38.5.2 ADC Clock

The ADC uses the ADC clock (ADCCLK) to perform conversions. The ADC clock frequency is selected in ADC_MR.PRESCAL.

To generate the ADC clock, the prescaler has two clock sources: peripheral clock and GCLK. The clock source is selected using ADC_EMR.SRCCLK.

Regardless of the prescaler clock source, peripheral clock must always be running in the PMC.

If GCLK is selected as a clock source, the ADC clock frequency is independent of the processor/bus clock. At reset, peripheral clock is selected.

If ADC_EMR.SRCCLK is cleared, the prescaler clock (presc_clk) is driven by the peripheral clock. If ADC_EMR.SRCCLK is set, the prescaler clock is driven by GCLK. The ADC clock frequency is between fpresc_clk/2, if PRESCAL is 0, and fpresc_clk/512, if PRESCAL is set to 255 (0xFF).

ADC_MR.PRESCAL must be programmed to provide the ADC clock frequency parameter given in the section “Electrical Characteristics”.