Enum sam4l::bpm::PowerModeControl::PSCM::Value [−] 

pub enum Value {
    WithCpuHalt,
    WithoutCpuHalt,
}

WARN: Undocumented!

According to the datasheet (sec 6.2, p57) changing power scaling requires waiting for an interrupt (presumably because flash is inaccessible during the transition). However, the ASF code sets bit 3 ('PSCM' bit) of the PMCON register, which is blank (not a '-') in the datasheet with supporting comments that this allows a change 'without CPU halt'

Variants