AN3023

Configuring ADC registers for conversion

Doc ID 16113 Rev 1

9/23

3

Configuring ADC registers for conversion

3.1

ADC setup

Before starting an ADC conversion, ADC setup is done using the following steps:

●

ADC clock selection (define conversion speed)

●

Channel selection

●

Power on the ADC (set ADON bit)

●

●

Conversion mode selection (single or continuous)

●

Trigger source selection and only after trigger enable

●

Apply trigger source or set ADON bit

3.1.1

ADC clock

The prescaler selection must ensure a maximum 4 MHz ADC frequency.

ADC conversion time depends on the ADC clock frequency, the sampling time duration and

the ADC resolution. The minimum ADC conversion time is 3.5 µs @ 4 MHz ADC clock

frequency.

Note:

It is recommended to change bits SPSEL[2:0] when the ADC is in power-down mode. This

is because, internally, there may be a glitch in the clock during this change. If this change is

made when ADC is not in power-down, the user is required to ignore the first converted

result.

3.1.2

ADC on/off control (power on)

The ADC can be powered on by setting the ADON bit in the ADC_CR1 register. When the

ADON bit is set for the first time, it wakes up the ADC from power-down mode.

After power ON, wait for stabilization time and then, to start a conversion, either set the

ADON bit again in the ADC_CR1 register or use an external trigger source (timers or

ADC_ETR pin).

At the end of conversion, the ADC remains powered on and the ADON bit has to be set only

once to start the next conversion. If the ADC is not used for a long time, it is recommended

to switch it off to decrease power consumption. This is done by clearing the ADON bit.

When the ADC is powered on, the output stage of the selected channel is disabled;

therefore it is recommended to select the channel first before powering on the ADC.

Note:

If any bit of the ADC_CR1 register other than ADON is changed at the same time, the

conversion is not triggered. This is to prevent triggering an erroneous conversion.

Moreover, be aware that any bit manipulation instruction on the ADC_CR1 register can start

a new conversion due to the fact that the ADON bit is read as ‘1’ and then written back as ‘1’

by this operation.