direction or bus and basically both MOS circuits are disconnected from VCC and Ground . That potential can be viewed as series high (off mosfets )resistances and hence you can say the voltage to be 2.5 .It wont sink or source current

Re: What's the tri-state? Why to use the tri-state in design

hey mystique,
will the addition of tristate buffers increase the current driving capabilities in the circuit??or is it jus to select ur signal???

regards

Re: What's the tri-state? Why to use the tri-state in design

hi ,
lets not complocate it..

Tristate is a state other than a 0 or 1.
Now u would ask y do we need another state when the two itself is difficult to mange.

1.Because As mystic was saying duplex lines practicall a commonplace in design exist only because of the concept called tristae.

2. Most important according to me and many people would be the pressence of this state greatly does away with excessive power consumption. When buses are not used then its better to assign them a Z value as it would consume very less power.

3. I agree with mystic that tristate buffers do improve driving capabilty. But dont think tristate is tristae buffers only.. They are just one application..

Any device controlled with a enable for operation only when its needed uses tristate.

Re: What's the tri-state? Why to use the tri-state in design

tristate used for both directions, in an out when there is complex logic less area to be consumed.

Re: What's the tri-state? Why to use the tri-state in design

Lets keep it clear guys...A '0' on a wireline means a current is being sinked totally...and no connect to supply power is there....the wire is at zero potential...somewhere a MOS connected to that wire is closed and making a node ground...similarly a '1' means somewhere a node is powering up the node...MOS is closed and connecting the reffered node to supply power...so effectively a '1' pumps in current and a '0' drains it out.... Now if we have tri-state buffer or for that matter tri- state imposed that means no current is being sourced or sinked...so effectively a tri-state is like nothing being there...we can say that shorting a node with a tri-stated(high impendance) node is like doing nothing at all to the referred node...it can be called as isolation...so how does it avoid contention on system buses...it does'nt cause any interference to values being imposed on the bus by other connected node... i hope this clarifies it all....

Re: What's the tri-state? Why to use the tri-state in design

at system io level we may have io buses. there we can use tristate logic for isolation.
and with in the chip we dont use tristate buses, to dont have synthesis issues.

correct me if i'm wrong.

Re: What's the tri-state? Why to use the tri-state in design

tristate switches are a kind of functionality we are using right from ttl logic design..first thing it is nothing to do specificaly with CMOS technology and specificaly use in bus designing.it is a third logic state u can say called 'Z'highimpedence which a kind of logic state which is not zero not one even an dit is strictly not '0' and not'1' .used basicaly in bus probbing

Re: What's the tri-state? Why to use the tri-state in design

how does tristate buffer increase the current drving capability?. can anyone explain it?

Re: What's the tri-state? Why to use the tri-state in design

tri-state buffer will not give a strong drive ability , it will consume more current and energy , so it need a stronger driver .

Re: What's the tri-state? Why to use the tri-state in design

I guess by now everyone must be sure what Tri-state is.
Just to be simplified see it as a valve which do not let the fluid (current in our case) flow from one side to other. So the line connected to the output side of the tri-state buffer is basically floating or high impedance state.
This is the link given by echo47 which clarifies all your doubt that you might have

Now the last bunch of quries were about if it has a drive capability. Yes sure it does.
Just like any other buffer Tri-state has a driving capability with the added functionality of being able to achive high impedance at its output when required(i.e when disabled)

If you wish to know whats inside it check the ckt below.
https://www.asic-world.com/digital/gates5.html

Now some were wondering about if we can use it or where we can use it.
1.As Anjali mentioned it is lot of time used near the output pads.

2.Plus it is also used in case of a common bus.Say lot of memory outputs from differnt memories(say 8) driving a single bus.YOu would traditionally like to use a mux.Hower if the width of each of busses is very big say 16 or 32 then the size of the mux will be honking big with inputs at 8x32.So if you want to avoid it emply tristate. But precaution must be taken in the case if you have a condition when nobody is driving the bus then you should have it pulled up.
This is similar to MUX being implemented using TRi-state buffers.
3. Since DC and other tools give errors this is not generally designed at the earlier stage but is included at the layout stages or P&R. Plus there are some testing issues with the tri-states employed on the chip.

Status
Not open for further replies.