Timer1 中的“OCR1x 更新”是什么意思 - Atmega32

Question

while going through the datasheet of the atmega32 , TIMER1 , modes of operation check the highlighted column in the image i added here

1-I don't understand what "Update" means (highlighted in the image).

2-What's meant by telling that the TOP value is fixed ??? can't be changed in the runtime or something else ??

Answer 1

Answer 1 OCR1A and OCR1B are the pins that timer produces PWM on. This tells about when something happens to those pins. If you read the section before that shows the waveforms and explains the different counting it explains it pretty well. Depending on the mode, update is either toggle or set or clear.

I'm looking at section 16.9 of the datasheet for the 328P. 19.9.1 lists normal mode as all 3 bits 0, the top line in the chart. It says that in normal mode counting is always up and overflows from TOP back to zero. TOP is always at the max of the counter. This is the simplest mode. In that mode the TOP value is OxFFFF (the MAX of the counter), and the PWM pins are updated immediately on a match with the compare register. The overflow flag is set at MAX, which is 0xFFFF for this timer.

It also describes CTC mode where the TOP value is set with one of the registers. And then it describes the different PWM modes and how they use TOP and BOTTOM and when they update. It's really more than I can reproduce here on this answer.

As one more example look at mode 1. That's a phase correct mode which according to section 16.9.4 means that the counter is counting from BOTTOM to TOP and then back down again. Looks like in mode 1 the TOP value is 0x00FF so it's an 8 bit timer. It updates the pin at TOP (and at compare match). If you read that section it explains how that affects the PWM pulses. It also says that it sets the overflow flag at BOTTOM, which makes sense because that is one full cycle of the timer.

Answer 2 If the top value is fixed then yes you can't change it. There are some modes where you can set the top value using one of the other registers. There are some modes where you can't. Again, the sections before all explain pretty well how all those different modes work.