In SX Microcontrollers, SX/B Compiler and SX-Key Tool, tdg8934 wrote:

The SX Key DEVICE setting shows INTERNAL 4MHz down to INTERNAL 32 KHz. Externally you can connect up to 75 MHz but not used rarely over 50 MHz for stability reasons I beleive. However, as a general rule, you should run your programs as slow as possible depending on the application requirements. I usually work with Internal 4 MHz because I don't have to use a resonator. However, if I have something that has alot of processing going on or something with serial communications, the internal 4 Mhz clock won't cut it so I have read on the forum here. If you are using DEBUG mode, you never want to have a resonator or crystal used and "will" damage the SX-Key. It's easier just to use the Internal 4 MHz setting. 

BTW: I have to do some stuff with my wife this morning so I hope to get back to the 8255 circuit later this morning.

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