Is there any resistance with analog-in for Arduino OSEP FIO?

Development Boards Arduino
1

Hello,

I’m having trouble reading in an analog signal using a fio and I’m wonder if there is any internal resistance that may impact the reading? Basically you have 12v across two resistors in sereis with 2M on the 12v end end and 1M on the other. Does anyone know why it may not pick up anything above 1v?

2

Is it a stable 12v? Or is that the max? It may also be the case where the current is too low for the ADC to register. Check with a multimeter to make sure the voltage doesn’t drop when you take a reading on the Fio. Even if there is internal resistance, it would not affect the reading.

3

Just hooked up a multimeter and I got 3.5 which isn’t a solid 4v, but it’s not the 0.5V I was getting from Fio. On that note, does Fio require I hook up AREF? Doesn’t it have a defaul 5v reference?

4

Could I have some basic code?

5 
#define SEND_EN 4
#define SEND_EN_CONF 13
#define SIO_RX 0
#define SIO_TX 1
#define TERM '~'

void sendSIO(char *data)
{
   Serial.print(data);
}

void disableSIO()
{
   Serial.end();
   pinMode(SIO_RX, INPUT);
   pinMode(SIO_TX, INPUT);
}

void enableSIO()
{
   Serial.begin(9600);
}

void fillString(char *data)
{
   int i;
   int temp[7];
   int heatV = ((analogRead(0) * 500.0)/1023.0);  // <--------- this is where I check for 12v
   float c;
   int val;
   
   for(i = 0; i < 7; i++)
   {
      val = analogRead(i+1);
     
      //23c = 681
      c = (((val / 1023.0) * 5.0) * 100.0) - 273.0;
      
      c -= 159.0; //calibration
      
      c *= (9.0/5.0);
      c += 32;
      
      //printed as int, so x10 for single decimal val
      c *= 10;
      temp[i] = c;
   }
   
   sprintf(data, "%4d %4d %4d %4d %4d %4d %4d %4d%c%c",
           temp[0], temp[1], temp[2], temp[3], temp[4], 
           temp[5], temp[6], heatV, TERM, '\0');
}

void setup()
{
   pinMode(SEND_EN, INPUT);
   pinMode(SEND_EN_CONF, OUTPUT);
   disableSIO();
}

void loop()
{
   char data[256];
   
   while(1)
   {
      if(digitalRead(SEND_EN))
      {
         digitalWrite(SEND_EN_CONF, 1);
         delay(5);
         if(digitalRead(SEND_EN))
         {
            fillString(data);
            enableSIO();
            delay(5);
            
            if(digitalRead(SEND_EN))
            {
               sendSIO(data);
            }
            
            while(digitalRead(SEND_EN)) {}
            delay(8);
            disableSIO();
         }
      }
      digitalWrite(SEND_EN_CONF, 0);
   }
}
6

Try a sketch that just prints out the raw analog value to a serial port for just the 12V. I need the hex or decimal(or binary) that it prints out. I’m trying to figure out whether the Fio is reading a wrong value or your code is manipulating the value in which you do not intend.

7

There there is an impedance recommendation for analog inputs.

Your selected resistor values cause the impedance to be is very high and therefore the current to be quite low… on the order of about 4uA. This is not really very close to what ATMEL recommends.

FROM ATMEL DATASHEET

The ADC is optimized for analog signals with an output impedance of approximately 10 kΩ or

less. If such a source is used, the sampling time will be negligible. If a source with higher impedance

is used, the sampling time will depend on how long time the source needs to charge the

S/H capacitor, with can vary widely. The user is recommended to only use low impedance

sources with slowly varying signals, since this minimizes the required charge transfer to the S/H

capacitor.

I suggest rethinking your voltage divider values and see if you can change your results.