11/29/2023 0 Comments Arduino millis catch overflow![]() To show them in your local time you can join the forum, and then set the 'time correction' field in your profile to the number of hours difference between your location and UTC time. The dates and times for posts above are shown in Universal Co-ordinated Time (UTC). The timer is configured to tick every 4 µs on a 16 MHz system (because of the prescaler of 64: 64. To get accurate results this ISR must not miss an overflow. Its purpose is to count Timer 0 overflows. One clock cycle is 62.5 nS, thus it "ticks" every 4 µs, and overflows every 256 * 4 µs). The code called by the (default) Timer 0 overflow interrupt vector (TIM0OVFvect) is used by millis and micros to help return their results. (It counts to 256, using a prescaler of 64. Also, micros has a resolution of 4 µs (not 1 µs) because of the way the timer is configured. However it wraps after around 71 minutes. If you are timing small intervals, micros() will be much more accurate, as that reads from the hardware directly, and does not suffer from this creeping error. I had be using millis in order to control the time step, I want to get the humidity and temperature each 1 minute (60000 seconds) between each print for several daysbut after some time, it get overflow in time1. (And since we are adding one every 42 overflows, that means we are adding one every 42 * 24 µs, which is every 1008 µs). Since 125 / 3 is 41.67 this happens every 42 overflows or so. When this happens it adds 1 to the millis counter (timer0_millis) and subtracts 125 from timer0_fract. It keeps doing that until timer0_fract is >= 125 (FRACT_MAX). ![]() Understanding millis() The Arduino millis() function will let you accomplish. In detail, it adds 3 (FRACT_INC) to a variable called timer0_fract each overflow. The analog input pins can be used as digital pins. At this point, of course, the count returned by millis() will "jump" as this extra compensating amount is added in. However the overflow interrupt, which is called every 1024 µs, keeps track of the amount it is out, and eventually adds one to the millis() count to catch up (and reduces the overflow amount to compensate). This will happen approximately every 42 overflow interrupts. In other words, millis() will run slow (it should update every 1000 µs but actually updates every 1024 µs). ![]() In other words, millis() will run slow (it should update every 1000 µs but actually updates every 1024 µs). The code eventually compensates so this inaccuracy is not cumulative over time. Therefore millis() will be out by 24 µs after the first interrupt, 48 µs after the second interrupt, and so on. There is an interrupt (Timer 0 overflow interrupt) called every 1024 µs which updates the variable used by millis(). Because of the way the library code works, the number returned by millis() will slowly drift.
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