Merge pull request #698 from Arnold-n/patch-2

Update millis.adoc

Author: per1234

Language/Functions/Time/millis.adoc

@@ -66,6 +66,8 @@ void loop() {
 === Notes and Warnings
 Please note that the return value for millis() is of type `unsigned long`, logic errors may occur if a programmer tries to do arithmetic with smaller data types such as `int`. Even signed `long` may encounter errors as its maximum value is half that of its unsigned counterpart.
 
+Reconfiguration of the microcontroller's timers may result in inaccurate `millis()` readings. The "Arduino AVR Boards" and "Arduino megaAVR Boards" cores use Timer0 to generate `millis()`. The "Arduino ARM (32-bits) Boards" and "Arduino SAMD (32-bits ARM Cortex-M0+) Boards" cores use the SysTick timer.
+
 --
 // HOW TO USE SECTION ENDS