Thursday, 18 April 2013

Digispark / RPi / HC-SR04 measurement - 2.

OK - now pretty much happy with the digispark / HC-SR04 interface.  Using a 'newping' library from here to control the interface which makes the digispark code simpler. - The code writes to the USB every time we get an input on the USB. - I will then use the Pi to send a 'request' to the digispark. - The digispark does the measurement (median / average of 20 pings), and returns the distance in cm.

Note that I had to comment out the 'timer' functions in the newping library to get it to compile for the digispark. - Think the Arduino has some functionallity which isn't available in the digispark.

HC-SR04 trigger connected to digispark pin 0.
HC-SR04 echo connected to digispark pin 2.

(Mine is a rev A digispark which means that the onboard LED is connected on Pin1, and I use this to flash when data is transmitted.)

Code for the digispark sketch.

#include <DigiUSB.h>
#include <NewPing.h>

// AJR - 17/4/13.  USing HC-SR04 to measure distance
// and output on the USB so it can be read on an RPi.
// Use with the receive python progs
// from the Digispark examples.
// DigisparkExamplePrograms-master.
// Utilising the NewPing library from
// Note that this will only compile if several of the timer functions are removed
// from the library.  I think there are some bits missing on the digispark which
// would otherwise be present in arduino.  One thing it is missing is memory!
// I keep running out of memory in the code space, and if I try to do too much here,
// then also run out. - Would like to have returned version string etc, but not
// enough room.... Ho, hum.

#define trigPin 0
#define echoPin 2
#define maxDist 300
// Pretty self explanatory below, but maxDist is the distance beyond
// which a ping is considered invalid.
NewPing sonar(trigPin, echoPin, maxDist);

void setup() {
  pinMode(1,OUTPUT); //give us some idea it's workng by flashing led
  pinMode(trigPin, OUTPUT);
  digitalWrite(trigPin, LOW);
  pinMode(echoPin, INPUT);

void measure() {
  int lastRead;
    // - This is where we call the NewPing library.  Don't think digispark has enough memory to do 50
    // but it seems reliable with 20.
    unsigned int uS = sonar.ping_median(20);
    DigiUSB.println(uS / US_ROUNDTRIP_CM);
    delay(50); //sleep for 0.5 seconds before sending again.

void get_input() {
  int lastRead;
  // when there are no characters to read, or the character isn't a newline
  while (true) { // loop forever
    if (DigiUSB.available()) {
      // something to read
      lastRead =;
          if (lastRead == '\n') {
              break; // when we get a newline, break out of loop
    // refresh the usb port for 10 milliseconds

void loop() {
  // print output
  // DigiUSB.println("Waiting for input...");
  digitalWrite(1, LOW);
  // Measure....

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