How to connect the tristate pin to GND


#1

Hi Sir,
I am using SN74HC245 buffer in my application. In my data sheet it has mentioned like don’t leave the unused pin as not connected. Kindly connected either vcc or gnd. But these pins are tristate pins. I could not able to connected in gnd. ERC error has occurred

. Kindly help me to resolve this.


#2

Are you sure the datasheet suggests to connect the output pin to ground? Typically only input pins are meant to be connected to ground.

I would never connect a tristate output pin directly to ground. What happens if you somehow activate it and get it to a state where it tries to pull the pin to high? So if the datasheet does indeed suggest to connect that pin to ground then i would put a resistor with high resistance in there (high enough to limit the current flow for the case where you have the pin on high to such a level that neither the pin nor the resistor get damaged.)


#3

Hi Sir,
I have enclosed the details mentioned in the datasheet. please find it and suggest me the correct status


#4

It clearly states “unused input”
tristate is an output type!

PS: i am not going to give you a definitive suggestion. I point you to the resources for you to be able to learn whats going on. From that you should be able to derive the correct way to do it.


#5

Hi Sir,
I have changed the Electrical pin type as bi-directional pin. I have resolved the issues but there is an another issue in one place errtype4 - bidirectional components connected to power output


#6

It is you who knows which pin and when can be input, Only when it is input it have to have defined state by external elements. If this buffer connects two buses and its direction is always switched that way, that pins which are input are driven by some other devices then you need not to do anything with it.
As this is HC to set the state you can use even 1M resistors.
The reason to set the state is that input of HC in undefined state can float makeing the input buffer to be in transition state when it takes from VCC current many, many times higher than when it is in defined state (when the supply current is really 0.000). In such buffer if top or bottom transistor is switched on current don’t flows, but during transition there are the state where both are on. If input floats that buffer can be all the time in such transition state.


#7

Hi sir,
I have a doubt. in previous i have enclosed the datasheet for pin functions.Their it has mentioned as i/o. Shall i use those pins has electrical type of bidirectional.


#8

This is a decision you need to make. If this is a user configurable part and the configuration fixes it to either input or output then make a project specific symbol with the electrical type set to whatever config you use in this project.

Otherwise use bidirectional and do not connect the pins directly to any power net as doing so could destroy your part if it is ever for some reason in a state where it tries to output on the pin you assumed to be not used.


By the way you never linked the datasheet so we are kind of flying blind here.


#9

This situation can happen independently of the type of the pin in the schematic.
Even if the unused pin is set to input and tied to GND, the programmer could, accidentaly or not, set the pin to output and use it.


#10

Hi Sir,
Please find the link, i have shared some photos about datasheet :https://forum.kicad.info/u/NishithaMCsn74hc245.pdf (2.2 MB)


#11

Hi Sir,
I am using 74HC245 octal bus transceiver with 3 state buffer. In that data sheet they have mentioned has based DIR it can decide the data will transfer from A to B or B to A. I have selected has DIR has high and CE is low… Now guide me electrical type of the pin


#12

Then do as @Rene_Poschl told you: decide what pin is an input, what pin is an output and set the input pins to input and the output pins to output in the schematic symbol (project specific).

You must also be sure than the DIR pin cannot be connected to low (I assume you have decided to connect DIR to high).


#13

Yes sir,
I have mentioned DIR will be HIGH


#14

If I mentioned A fully as input and B fully as output. Then for unused inputs pins A to be used to connect in GND then what about the unused outputs pins in B, shall i connect as a unconnected pin


#15

This is not a Kicad question anymore.

From the vendor’s datasheet you should know how the final circuit must be connected. Then, make the symbol and connect the pins according with your design.


#16

You should never leave unused inputs open, whether analog circuits or digital, without verifying from the datasheet it can be done safely.

The easiest way out is to simply add pull-up or pull-down resistors.
These are very common and for data busses these are cheaply available as 9-pin resistor networks (8 resistors, and 1 pin common to all resistors)
https://duckduckgo.com/?q=sip+resistor+network&iax=images&ia=images


#17

Rene is correct. Leave the TriState pin OPEN.

Unused INPUTS (only) should not be left floating as the input wants to be either logic hi/low. Unterminated, it may cause oscillation of overheating (and likely failure) of the chip as the levels are attempting to operate in the linear range.

An unused OUTPUTs are not to be connected to anything as it will drive current into the low impedance load causing the device to overheat and fail.

Converting the TriState to Bidirectional is a bad idea, it only masks the problem.

This is not meant as an insult. You are confusing a KiCad error detection for a lack of understanding of what the circuit should be. Please learn more about the internal operation of the chip and logic circuits in general, then KiCad will take care of things for you. Finding a work around for the KiCad error will not make your circuit work.


#18

There is no good reason a bidirectional pin can’t be attached to the same power net as the power / ground pins of the device. Of course schematic tools don’t know about loops and how unlike an ideal ground a “ground” can be, so latchup on the real deal, if things ever get that far, is on you.

If you don’t like the ERC, change or disable the rule. If you can do that to just the one the part, so much the better.

If you have a “presistor” (parasitic resistor) element that netlists as a short for LVS but a trivial value for simulation, there’s your pony. No idea whether, for this tool. Common in IC design platforms.


#19

A typical usecase for bidirectional is to mark the GPIO of a µC. If you then have a programming error and configure it as an output then you can easily fry your GPIO bank that way.

In other words: In general it is not ok to connect a bidirectional pin directly to any fixed potential.


#20

But if you are enough carefull to make no such basic programming errors you can connect all unused microcontroller I/O pins to GND makeing your product more resistant to ESD. I am thinking about indirect ESD but if the device is small also direct ESD can find a way to microcontroller pins.
For ESD 10k resistor typically used to not left unused I/O floating is like infinity.
In one our device (pendrive size) I have used only 2 microcontroller I/O pins (USB, quartz not needed as synchronised to USB) and all rest directly connected to GND.