Posted on January 11, 2014 at 17:28Hello
I am trying to implement indirect voltage measurement in a battery operated application.
Our MCU supports VREF+ and VREF+ is derived from the battery voltage.
Therefore - VREFINT must be used to extract VREF+.
The trouble is I keep getting a wrong digital value when measuring VREFINT via ADC (I get a digitized value of around 600 for the first measurement and from the second measure and forth I receive a value around 1400).
Thus the VREFINT I get is ~1.12V (when neglecting the first measurement).
VREF+ is stable at 3.3V during the measurements.
The ADC is configured to a sampling time of 384 cycles
The code for indirect VREF+ measurement is based on AN3964 (see below).
Please be so kind to share your thoughts as to why the ADC measurement deviates.
Thank you very much
/Gil
void Indirect_Input_BatterySampling( void )
{
static U16 val_vrefint; //for vref int (1.2 volt via channel17
static U16 val_bat_mon; // bat volt in units via adc
static U32 v_bat; // bat in volt
static U32 vref_plus;
// For Indirect VREF+ calculation
while((PWR->CSR & PWR_CSR_VREFINTRDYF) == RESET)
{
}
ADC_TempSensorVrefintCmd(ENABLE); // ENABLE VREFINT
val_vrefint = Adc_GetAdcValue ( ADC_Channel_17 ); // Measured value of VREFINT via ADC.
val_bat_mon = Adc_GetAdcValue (ADC_BATTERY );
vref_plus = (vrefcal*3*10000) / val_vrefint;
v_bat = ((val_bat_mon * vref_plus) / 4096) * 2;
}
#stm32l #vrefint
