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

A rule of thumb:

When using any other clock source but the internal RC clock, add an IDC_CAL IRC_FAST or IDC_CAL IRC_SLOW directive to your code, just to keep the SX-Key IDE from performing an IRC calibration process when you select the "Program", "Run", or "Debug" options from the menu, just to avoid the extra time required for the calibration process. The IDE will perform the calibration whenever it "sees" an IRC_CAL IRC_4MHZ directive in the code, no matter if you have specified the internal RC clock (DEVICE OSC4MHZ) or not.

When you want to use the internal RC clock, use the DEVICE OSC4MHZ directive (or any other one also activating the clock prescaler). You may still may still leave the IRC_CAL IRC_FAST or IRC_CAL IRC_SLOW directive in your code if precision of the internal RC clock is not an issue. In case you want to have the internal RC clock adjusted to 4 MHz as close as possible, use an IRC_CAL IRC_4MHZ directive instead. When the SX is programmed, some extra time is required to allow the SX-Key IDE to perform this calibration. Please keep in mind that the internal RC clock (even if calibrated) may be not stable/precise enough for certain purposes, like executing a UART VP, etc.

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