Update readme. Fix #102

README.md

@@ -126,22 +126,27 @@ For additional functions, please refer to [this example sketch](examples/AllFunc
 
 ## Troubleshooting
 
+### Diagnostics sketch
+
+Use this sketch to diagnose your SIM card and GPRS connection:  
+  File -> Examples -> TynyGSM -> tools -> [Diagnostics](https://github.com/vshymanskyy/TinyGSM/blob/master/tools/Diagnostics/Diagnostics.ino)
+
 ### SoftwareSerial problems
 
 When using ```SoftwareSerial``` (on Uno, Nano, etc), the speed **115200** may not work.  
 Try selecting **57600**, **38400**, or even lower - the one that works best for you.  
 Be sure to set correct TX/RX pins in the sketch. Please note that not every Arduino pin can serve as TX or RX pin.  
 **Read more about SoftSerial options and configuration [here](https://www.pjrc.com/teensy/td_libs_AltSoftSerial.html) and [here](https://www.arduino.cc/en/Reference/SoftwareSerial).**
-
+  
 ### ESP32 HardwareSerial
 
 When using ESP32 `HardwareSerial`, you may need to specify additional parameters to the `.begin()` call.
 Please [refer to this comment](https://github.com/vshymanskyy/TinyGSM/issues/91#issuecomment-356024747).
 
-### Diagnostics sketch
+### SAMD21
 
-Use this sketch to diagnose your SIM card and GPRS connection:  
-  File -> Examples -> TynyGSM -> tools -> [Diagnostics](https://github.com/vshymanskyy/TinyGSM/blob/master/tools/Diagnostics/Diagnostics.ino)
+When using SAMD21-based boards, you may need to use a sercom uart port instead of `Serial1`.
+Please [refer to this comment](https://github.com/vshymanskyy/TinyGSM/issues/102#issuecomment-345548941).
 
 ### Broken initial configuration