Therefore, one cycle can be said to take 625 ns. Or what is the same, in this case, when 16 million cycles have passed, one second has passed. Cycle: is each one of the repetitions of the signal that occur per unit of time.And the frequency is the inverse of the period, so f = 1/T. For example, T=1/C, which would result in 1/16000000 = 0.0000000625, the time it would take for each cycle to complete.
Period: is represented by the T, and is measured in seconds, and is the inverse of the cycles. Clock frequency: is the number of cycles per second that it is capable of developing, in the case of Arduino it is 16 Mhz, or what is the same, the clock signal oscillates 16.000.000 times in a second (cycles). To work with an Arduino Timer, it is vital to know how all this works electronically in the MCU of this development board: Timer 3, 4, 5 (only on Arduino MEGA): all 16-bit. Used for the tone() function, if not used, it can be used freely for your application. timer 2: 16-bit, and can range from 0 to 65.525 (65.536 possible values). Used by the Servo library in UNO (Timer 5 for MEGA). Its modification is not recommended so as not to alter the programs. Used by functions like delay(), millis(), and micros(). timer 0: 8-bit, can count from 0 to 255 (256 possible values). There are 3 timers on the plates Arduino UNO, although there may be more on other top plates: How many timers does it have? Types of Timers CTC (Clear timer on compare match): counts the time inside a counter and when it reaches the value specified in a register of the timers, the interruption is executed. PWM signal: You can control the Arduino pins (~). The Arduino Timer has 2 operating modes, being able to use it in: That is why it is also important to know about these interruptions. In other words, the Arduino Timer is a function that is triggered at a certain time, executing an interrupt function. The timer is related to interruptions of Arduino, since they will be executed through them to attend some specific task. It will be timing as the MCU continues to execute other instructions simultaneously. Imagine that you use the delay() function, this will block execution on the Arduino MCU until the specified time elapses and then continue with the program, but the timer will not block. The instructions need X cycles to execute, not all of them are executed in the same clock cycles, for example, the 16-bit ones need more cycles in this AVR architecture. It has to be set to 1 to enable the interrupt when TCNT register value reaches OCRA value.As a Arduino UNO It has an MCU chip that works at 16 Mhz, 16.000.000 could be executed every second. 
Generally, only the 0CIE0A bit is important here. The value of TCNTx is continuously compared with OCRxA and when the values match, the interrupt is generated TIMSKx This is the register which holds the number of ticks required for generating the required delay. It’s value is compared with the Output Compare Register (in CTC mode), and an interrupt is generated when the values match. This register is where the actual counting happens. The CS00, CS01 and CS02 bits of TCCR0B are used to set the prescaler of the clock. This gives you more control on the time interval. In the CTC (Clear Timer on Compare) mode, the interrupt is generated when the value of the counter reaches the value set in OCRA register. If you have an 8-bit counter, you set its value to 0, wait it to reach 0xFF (0xFFFF for 16 bit) and then the interrupt is generated. In normal more, you don’t care about the exact time interval. Generally, one of Normal or CTC modes is used. The TCCR0A register (primarily the WGM00 and WGM01 bits, along with WGM02 bit from TCRR0B register), control the counting sequence of the counter. If you are using Timer0, you will be concerned with TCCR0A and TCCR0B registers.
You can find the datasheet here − TCCRxA and TCCRxB We will also provide the page numbers of the ATmega328 (used in Arduino Uno) datasheet wherein you can find detailed information on these registers. In this article, we will just introduce the registers relevant to timer operations and explain their significance. But what if we wish to generate timer interrupts without a third-party library? In that case, you will directly have to meddle with the timer registers in Arduino. In a previous article, we used the TimerOne library to add timer interrupts to Arduino.
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