Don't be angry at me :P
First, I have to say for a new user needing to check PinNames.h or tracing schematics isn't what you like to be doing ;). Also I know the way the mbed library is build, but for a new user it can take quite some time to find PinNames.h.
PTA1 and PTA2 are on arduino shields the 'default' pins for serial connection, all others need a software serial. On the arduino the USBTX and USBRX also share those pins, since their atmega has only one serial port, so they didn't have much choice. The Obvious downside is that you cannot use both a serial connection with your PC and with a shield, but on a limitted device as an atmega328 you have to make compromises.
Then you got the KL25Z. I understand they want to make it like an arduino, but that doesn't mean imo you should also copy its weaknesses when you dont have to. (You cant do much about the stupid pin spacing for example). Considering PTA1 + PTA2 and USBTX + USBRX are the most used serial pins when you want something arduino compatible, imo it makes sense to me to put them on a different hardware UART, so you can both communicate serial with an arduino shield, and still send debug information to your PC. Considering the KL25 has 3 UARTs and a gazilion possible ways of connecting them to pins this shouldn't be a problem.
If you really want it to be like arduino you could also connect them to one UART but with different physical pins. Not what I personally would do, but then at least you can disable the UART on PTA1 and PTA2 while still communicating with your PC.
From just looking at the pinout of the KL25 and without being hindered by any info on how it should be routed on the PCB, I would connect PTC3 and PTC4 as USBTX and USBRX. (Or them to the place of PTA1 and PTA2, and keep them as USBTX and USBRX). Since these pins are on the inside row they are less important: standard shields don't use them. You could then either keep them connected to USBTX and USBRX, or just make those pins NC, something to say for both options.
But the main upsite is that this way you can debug to your PC while also using the default hardware serial port of arduinos.
Hello All, When making a USB serial object on KL25Z:
pins PTA1 and PTA2 are overwritten to be serial communication too. This is very unclear from the communication, and to me a waste of resources; I want to use those pins!
My suggestion: