RM0444 Rev 5 347/1390
RM0444 Analog-to-digital converter (ADC)
403
If the main voltage regulator enters low-power mode (such as Low-power run mode), this
buffer is disabled and the ADC cannot be used.
ADC Voltage regulator enable sequence
To enable the ADC voltage regulator, set ADVREGEN bit to 1 in ADC_CR register.
ADC voltage regulator disable sequence
To disable the ADC voltage regulator, follow the sequence below:
1. Make sure that the ADC is disabled (ADEN = 0).
2. Clear ADVREGEN bit in ADC_CR register.
15.3.3 Calibration (ADCAL)
The ADC has a calibration feature. During the procedure, the ADC calculates a calibration
factor which is internally applied to the ADC until the next ADC power-off. The application
must not use the ADC during calibration and must wait until it is complete.
Calibration should be performed before starting A/D conversion. It removes the offset error
which may vary from chip to chip due to process variation.
The calibration is initiated by software by setting bit ADCAL = 1. Calibration can only be
initiated when the ADC voltage regulator is enabled (ADVREGEN = 1 and t
ADCVREG_SETUP
has elapsed) and the ADC is disabled (when ADEN = 0). ADCAL bit stays at 1 during all the
calibration sequence. It is then cleared by hardware as soon the calibration completes. After
this, the calibration factor can be read from the ADC_DR register (from bits 6 to 0).
The internal analog calibration is kept if the ADC is disabled (ADEN = 0). When the ADC
operating conditions change (V
DDA
changes are the main contributor to ADC offset
variations and temperature change to a lesser extend), it is recommended to re-run a
calibration cycle.
The calibration factor is lost in the following cases:
• The power supply is removed from the ADC (for example when the product enters
STANDBY or VBAT mode)
• The ADC peripheral is reset.
The calibration factor is lost each time power is removed from the ADC (for example when
the product enters Standby or V
BAT
mode). Still, it is possible to save and restore the
calibration factor by software to save time when re-starting the ADC (as long as temperature
and voltage are stable during the ADC power-down).
The calibration factor can be written if the ADC is enabled but not converting (ADEN = 1 and
ADSTART = 0). Then, at the next start of conversion, the calibration factor is automatically
injected into the analog ADC. This loading is transparent and does not add any cycle
latency to the start of the conversion.
Software calibration procedure
1. Ensure that ADEN = 0, ADVREGEN = 1 and DMAEN = 0.
2. Set ADCAL = 1.
3. Wait until ADCAL = 0 (or until EOCAL = 1). This can be handled by interrupt if the
interrupt is enabled by setting the EOCALIE bit in the ADC_IER register
4. The calibration factor can be read from bits 6:0 of ADC_DR or ADC_CALFACT
registers.
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