Here is the Timer #0 overflow interrupt handler with the above substitutions made and a little cleanup just to make the code easier to understand (the original code is listed at the bottom of the post): #define FRACT_INC ((MICROSECONDS_PER_TIMER0_OVERFLOW % 1000) > 3)į_CPU is the oscillator frequency, and is defined during sketch compilation. about - 8 and 16 MHz - this doesn't lose precision.) by three to fit these numbers into a byte. 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. 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. There are a number of different PWM patterns already programmed into the code, what I'd like to do is be able to assign more than one device to a single pattern, and start/stop the patterns at different times to different devices.First, some background information. I'm working on a project where I could have up to six devices connected to an arduino, it might be the case that not all of them will be attached at the same time, and all of them need to be able to receive PWM signals but without using delay().
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