I recently purchased an nRF8001 Bluefruit LE and I am now trying to read/send data with an app. But I am not sure how I can write/send a double value. I want to send two double values at once and to successfully send this, do I have to convert these doubles to a byte array? If so I would very much appreciate how that would look!

This is documentation on how my module works: https://learn.adafruit.com/getting-started-with-the-nrf8001-bluefruit-le-breakout/nrf-uart-in-detail

Under the headline Writing data they give examples on how to send data but as far as I can see (I am a bit new to Arduino code) in their example they only send an int and a string.

I know how to read and set up the module, so it is only the writing/sending doubles that I am struggling with.

These are the two doubles that I wish to send and I control it with:

double valueOne;
double valueTwo;

The Bluetooth control:


So if I for example do this:

BTLEserial.write(valueOne, valueTwo);

I get this message: no matching function for call to 'Adafruit_BLE_UART::write(double&, double&)'

So do I need to convert these two double values to a byte array and if so how would that look?

Appreciate every help, tips, code! :)

  • After the first successful message received, did you see the location refreshed on the serial monitor by the Serial.println(gps.location.lat(),6); ?
    – J. Piquard
    Commented Jan 3, 2017 at 21:59
  • I see that once yes and also the Serial.println(gps.location.lng(),6); once
    – Martman
    Commented Jan 3, 2017 at 22:01
  • Also the Serial.println(F("* Connected!")); once
    – Martman
    Commented Jan 3, 2017 at 22:01
  • Seeing once Serial.println(F("* Connected!")); is normal, because the message is sent only when the status change. But are you sure that your gps.location.isUpdated() is true ?
    – J. Piquard
    Commented Jan 3, 2017 at 22:07
  • I removed gps.location.isUpdated() in the if-statement but it still stops right when I connect my app to the bluetooth. I also added a Serial.println("test"); right at the top of the while loop and that stops once i connect the app with the bluetooth as well so there must be something else.
    – Martman
    Commented Jan 3, 2017 at 22:10

2 Answers 2


As the link "Adafruit - nRF UART In Detail" describes, the function write(sendbuffer, sendbuffersize); is waiting for a buffer of uint8_t followed by the size of the message.

Here are the step--by-step solution to send the 2 doubles by using that function.

Step1 - convert the floating-point values into a char string.

In the Arduino environment, the standard format-specifier %f or %lf is not well-managed and the function dtostrf() is recommended.

The format is char * dtostrf (double __val, signed char __width, unsigned char __prec, char *__s) where:

  • __val is the floating-point value to be converted,
  • __s is the array of char to store the result,
  • __width is the number of characters to store the result,
  • __prec is the number of digit

The conversion is:

char sValueOne[16], sValueTwo[16];
char sBuffer[33];

strcpy(sBuffer,dtostrf(valueOne, 15, 8, sValueOne));
strcat(sBuffer,dtostrf(valueTwo, 15, 8, sValueTwo));

Step2 - write the buffer over the BTLEserial object.

BTLEserial.write((uint8_t *)sBuffer, strlen(sBuffer));

Alternate output solution using BTLEserial.print() function.

  • so should I use a strcpyon the sValueOne and a strcaton sValueTwo? What does these strcpy and strcpy mean? What type of function is it? Btw, thanks a lot for the answer!! I learned a lot with your example
    – Martman
    Commented Jan 3, 2017 at 20:04
  • Both functions strcpy() and strcat() are used to copy char strings. The first is used to initialize the buffer char sBuffer[33] and the second is used to concatenate the extra string to the existing. If valueOne= 8.89 and valueTwo=9.88, the first string is sValueOne = "8.89000000" and the second string is sValueTwo = "9.98000000" and the output buffer is sBuffer = "8.89000000,9.98000000".
    – J. Piquard
    Commented Jan 3, 2017 at 20:14
  • How often can I write code? Is it possible to do it each 0.5 second?
    – Martman
    Commented Jan 3, 2017 at 20:14
  • Ok. Thanks a lot for explaining that! my both doubles cointains 6 numbers after. I send coordinates looking like this: double latvalue; double lngvalue; latvalue = (gps.location.lat(),6); lngvalue = (gps.location.lng(),6);
    – Martman
    Commented Jan 3, 2017 at 20:15
  • The period is highly depending of the real baudrate of your BTLEserial device. The conversion is enough fast for two samples per second.
    – J. Piquard
    Commented Jan 3, 2017 at 20:19

I didn't try this, but since Adafruit_BLE_UART inherits from Stream, which in turn inherits from Print, you should be able to

  • Codes uploads without any trouble! However the while-loop stops with my current code when I use your code as well. It works once (when I connect to the bluetooth with my phone) I edited the post with my source code if you want to check it out.
    – Martman
    Commented Jan 3, 2017 at 21:43
  • If i completely remove write or print and connect to the bluetooth with my app the while loop keeps going but if i add your code or j.piquards code the while loop stops right away when i connect the app with the bluetooth
    – Martman
    Commented Jan 3, 2017 at 23:02

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