Displays the system clock frequency error in parts per million (ppm).
It will always be a positive value. This error adjustment happens when
the system reboots or the NTP is enabled for the first time and it may
take up to fifteen minutes to complete.
Each system clock oscillator interrupts to update its clock. If for
example, it interrupts one million time per second then it has
resolution of microseconds. This is also frequency of the clock update.
Frequency error represents failed to update by parts per millionth of a
second.
For example, if we have frequency error of 1ppm, it means clock is ahead
by 0.864 second in a day and that much has to be corrected in a day for
clock to be accurate. Frequency error varies with temperature. Due to
this system clock has to be corrected when system boots based on
temperature it has to operate.