Frequency sensitivity is a specification that often times is overlooked. This specification provides an indication of how efficient the MCU will be when it is active and drawing the most current.  Not all MCU manufacturers follow the same curve for frequency sensitivity so it is important to check the data sheets in order to make valid comparisons. For example, the frequency sensitivity of the C8051F99x is less than 150 μA/MHz at 24.5 MHz. Comparison reveals that some competing devices cannot operate at that frequency; if they can, the frequency sensitivity goes up which consumes more current. Competing devices may claim low frequency sensitivity numbers, however, that efficiency comes from a much lower frequency. The lower frequency increases the ON time which requires all of the other system components to be active as well. This is one of the aspects of low power design that is not always obvious when choosing an MCU, but is a very important consideration.