the fractional number of milliseconds per timer0 overflow. #define MILLIS_INC (MICROSECONDS_PER_TIMER0_OVERFLOW / 1000) the whole number of milliseconds per timer0 overflow #define MICROSECONDS_PER_TIMER0_OVERFLOW (clockCyclesToMicroseconds(64 * 256)) #define clockCyclesPerMicrosecond() ( F_CPU / 1000000L ) But first, let’s do some math so we can substitute these numbers for the following MACROs (these macros are embedded inside several Arduino hardware files): Next we need to examine the Timer #0 interrupt handler, which is made slightly more complicated because of the decimal part of the 1.024ms roll-over period. It’s important to realize the interrupt doesn’t occur exactly each millisecond. Taking the above math into account, this occurs every 1/16,000,000(oscillator) * 64(prescale) * 256(roll over) = 0.001024 seconds, or every 1.024 ms. Therefore a Timer #0 “overflow interrupt” occurs each time the timer’s counter (TCNT0) rolls over.
#Arduino millis catch overflow code
The Arduino code enables the Timer #0 overflow interrupt (again, found inside the wiring.c file):ĪTMEL ATMega328 Datasheet page 112 specifies the Timer #0 interrupt enable bit: This subroutine is typically called an “interrupt handler.” The interrupt handler that we are interested in is called the Timer #0 Overflow Interrupt. When an interrupt occurs, the Arduino halts execution of the running program and then calls the specific interrupt subroutine.
The Timer #0 overflow can trigger an interrupt. Take note the counter ticks off 256 times, not 255 times (because 0 + 255 = 256). This “roll over” is called an “overflow” in microcontroller parlance.
Therefore, when the timer attempts to count to 256, it “rolls over” to 0. The maximum 8-bit value the timer can hold is 255 (hexadecimal 0xFF). Timer #0 has an 8-bit counter register (TCNT0) which is incremented by 1 every 0.004 milliseconds. For example, with an Arduino clock cycle occurring every 1/16,000,000 of a second, applying a 64 prescale means Timer #0 is ticking at 64 times the base oscillator rate, or 64*1/16,000,000, which is every: While technically a prescaler divides the timer, I like to think of it as a multiplier. this combination is for the standard 168/328/1280/2560ĪTMEL ATMega328 Datasheet page 111 specifies the Timer #0 prescale bits: If we examine how the Arduino sets up Timer #0, we find it sets a prescale factor of 64 (from the wiring.c file): Before utilizing any timer, several registers must be set. The Atmel ATmega168/328 based Arduino has 3 timers, of which the millis function uses the microcontroller’s Timer #0. Therefore, on the Arduino, each clock cycle is 1/16,000,000 of a second, which is: A clock cycle is roughly the time it takes for one instruction cycle (there are exceptions). A 16MHz oscillator results in the microcontroller having a clock cycle once every 16-millionth of a second. The typical Arduino has a 16MHz oscillator.
0 kommentar(er)
