![]() ![]() But we still can generate interrupts or generate signals on OC0B pin. I mean TCNT0 value will be cleared only when it matches with OCR0A value, while OCR0B will not affect timer counter. As datasheet says, CTC mode works only with OCR0A. ![]() Let’s leave PWM generating for another time and do one fascinating job with both units in CTC mode. You can find lots of usages of second Output Compare Unit, especially when talking about PWM modes. Using a second Output Compare Unit in CTC mode For this, we would need to generate OCF0A interrupts and write new compare values. If we would like to change the signal frequency, we would have to change the OCR0A value. In this example, we see that we all need to set the timer once and forget. and make toggle PD6/OC0A pin on compare match Place TOP timer value to Output compare register In this mode, timer clears its value each time it reaches the value stored in the OCR0A register. In the following example, we are going to use Clear Timer on Compare or simply CTC mode. This can be done only in hardware while giving AVR core full 100% resources. Also, we had to toggle pin value by occupying processor time. For this, we had to create an interrupt and do this in software. We had to reset timer value to 5 each time an overflow occurred. In the previous example, we tried to generate a 1kHz signal in normal timer mode. The timer can do this entirely in hardware in CTC mode. Resetting timer value requires some software intervention. Especially if you need to generate some output signals. Normal timer mode isn’t practical and is not used because it is inefficient. As we know timer in this mode counts by incrementing its value we will need to restore initial timer value TCNT0=255-250=5 in overflow service routine. In this case, we will need to do timer 250 counts before generating TOV0 interrupt. First of all, let’s calculate timer resolutions and timer values for 1ms for each prescaller: Prescallerįrom the table above, we see only one choice – to run a timer with prescaller 64. Atmega328 is running at FCPU=16MHz clock speed. Let’s program a timer to generate an interrupt every 1 ms. When timer overflows, it may be set to generate TOV0 interrupt. In this mode timer value can be changed any time. This is the simplest timer mode when the timer counts to top 0xFF value and then overflows to 0, continuing same sequence again. Better focus on some operation modes by studying examples. It would take lots of space to go into details describing these registers – besides, all info is nicely plotted in datasheets. These are used to set timer operation modes, set prescallers, and start timer itself. And, of course, there are two more essential registers, TCCR0A and TCCR0B. TIFR0 register holds bits indicating interrupt request signals. Each timer interrupt can be enabled individually in TIMSK0 register by setting bits to one. As you see Timer/Counter0 register, TCN0 has two 8 bit double buffered Output Compare Registers (OCR0A and OCR0B) associated that can be used to generate two different waveforms on microcontroller pins OC0A and OC0B and two interrupts (OCF0A and OCF0B) as well. ![]()
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