Declare an unsigned long (value that will be returned).It is very simple and can be broken down into: in the middle of a write to timer0_millis) disable interrupts while we read timer0_millis or we might get an Moving swiftly on, the pertinent code can again be found in wiring.c: I’ll say it again: Reseting/modifying the millis() count MAY break other libraries or possibly your existing code. Now that we understand how millis() actually works we can go about resetting it. This means you might get unexpected behaviour if you do things like “ if(millis()=1043)” as millis() might never return that value. It’s also worth pointing out that this implementation means that millis will be incremented by 2 (or 3 using an 8MHz oscillator) when the microsecond error is factored in. If we've accrued more than 1ms of error increment the millis value and deduct a millisecond from our error increment extra microseconds by correct amount (volatile variables must be read from memory on every access) copy these to local variables so they can be stored in registers Here is the code for the interrupt handler: using 8 bit math on an 8 bit microprocessor) in the interrupt handler since it runs so frequently. I assume that this is done to reduce the number of instructions (i.e. FRACT_MAX, therefore, is the threshold at which you have accumulated 1 millisecond of error, bit shifted to also fit into one byte. FRACT_INC is the “extra” 24 microseconds bit-shifted to fit into one byte. This is where FRACT_INC and FRACT_MAX come in. If this was ignored then millis would lose roughly 2000 seconds a day. However this leaves 24 microseconds unaccounted for every millisecond. The millis value is incremented, therefore, by 1. So by the calculations above we can see that it takes 1024 microseconds for Timer0 to overflow. (64 * 256) / (16000000Hz/1000000L) = 1024įor the moment I will just focus on the values for a 16MHz clock to avoid confusion. Here are the relevant numbers for both 16MHz and 8 Mhz. #define clockCyclesToMicroseconds(a) ( (a) / clockCyclesPerMicrosecond() ) #define clockCyclesPerMicrosecond() ( F_CPU / 1000000L ) To work out the values we also need the following #defines: #define FRACT_INC ((MICROSECONDS_PER_TIMER0_OVERFLOW % 1000) > 3) #define MILLIS_INC (MICROSECONDS_PER_TIMER0_OVERFLOW / 1000) #define MICROSECONDS_PER_TIMER0_OVERFLOW (clockCyclesToMicroseconds(64 * 256)) First off there are some #defines that are used to calculate various values depending on the clock frequency being used. To find out how this is implemented, you need to look in wiring.c which is found in the arduino folder “ hardware/arduino/cores/arduino” or thereabouts. ![]() When this happens, an interrupt is triggered and the associated interrupt handling code is run. Since the register is only 1 byte, after 256 ticks the register overflows. Every time it ticks, the register containing the number of times Timer0 has ticked is incremented. So how does millis actually work? Millis uses Timer0 which is configured to “tick” every 64 clock cycles. I think it’s extremely unlikely that you are going to permanently brick your Arduino, but you should understand what it is that you are doing. ![]() That being said, sometimes it may be appropriate and it provides an interesting insight into some of the core Arduino code.īefore trying any of the following please remember that a lot of libraries use millis() internally and changing its behaviour MAY BREAK THINGS. 99 % of the time, such as when dealing with millis overflow, it really isn’t necessary. This seems to be something that people ask how to do fairly frequently.
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