STM32F103RET6 ADC reads voltage with no voltage at pin

[charlie2]

Posted on April 06, 2014 at 20:01

If i read any of the ADC's i see a voltage of about 1736mv it changes a little depending on the sampling speed, i've used non continuous reads, and continuous DMA , both mostly the same, reads the same in circuit or out. if i put 2v6 on it, it'll read it exactly correct, same at 0v

i've used stm32lib, in different ways but always the same. i have all the vss gnd, vdd 3v3, vdda via a ferrite to 3v3 with a 10uF and 100nF cap to vssa. 72mhz speed with external 8mhz xtal. am i just completely misunderstanding something? input voltages are stable schematic http://i.imgur.com/AfaWwpng its a larger project, i cut out the adc part, i can make a simpler full example if nothing obvious or misunderstand on my part.

ADC_RegularChannelConfig ( ADC1, ADC_Channel_0, 1, ADC_SampleTime_55Cycles5 );
ADC_SoftwareStartConvCmd ( ADC1 ,ENABLE ); //Start the conversion
while ( ADC_GetFlagStatus ( ADC1, ADC_FLAG_EOC ) == RESET ); //Processing the conversion
return ADC_GetConversionValue ( ADC1 ) /4; //Return the converted data

adc setup

void ADC_Configuration ( void )
{
ADC_InitTypeDef ADC_InitStructure;
#if DMA_ADC == 1
DMA_InitTypeDef DMA_InitStructure;
memset ( &DMA_InitStructure,0,sizeof ( DMA_InitStructure ) );
memset ( &ADC_InitStructure,0,sizeof ( ADC_InitStructure ) );
/* Enable DMA1 clock */
RCC_AHBPeriphClockCmd ( RCC_AHBPeriph_DMA1, ENABLE );
/* Enable ADC1 clock */
RCC_APB2PeriphClockCmd ( RCC_APB2Periph_ADC1, ENABLE );
/* DMA1 channel1 configuration ---------------------------------------------*/
DMA_DeInit ( DMA1_Channel1 );
DMA_InitStructure.DMA_PeripheralBaseAddr = ADC1_DR_Address;
DMA_InitStructure.DMA_MemoryBaseAddr = ( u32 ) &ADC_Ampl[0];
DMA_InitStructure.DMA_DIR = DMA_DIR_PeripheralSRC;
DMA_InitStructure.DMA_BufferSize = 2;
DMA_InitStructure.DMA_PeripheralInc = DMA_PeripheralInc_Disable;
DMA_InitStructure.DMA_MemoryInc = DMA_MemoryInc_Enable;
DMA_InitStructure.DMA_PeripheralDataSize = DMA_PeripheralDataSize_HalfWord;
DMA_InitStructure.DMA_MemoryDataSize = DMA_MemoryDataSize_HalfWord;
DMA_InitStructure.DMA_Mode = DMA_Mode_Circular;
DMA_InitStructure.DMA_Priority = DMA_Priority_Low;
DMA_InitStructure.DMA_M2M = DMA_M2M_Disable;
DMA_Init ( DMA1_Channel1, &DMA_InitStructure );
/* Enable DMA1 channel1 */
DMA_Cmd ( DMA1_Channel1, ENABLE );
/* Disable the DMA1 Channel1 Transfer complete interrupt */
DMA_ITConfig ( DMA1_Channel1, DMA_IT_TC, DISABLE );
#else
/* Enable ADC1 clock */
RCC_APB2PeriphClockCmd ( RCC_APB2Periph_ADC1, ENABLE );
#endif
/* ADC1 configuration ------------------------------------------------------*/
ADC_InitStructure.ADC_Mode = ADC_Mode_Independent;
#if DMA_ADC == 1
ADC_InitStructure.ADC_ScanConvMode = ENABLE;
ADC_InitStructure.ADC_ContinuousConvMode = ENABLE;
#else
ADC_InitStructure.ADC_ScanConvMode = DISABLE;
ADC_InitStructure.ADC_ContinuousConvMode = DISABLE;
#endif
ADC_InitStructure.ADC_ExternalTrigConv = ADC_ExternalTrigConv_None;
ADC_InitStructure.ADC_DataAlign = ADC_DataAlign_Right;
ADC_InitStructure.ADC_NbrOfChannel = 2;
ADC_Init ( ADC1, &ADC_InitStructure );
/* ADC0 regular channel configuration */
ADC_RegularChannelConfig ( ADC1, ADC_Channel_0, 1, ADC_SampleTime_239Cycles5 );
/* ADC1 regular channel configuration */
ADC_RegularChannelConfig ( ADC1, ADC_Channel_1, 2, ADC_SampleTime_239Cycles5 );
#if DMA_ADC == 1
/* Enable ADC1 DMA */
ADC_DMACmd ( ADC1, ENABLE );
#endif
/* Enable ADC1 */
ADC_Cmd ( ADC1, ENABLE );
/* Enable ADC1 reset calibaration register */
ADC_ResetCalibration ( ADC1 );
/* Check the end of ADC1 reset calibration register */
while ( ADC_GetResetCalibrationStatus ( ADC1 ) );
/* Start ADC1 calibaration */
ADC_StartCalibration ( ADC1 );
/* Check the end of ADC1 calibration */
while ( ADC_GetCalibrationStatus ( ADC1 ) );
}

memset ( &GPIO_InitStructure,0,sizeof ( GPIO_InitStructure ) );
GPIO_InitStructure.GPIO_Pin = GPIO_Pin_0 | GPIO_Pin_1 ;
GPIO_InitStructure.GPIO_Mode = GPIO_Mode_AIN;
GPIO_InitStructure.GPIO_Speed = GPIO_Speed_50MHz;
GPIO_Init ( GPIOA, &GPIO_InitStructure );

thanks,

[waclawek.jan]

Posted on April 06, 2014 at 20:29

For proper working, the ADC needs to be fed from a voltage source with some finite output impedance (look at the respective pages in datasheet) so that it can charge the sampling capacitor in the ADC in the given sampling time. If you leave an input floating, the capacitor holds the voltage to a certain extent (''copies'' voltage of the last non-floating pin switched through the input mux), subject to parasitic charging/discharging to adjacent circuits.

In other words, yes, this is normal and expected. JW

[charlie2]

Posted on April 06, 2014 at 21:38

thanks! that is what i suspected, i guess i'll have to do some redesign on the RF stage, since it showing the same in circuit too.