Serial Peripheral Interface SPI SPI Control Register SPCR data order if set, LSB is transmitted first interrupt enable if set, interrupt occurs when SPI. Arduino Timer Interrupts 6 Steps with PicturesThe Uno has three timers called timer. Each of the timers has a counter that is incremented on each tick of the timers clock. CTC timer interrupts are triggered when the counter reaches a specified value, stored in the compare match register. Once a timer counter reaches this value it will clear reset to zero on the next tick of the timers clock, then it will continue to count up to the compare match value again. By choosing the compare match value and setting the speed at which the timer increments the counter, you can control the frequency of timer interrupts. KeyStone Architecture Serial Peripheral Interface SPI User Guide SPRUGP2AMarch 2012 www. Submit Documentation Feedback Release History. The first parameter Ill discuss is the speed at which the timer increments the counter. You Set Me Angie Miller'>You Set Me Angie Miller. The Arduino clock runs at 1. MHz, this is the fastest speed that the timers can increment their counters. At 1. MHz each tick of the counter represents 11. In many situations, you will find that setting the counter speed to 1. MHz is too fast.   Timer. Timer. 1 is a 1. 6 bit timer, meaning it can store a maximum counter value of 6. Once a counter reaches its maximum, it will tick back to zero this is called overflow. This means at 1. MHz, even if we set the compare match register to the max counter value, interrupts will occur every 2. Bohemian Rhapsody Notes For Piano Pdf Sheets there. Clearly, this is not very useful if you only want to interrupt once a second. Instead you can control the speed of the timer counter incrementation by using something called a prescaler. A prescaler dictates the speed of your timer according the the following equation timer speed Hz Arduino clock speed 1. MHz prescaler. So a 1 prescaler will increment the counter at 1. MHz, an 8 prescaler will increment it at 2. MHz, a 6. 4 prescaler 2. Hz, and so on.   As indicated in the tables above, the prescaler can equal 1, 8, 6. Ill explain the meaning of CS1. CS1. 1, and CS1. 0 in the next step. Now you can calculate the interrupt frequency with the following equation interrupt frequency Hz Arduino clock speed 1. Hz prescaler compare match register 1the 1 is in there because the compare match register is zero indexedrearranging the equation above, you can solve for the compare match register value that will give your desired interrupt frequency compare match register 1. Hz prescaler desired interrupt frequency 1remember that when you use timers 0 and 2 this number must be less than 2. Hz compare match register 1.