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26.3.4 Warm-up Time
After power-on, the ADC requires some time for internal bias currents and references to settle prior to starting a conversion. This time
period is called the warm-up time. Warm-up timing is performed by hardware. Software must program the number of ADC_CLK cycles
required to count at least 1 µs in the TIMEBASE field of the ADCn_CTRL register. TIMEBASE only affects the timing of the warm-up
sequence and is not dependent on adc_clk_sar. When enabling the ADC or changing references between samples, the ADC is auto-
matically warmed up for 5 µs (5 times the period indicated by TIMEBASE).
Normally, the ADC will be warmed up only when samples are requested and is shut off when there are no more samples waiting. How-
ever, if lower latency is needed, configuring the WARMUPMODE field in ADCn_CTRL allows the ADC and/or reference to stay warm
between samples, reducing the warm-up time or eliminating it altogether.
Figure 26.4 ADC Analog Power Consumption With Different
WARMUPMODE Settings on page 851
shows the effects on analog power consumption in scenarios using different WARMUPMODE
settings.
The user can program which reference should be kept warm in the CHCONREFWARMIDLE bitfield in the ADCn_CTRL register. By
default the scan mode reference is kept warm. The user can also choose to keep the single channel mode reference warm or to keep
the last used reference warm. If the default setting is kept (scan mode reference is to be kept warm) and if the single-mode reference
setting is different than scan-mode, then single mode conversions will first warmup its reference for 5 µs before a conversion can begin.
Various warmup modes are described here:
• NORMAL: This is the lowest power option for general-purpose use and low sampling rates (below 35 ksps). The ADC and referen-
ces are shut off when there are no samples waiting. The ADC does not consume any power when it is shut down. A 5 µs warmup
time will be initiated prior to every conversion. Figure a in
Figure 26.4 ADC Analog Power Consumption With Different WARMUP-
shows this mode.
• KEEPINSTANDBY: This mode is suitable for infrequent sampling of lower impedance inputs, and is the lowest power option for sam-
pling rates between about 35 and 125 ksps. It may also be useful for lower sampling rates where latency is important. The reference
selected for scan mode is kept warm, but the ADC is powered down. The ADC will initiate a 1 µs warmup period before a conversion
begins. Because the reference is kept warm, the ADC will consume a small amount of standby current when it is not converting.
Figure b in
Figure 26.4 ADC Analog Power Consumption With Different WARMUPMODE Settings on page 851
shows this mode.
• KEEPINSLOWACC: This mode is useful for high-impedance inputs which are sampled infrequently. It is similar to KEEPINSTAND-
BY, but continuously tracks the input, keeping the input multiplexer connected to the APORT bus. This mode consumes little more
power than KEEPINSTANDBY mode (about 2 µA extra) when a conversion is not in progress. This allows the user to avoid pro-
gramming long acquisition time that would otherwise be necessary for high-impedance inputs when ADC wakes up to full power
mode, thereby reducing the total current consumption per conversion.
• KEEPADCWARM: This mode provides the lowest latency and allows for maximum sampling rates. The ADC and reference circuitry
remain powered on even when conversions are not in progress. Figure c in
Figure 26.4 ADC Analog Power Consumption With Dif-
ferent WARMUPMODE Settings on page 851
shows this mode. This mode consumes the most power, but as soon as a trigger
event occurs, the acquisition and conversion begin with no warm-up time. Note that if KEEPADCWARM mode is set and HFXO is
selected as the ADC clock source, the HFXO will remain on in EM2.
When KEEPADCWARM is chosen, ADC is termed as being in continuous operation. When any other warmup mode is chosen, ADC is
termed to be in duty-cycled operation.
When entering EM2 Deep Sleep or EM3 Stop, if the ADC is not going to be used, it should be returned to an idle state and WARMUP-
MODE in ADCn_CTRL written to 0. Refer to
for more information on placing the ADC in an idle state.
If the ADC is going to be used in these low energy modes, the user can use any of the WARMUPMODE settings, but should be mindful
of the power consumption that comes along with the different mode settings. For EM2 Deep Sleep or EM3 Stop operation, the ADC
clock source must be configured to use AUXHFRCO.
Reference Manual
ADC - Analog to Digital Converter
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