In SX Microcontrollers, SX/B Compiler and SX-Key Tool, g_daubach wrote:

The most important reason why the OSC1/OSC2 pins are used for serial programming and debugging is not to make the programming port more secure. Those two pins are the only ones that can be disconnected in a target system without influencing any other function. If some I/O pins would be used for this purpose (like in the parallel programming mode), they could not be used for general I/O.

Programming the SX is not just a matter of simply taking the UART data from the FTDI chip, and translating it into OSC1/2 toggles. You can find more information about chip programming here: 

[url]http://www.parallax.com/dl/docs/prod/sx/DeviceProgramming.pdf[/url]

If you look at the SX-Blitz USB, besides the FTDI chip you will also find an SX20 plus a bunch of other components. The SX20 handles the communication between the PC and the SX-Blitz, and it also handles the programming communications with the SX on the target system. It is also is used to generate the programming voltage Vpp (about 12V) that is required. On the SX-Key, you will also find a clock generator chip that is required to provide clock frequency you have selected for the target system. For debugging, it is also necessary that a little utility program is loaded into the target SX. The SX-Key automatically does this when you program an SX device for debugging in order to handle the debugging communication with the SX-Key IDE.

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