Change Digispark clock speed, run at low voltage?
You can set the clock prescaler at run time. For example:



#include &lt;avr/power.h&gt;

void setup()
{
    clock_prescale_set(clock_div_8);
    // etc...
}


More details in the documentation from
avr-libc.

Note that with this method your Digispark will still be overclocked when
booting, so it might not be 100% reliable. But it will be overclocked
only for a very short time.