STM32F072 temp sensor counts wrong way !!

[dave239955]

Posted on December 18, 2014 at 21:25

Hi I'm just playing around with the discovery board ST32F0

so I wrote a quick and dirty C app in CoIDE, started a ADC conversion etc and read in the on chip temp sensor. Thats all works fine ( or so it seems) even though I,m not calibrating it, nor compensating for the 3v versus 3v3 issue.What I get at rest on the board is decimal 1900-1927But strangely it counts lower when I put my finger on the chip and if I hit the chip with some freezer spray it counts higher  ( like 20xx-21xx)weird ???any ideas thanksdave code below ( parts from a  CoIDE demo)---------------

&sharpinclude

<stdio.h>

&sharpinclude

<stm32f0xx_gpio.h>

&sharpinclude

<stm32f0xx_rcc.h>

&sharpinclude

<stm32f0xx_adc.h>

int

main(

void

)

{

GPIO_InitTypeDef

gpio;

ADC_InitTypeDef

      ADC_InitStructure;

 RCC_AHBPeriphClockCmd(RCC_AHBPeriph_GPIOC,

ENABLE

); 

// Peripheral IO clock

 RCC_APB2PeriphClockCmd(RCC_APB2Periph_ADC1,

ENABLE

);

// ADC Clock

 GPIO_StructInit(&gpio);

 gpio.

GPIO_Pin

= GPIO_Pin_9;

// Green LED

 gpio.

GPIO_Mode

=

GPIO_Mode_OUT

;

 gpio.

GPIO_OType

=

GPIO_OType_PP

;

 gpio.

GPIO_PuPd

=

GPIO_PuPd_NOPULL

;

 gpio.

GPIO_Speed

= GPIO_Speed_50MHz;

 GPIO_Init(GPIOC, &gpio);

 gpio.

GPIO_Pin

= GPIO_Pin_7;

// Blue LED

 GPIO_Init(GPIOC, &gpio);

/* Initialize ADC structure */

 ADC_StructInit(&ADC_InitStructure);

 ADC_InitStructure.

ADC_ContinuousConvMode

=

ENABLE

;

 ADC_InitStructure.

ADC_DataAlign

= ADC_DataAlign_Right;

 ADC_InitStructure.

ADC_Resolution

= ADC_Resolution_12b;

 ADC_InitStructure.

ADC_ScanDirection

= ADC_ScanDirection_Upward;

 ADC_InitStructure.

ADC_ExternalTrigConvEdge

= ADC_ExternalTrigConvEdge_None;

 ADC_Init(ADC1, &ADC_InitStructure);

 ADC_ChannelConfig(ADC1,ADC_Channel_16,ADC_SampleTime_239_5Cycles);

 ADC_TempSensorCmd(

ENABLE

);

 ADC_Cmd(ADC1,

ENABLE

);

 ADC_StartOfConversion(ADC1);

printf

(

''Hello

Dave

!\r\n''

);

while

(1)

 {

static int

i;

static

int

led_state=0;

  uint16_t chip_temp;

for

(i=0; i<100000; ++i);

  GPIO_WriteBit(GPIOC, GPIO_Pin_9, led_state ?

Bit_SET

:

Bit_RESET

);

  led_state = !led_state;

  GPIO_WriteBit(GPIOC, GPIO_Pin_7, led_state ?

Bit_SET

:

Bit_RESET

);

while

(ADC_GetFlagStatus(ADC1, ADC_FLAG_EOC) ==

RESET

);

// wait for conversion

  chip_temp = ADC_GetConversionValue(ADC1);

printf

(

''Chip

Temp

%d\r\n''

, chip_temp);

 }

}

#adc #discovery #stm32f0 #temp

[Uwe Bonnes]

Posted on December 18, 2014 at 22:20

The on-chip sensor delivers a temperature proportional voltage. The on-chip parameter contain measured voltage at 30 and 110 degree and VDDA 3.3 V. With these values you have to calculate the temperature from the measured voltage.

[Tesla DeLorean]

Posted on December 18, 2014 at 22:41

''A thermistor is a passive device that changes its resistance with temperature. If the temperature-resistance characteristic is known, it can be used as a temperature sensor by measuring the resistance, or more precisely, the voltage across it. Thermistors are classified in to two types: NTC (negative temperature coefficient) and PTC (positive temperature coefficient). A NTC thermistor decreases its resistance while the temperature rises, and a PTC does the opposite.

http://embedded-lab.com/blog/?p=1019

Still weird? Aren't the set points stored in System Memory, so as to define the actual slope?

[raptorhal2]

Posted on December 18, 2014 at 23:17

See Section 12.9 in the reference manual. The formula is:

Temperature (in °C) = {(V25 – VSENSE) / Avg_Slope} + 25 As VSENSE decreases, calculated Temperature increases. The confusing part is that the formula assumes an unsigned slope. Cheers, Hal