Using the microsecond clock

[matthewd139]

I want to either use a delay or preferably an interval timing, similar to the Arduino blink without delay tutorial, on a RPi board.

What function would I need to to insert into my loop to have an interval of 200 microseconds?

[jbeale]

Normal Linux scheduling latency and jitter will cause a nominal 200 microsecond delay to be uncertain by some number of milliseconds, unless you take special measures such as turning off all interrupts (which in turn prevents many other things like USB and network from working).

A 200 microsecond delay is an example of "hard real time" that normal Linux systems don't do very well at. Usually a microcontroller (eg. the Arduino you mentioned) is more suited to timing requirements at that level. They are pretty darn cheap.

[joan]

From C use usleep(200).

From Python use time.sleep(0.0002).

You need to bear in mind that these will be minimum delays, occasionally they may be much longer.

[matthewd139]

So I just insert time.sleep(0.0002) at the end of my python loop and that would work?

Code: Select all

#include <SD.h>   //Includes the SD library

//=== Governing test conditions ===//

const char textFile [] = "Data.txt"; //Defines the file name for the data

const byte timeOfTest = 3;  //Sets the length of time that data will be written for in minutes

const byte testDataReadingInterval = 150;   //Sets interval between readings in microseconds

//=== Define variables used with the SD card ===//

const byte bodyGaugePin[] = {A0, A1, A2, A3, A4, A5, A6, A7};       //Defines the analog inputs
const byte legGaugePin[] = {A8, A9, A10, A11, A12, A13, A14, A15};  //Defines the analog inputs

int SdCsPin = 53;  //SD card chip select pin

//=== Initial setup ===//

void setup()
{
  Serial.begin(115200);   //Initialises Serial bus connection with 115200 a Buad rate
  
  //This tells Arduino that all input signals can move over a range of 1.1 V; this will be written
  analogReference(INTERNAL1V1); //in 10-bit resolution (2^10 = 1024; [1.1 / 1024 = 1 mV per bit])

  Serial.println("Initialising Card");

  //Chip Select Pin is an output
  pinMode(SdCsPin, OUTPUT);

  if (!SD.begin(SdCsPin)) // This states that if the SD card function has not started, then print text
  {
    Serial.println("Card Failure");
    Serial.println("Check SD card is inserted");
    while (!Serial.available()) {}
  }
  Serial.println("Card Ready");

  Serial.println();
  Serial.println("Begin writing test data");
  Serial.print("Data will be logged for ");
  Serial.print(timeOfTest);
  Serial.println(" minutes");
}

//=== Writes test data to SD card ===//

void WriteTestData() {  
  // Converts the digital bit reading back into mV for body gauges
  int BG_V1 = ((4.96 / 1024) * (analogRead(bodyGaugePin[0]) + 1.0)) * 1000;
  int BG_V8 = ... // This was previously used, but the ADC chips will write to the digital pins now

  // Converts the digital bit reading back into mV for leg gauges
  int LG_V1 = ((4.96 / 1024) * (analogRead(legGaugePin[0]) + 1.0)) * 1000;
  int LG_V8 = ...

  File testData = SD.open(textFile, FILE_WRITE); // Opens the text file to write data
  if (testData) {
    testData.print(abs(BG_V1));   // This determines which inputs will be logged
    testData.print("     ");      //
    testData.println(abs(BG_V8));
    testData.close();
  }
  else {
    Serial.println("Cannot write data to SD card"); //If data is unable to write, an error will appear
    while (!Serial.available()) {}
  }
}

//=== Main program loop ===//

void loop() {
  unsigned long time = micros();

  if (time <= timeOfTest * 60000000) { // Sets how long test will run for in microseconds, 60000000 = 60 seconds
    WriteTestData();
  }
  else {
    Serial.println();
    Serial.println("End of data collection");
    Serial.println("Press upload to collect more data");
    while (1) {}
  }

  delayMicroseconds(testDataReadingInterval);       // time between readings
}

I programmed this for an Arduino, its OK if the timings not exact, just as long as I can log data with enough resolution