https://devzone.nordicsemi.com/f/nordic-q-a/8959/calling-twi-driver-from-multiple-files-using-nrf_drv_twiHi, I use nrf51822 with SDK8.1, S110 and gcc with Makefile. I want to use nrf_drv_twi in both my main “.c” file as well as a library “.c” file that is included in the build. So in my main file I have: #include &lt;nrf_drv_twi.h&gt; //create TWI hardwareen-USTelligent Community 13Sat, 29 Aug 2015 01:40:22 GMThttps://devzone.nordicsemi.com/thread/33002?ContentTypeID=1Sat, 29 Aug 2015 01:40:22 GMT137ad170-7792-4731-bb38-c0d22fbe4515:99243e4e-a147-4e20-9922-94a4d65f4462RK<p>This is a basic C question. Either pass the twi instance to your library functions as a parameter, ie change the function from</p> <pre><code>void some_function( char some_data ); </code></pre> <p>to</p> <pre><code>void some_function( nrf_drv_twi_t *twi_ptr, char some_data ); </code></pre> <p>or declare it in a header file marked extern</p> <pre><code>extern nrf_drv_twi_t twi; </code></pre> <p>and define it in one of the C files</p> <pre><code>nrf_drv_twi_t twi; </code></pre> <p>it&#39;s now an external symbol you can use from anywhere</p><div style="clear:both;"></div>