Measuring 30b CPU speed reduction

[Marcus von Cube]

I might be able to add to the confusion.

The RC oscillator runs at 2 MHz, not 4 MHz. When idling, the 34S divides this by 64 to get roughly 32 KHz (the slow clock didn't work as expected so I chose to scale down the RC clock instead).

SLOW mode runs at approximately 20 MHz, FAST mode at approximately 37 MHz. Wait states (2 of them) are only set for the fast clock, together with a core voltage of 1.8V, instead of 1.55V at zero wait states when running at 20 MHz speed or slower. The frequencies mentioned here are accurate for a crystal equipped device only.

To manage power, the 34S does not step down the CPU speed but steps it up in a controlled manner. When entering digits 2 MHz is enough (unless some of the recent UI enhancements are activated which override this setting to get better responsiveness) . When a calculation starts, the speed is increased to 20 MHz after a short threshold (below one second). If the calculation takes longer, the speed is set to the maximum of 37 MHz. If the supply voltage is 2.5V or less, the maximum frequency is always limited to 20 MHz.

A device as complex as the 34S with a software that is mostly optimized for space, not speed, does not run very well at just 2 MHz. We might add a 10 MHz option but not less. If you want to play with the speed options look for set_speed in main.c and throughout the code.