38.5.8 Sleep Mode and Conversion Sequencer

The ADC Sleep mode maximizes power saving by automatically deactivating the ADC when it is not being used for conversions. Sleep mode is selected by setting ADC_MR.SLEEP.

Sleep mode is managed by a conversion sequencer, which automatically processes the conversions of all channels at the lowest power consumption.

This mode can be used when the minimum period of time between two successive trigger events is greater than the start-up period of the ADC. Refer to the section “Electrical Characteristics”.

When a start conversion request occurs, the ADC is automatically activated. As the analog cell requires a start-up time, the logic waits during this time and starts the conversion on the enabled channels. When all conversions are complete, the ADC is deactivated until the next trigger. Events triggered during the sequence are ignored.

The conversion sequencer allows automatic processing with minimum processor intervention and optimized power consumption. Conversion sequences can be performed periodically using the internal timer (ADC_TRGR) or the PWM event line. The periodic acquisition of several samples can be processed automatically without any intervention of the processor via the DMA channel.

The sequence can be customized by programming ADC_SEQR1 and ADC_SEQR2 and setting ADC_MR.USEQ. This sequence is limited to 16 channels, from 0 to 15. The user can choose a specific order of channels and can program up to 16 conversions by sequence. The user is free to create a personal sequence by writing channel numbers in ADC_SEQR1 and ADC_SEQR2. Not only can channel numbers be written in any sequence, channel numbers can be repeated several times. When ADC_MR.USEQ is set, ADC_SEQR1.USCHx and ADC_SEQR2.USCHx are used to define the sequence. Only enabled USCHx fields will be part of the sequence. Each USCHx field has a corresponding enable, CHx-1, in ADC_CHER.

If 16 channels are used on an application board, there is no restriction of usage of the user sequence. However, if some ADC channels are not enabled for conversion but rather used as pure digital inputs, the respective indexes of these channels cannot be used in the user sequence fields (see ADC_SEQR1 and ADC_SEQR2). For example, if channel 4 is disabled (ADC_CHSR[4] = 0), ADC_SEQRx fields USCH1 up to USCH16 must not contain the value 4. Thus the length of the user sequence may be limited by this behavior.

As an example, if only four channels (CH0 up to CH3) are selected for ADC conversions, the user sequence length cannot exceed four channels. Each trigger event may launch up to four successive conversions of any combination of channels 0 up to 3, but no more (i.e., in this case the sequence CH0, CH0, CH1, CH1, CH1 is impossible).

A sequence that repeats the same channel several times requires more enabled channels than channels actually used for conversion. For example, the sequence CH0, CH0, CH1, CH1 requires four enabled channels (four free channels on application boards) whereas only CH0, CH1 are really converted.