Joystick reading erroneous values

Question

Under normal operation, the value of a joystick's ADC range on my board is from 0 to 1023 (10-bits). However, my graph for either axis is not continuous. Rather, there is more than one min and maximum for both axis. I am running this thread on a MSP432P401R however the code isn't any different than an Arduino.

int jumpFlag;
int backwardsFlag;
int forwardsFlag;
int selPin = P5_1; //Select digital pin location
int xOutPin = A14;
int yOutPin = A13;
void setupPlayerActions() {
 pinMode(selPin, INPUT_PULLUP);
 attachInterrupt(digitalPinToInterrupt(selPin),jumpISR,RISING);
 //pinMode(xOutPin, ); //Analog X pin setup
 //pinMode(xOutPin, ); //Analog Y pin setup
}
void jumpISR() {
 Serial.println("Jump Detected!");
 jumpFlag = 1;
}
void loopPlayerActions() {
 int x_adc_val, y_adc_val; 
 int leftThreshold=50,rightThreshold=900;
 float x_volt, y_volt;
 x_adc_val = analogRead(xOutPin);
 y_adc_val = analogRead(yOutPin);
 x_volt = ( ( x_adc_val )); /*Convert digital value to voltage */
 y_volt = ( ( y_adc_val)); /*Convert digital value to voltage */
 if (x_volt < leftThreshold){
 Serial.println("Move Backwards");
 }
 else if (x_volt < rightThreshold){
 Serial.println("Move Forwards");
 }
 Serial.println("X_Voltage = ");
 Serial.print(x_volt);
 Serial.print("\t");
 //Serial.print("Y_Voltage = ");
 //Serial.println(y_volt);
 delay(100);
 //delay(250);
}

The graph goes to zero for all positive x and y integers however it briefly reaches a maximum past a small value on the -x and -y axis then sinks to about ~420. What could be causing this?

• Edgar Bonet: Re "if (x_volt < rightThreshold)": do you mean if (x_volt > rightThreshold)?
• thediyer: @EdgarBonet will not change the fact the graph of the potentiometer is not linear
• Sim Son: Can you show the graph?
• Juraj: it could be a problem of analogRead implementation or the ADC itself. for example for AVR it is recommended to wait a little or do a dummy reading between consequent readings on the same ADC.

Answer 1

A 10 bit ADC may not have 10 bits of resolution for a given sample. It might only have 5 or 6 bits due to random noise and other factors. Consult the chip manufactures specifications / recommendations if you need a specific number.

What can be done?

• If response time is not an issue, consider averaging the ADC reading. A common coding practice is to use an exponential moving average.
• Consider a better ADC. Many times a dedicated ADC chip (not integrated into a processor) will have better specifications.

For more in depth reading this NXP (aka: Motorola, Freescale) application note discusses "How to Increase the Analog-to-Digital Converter Accuracy in an Application".