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I created a (for me) fairly big (multisheet) circuit diagram in CircuitMaker. However, I get several warnings/errors after clicking Outputs/Electrical Rules Check. One of them is below; how can I fix it?

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[Warning: Net U13_16 contains IO Pin and Output port objects (Port GPIO_LATCH)]

And in another sheet where I use the GPIO_LATCH, I get the error:

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[Warning: Net NetU26_12 has no driving source (Pin U26-12, Pin U27-12, Pin U28-12, Pin U29-12)]

Note the GPIO_LATCH is used for SPI and need to be connected from one GPIO pin from the MCU to 4 daisy chained 74HC595 multiplexers.

I also tried flipping the type from input to output and some combinations, but in all cases I get some warning(s).

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The warning tells you pretty straight-forward what the issue is: An I/O-pin is connected to an Output Port. It's one of those warnings that in many cases could be disabled, since it might contribute more clutter than it provides useful information.

More specifically, the issue is that the pin on your schematic symbol is an Input/Output-pin, which Altium considers to be both input and output, instead of one of them. When you then connect this to an Output Port (arrow pointing away from connection), it warns you that there might be an issue.

The annoying part of this is that many standard / manufacturer-created symbols use IO-pins to indicate user configurable pins on MCUs.

To get rid of the warning, either edit your symbol to be either an input, output or non-directional pin, use a generic Port, or put a No ERC-marker on the connection.

Also, you should ensure that only the driving source has an Output Port-object, and the receivers have an Input Port-object. It doesn't really matter electrically, but it provides clarity when reading the schematic.

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