Clarification TIMx use by AVR architectures

Author: Arnold-n

Language/Functions/Time/millis.adoc

@@ -64,7 +64,9 @@ void loop() {
 
 [float]
 === 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. millis() uses timer0's overflow interrupt.
+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.
+
+AVR-based architectures program a timer to implement millis(), reconfiguring these timers may result in inaccurate millis() readings. ArduinoCore-AVR relies on TIM0 which is programmed to generate an interrupt every 16384 clock cycles (clock divider set to 64, overflow every 256 ticks). The same holds for ArduinoCore-mega-AVR, except that it uses TIMx if MILLIS_USER_TIMx (0<=x<=3) is defined.
 
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 // HOW TO USE SECTION ENDS