[xxpor]

I know next to nothing about microcontrollers, so this might be a dumb question, but how do they do the initial flashing is they can't run a serial port? Are they hooked up to something external before going on the final PCB? Do they not need a precise clock to read the onboard program from whatever's storing it?

[michaelt]

Even if you've got a really inaccurate clock, you can still accept synchronous protocols like SPI and I2C where the bus master provides a clock signal.

Every chip brand would have their own protocol and provide their own programming hardware that could speak it.