can I create a single project and have each single schematic included as each circuit has signals that are common to all like clocks etc?
Yes. KiCad has hierarchical sheets which allow you to embed schematics within schematics. It also has three types of net names: net label, hierarchical label, and global label.
To draw a hierarchical schematic, start with the
Create hierarchical sheet button on the right toolbar. Double-click on the box to enter. To have an input/output signal on this block, make a hierarchical label. Once you've drawn your subcircuit, you can leave the sheet with the
Leave sheet button on the top toolbar. In the higher-level circuit, right-click on the box representing the subcircuit and click
Import sheet pins to bring your signals out of the block and into the higher-level circuit.
Global labels are nets shared by every sheet. These include power labels placed as power ports (e.g. +3V3, GNDPWR), and any you have defined as global labels using
Place global label (on the right toolbar).
when I run the ERC I get errors saying input pins are not driven by output pins. These circuits are all powered remotely by using pins on the PCB. I have heard about using the PWR_FLAGs but how do they work?
A PWR_FLAG is a special single-pin symbol that acts as a
Power output device. ERC is simply ensuring that all
Power input pins are connected to a
Power output pin, i.e., that something is actually providing the power those pins are requesting. Your connection / terminal blocks will usually only have
Passive pins, though, so ERC assumes that pin will not provide power. If you put a PWR_FLAG somewhere on that net, ERC will see a
Power output and be happy.
You can see (and set) the exact ERC rules (which connections are errors, which are warnings, and which are OK) under the ERC dialog's Options tab.
If a symbol already in your circuit has its own
Power output terminal (e.g. a switched-mode power supply or linear regulator) then you won't need a power flag on that net.
You can attach the PWR_FLAG to any point on a net, if you know that net is powered from somewhere outside your circuit. Personally, I prefer to put PWR_FLAGs beside the actual connection that power comes in from the outside world; this just makes the schematic a little bit more readable. I often see a PWR_FLAG just connected directly to a +3V3 net off in the corner somewhere, which still works for ERC but isn't as clear to the reader.