NRF52811-QCAA-R datasheet(262/444 Pages) NORDIC

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Peripherals

6.17.5.2 Continuous mode

Continuous sampling can be achieved by using the internal timer in the ADC, or triggering the SAMPLE task

from one of the general purpose timers through the PPI.

Care shall be taken to ensure that the sample rate fulfils the following criteria, depending on how many

channels are active:

fSAMPLE< 1/[tACQ + tconv]

The SAMPLERATE register can be used as a local timer instead of triggering individual SAMPLE tasks. When

SAMPLERATE.MODE is set to Timers, it is sufficient to trigger SAMPLE task only once in order to start the

SAADC and triggering the STOP task will stop sampling. The SAMPLERATE.CC field controls the sample rate.

The SAMPLERATE timer mode cannot be combined with SCAN mode, and only one channel can be

enabled in this mode.

A DONE event signals that one sample has been taken.

In this mode, the RESULTDONE event has the same meaning as DONE when no oversampling takes place.

Note that both events may occur before the actual value has been transferred into RAM by EasyDMA.

6.17.5.3 Oversampling

An accumulator in the ADC can be used to average noise on the analog input. In general, oversampling

improves the signal-to-noise ratio (SNR). Oversampling, however, does not improve the integral non-

linearity (INL), or differential non-linearity (DNL).

Oversampling and scan should not be combined, since oversampling and scan will average over input

channels.

The accumulator is controlled in the OVERSAMPLE register. The SAMPLE task must be set 2OVERSAMPLE

number of times before the result is written to RAM. This can be achieved by:

• Configuring a fixed sampling rate using the local timer or a general purpose timer and PPI to trigger a

SAMPLE task

• Triggering SAMPLE 2OVERSAMPLE times from software

• Enabling BURST mode

CH[n].CONFIG.BURST can be enabled to avoid setting SAMPLE task 2OVERSAMPLE times. With BURST = 1 the

ADC will sample the input 2OVERSAMPLE times as fast as it can (actual timing: <(tACQ+tCONV)×2OVERSAMPLE).

Thus, for the user it will just appear like the conversion took a bit longer time, but other than that, it is

similar to one-shot mode. Scan mode can be combined with BURST=1, if burst is enabled on all channels.

A DONE event signals that one sample has been taken.

In this mode, the RESULTDONE event signals that enough conversions have taken place for an

oversampled result to get transferred into RAM. Note that both events may occur before the actual value

has been transferred into RAM by EasyDMA.

6.17.5.4 Scan mode

A channel is considered enabled if CH[n].PSELP is set. If more than one channel, CH[n], is enabled, the ADC

enters scan mode.

In scan mode, one SAMPLE task will trigger one conversion per enabled channel. The time it takes to

sample all channels is:

Total time < Sum(CH[x].tACQ+tCONV), x=0..enabled channels

A DONE event signals that one sample has been taken.

4454_140 v1.0

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