Hi,
I found one opensource KiCAD project and I want to modify schematics and PCB files a little bit. By default ERC returns four “Pins of type Power output and Tri-state are connected” errors:
If I remove PWR_FLAG, those four errors disappear but another error appeared: “Input pin not driven by any Output Power pins”, the problem is with those B4 to B7 pins.
How to solve this problem? The connection is valid for this 74LVC234 IC.
You could change the pin conflict map entry for Power Output Pin → Tri-State Pin from error to warning or OK.
It’s a warning, not an error, no? (I didn’t try it).
In that case, ignore it, or turn the warning off.
EDIT: scratch this, JamesJ’s answer is better.
Or create your own version of the symbol with a different pin type (such as Input Pin).
It’s an interesting issue. The '243 and '245 types are transceivers with tri-state capability.
Perhaps it would be more correct to define the data pins as bidirectional instead?
In general, yes (given that CE is tied low so the high Z state will never be invoked), but here the direction pin is also tied low. Depends quite how bespoke you want to be!
ERC is just a simple comparison of pintypes according to the matrix shown by JamesJ. It does not have spice like features. there is no model in KiCad for the 74LVC245, and KiCad does not know how the direction and Chip Enable influence the behavior of the other pins. But you are smarter, and can disable some of those warnings once you have confirmed that the circuit works as intended.
Some people have the opinion that ERC is not powerful enough to actually be useful, and they just never run ERC.
Long ago I was taught that connecting tri-state pins of a '245 directly to power was dangerous design. 10k pull down resistors were the approved approach.
Very true and I agree with it.
But then, why does a bidirectional pin only throw a warning?
The ERC error/warning/ignore table is settable by the user
This is what Ti “prints” in their LVxx user manual:
Proper Termination of Unused Inputs and Bus Hold
A characteristic of all CMOS input structures is that any unused inputs should not be left floating; they should be tied high to
VCC or low to GND via a resistor. The value of the resistor should be approximately 1kΩ. If the inputs are not tied high or low
but are left floating, excessive output glitching or oscillations can result due to induced voltage transients on the parasitic lead
inductance inherent to the device input and output structure.
Source, page 43 of:
I also like the idea of the bus hold feature added to devices with an extra H in their type number.
Technically it is an error from ERC
So, it better to connect unused inputs to GND via 1K resistors?
I’m just regurgitating what other, more intelligent people have experienced over the last 60 or so years.
A smal experiment with adding resistors for unused inputs removed those errors:
The remaining error is: “Input Power pin not driven by any Output Power pins”
Your +3.3V / GND nets probably don’t have PWR_FLAGs on them
Ok, no more errors, only the warning: “Warning: Pins of type Power output and Bidirectional are connected”
I suspect that the pin types for pins 1 and 19 are incorrect (on the assumption they are both actually input-only).
There’s probably another bidirectional pin on another device connected to a power line then.
