The Electrical Rules Check tries to ensure that everything that is connected to a single net fits together. The quick summary is that a valid net has
- either power (
power_out) or data (
output) pins, not a mixture of them
- at least one pin that can drive the net (
power_out for power nets,
output for data nets)
- at most one pin that unconditionally drives the net (
So a power network needs exactly one
power_out connected to it.
A known limitation is that connectors' pins are defined as
passive, because that doesn't immediately lead to a conflict with anything else connected to the same net, so the connector that the power supply is connected to cannot be marked as having a
power_out pin to placate the ERC.
PWR_FLAG pseudo-component provides a single
power_out pin, so you can avoid the warning here. The ERC will complain if the same net has two
PWR_FLAGs, or a
PWR_FLAG and some other component with a
power_out pin (such as a linear regulator).