28.6.3 Prescaler

The ADC is clocked by GCLK_ADC. There is also a prescaler in the ADC to enable conversion at lower clock rates.

Refer to CTRLB for details on prescaler settings.

Figure 28-2. ADC Prescaler
The propagation delay of an ADC measurement depends on the selected mode and is given by:
  • Single-shot mode:
    PropagationDelay = 1 + Resolution 2 + DelayGain f CLK + − ADC
  • Free-running mode:
    PropagationDelay = Resolution 2 + DelayGain f CLK + − ADC
    Table 28-1. Delay Gain
    Delay Gain (in CLK_ADC Period)
    INTPUTCTRL.GAIN[3:0] Free-running mode Single shot mode
    Name Differential Mode Single-Ended Mode Differential mode Single-Ended mode
    1X 0x0 0 0 0 1
    2X 0x1 0 1 0.5 1.5
    4X 0x2 1 1 1 2
    8X 0x3 1 2 1.5 2.5
    16X 0x4 2 2 2 3
    Reserved 0x5 ... 0xE Reserved Reserved Reserved Reserved
    DIV2 0xF 0 1 0.5 1.5