18.10 Timer/Counter Timing Diagrams

The Timer/Counter is a synchronous design and the timer clock (clkTn) is therefore shown as a clock enable signal in the following figures. The figures include information on when Interrupt Flags are set, and when the OCRnx Register is updated with the OCRnx buffer value (only for modes utilizing double buffering). The first figure shows a timing diagram for the setting of OCFnx.

Figure 18-9. Timer/Counter Timing Diagram, Setting of OCFnx, no Prescaling
Note: The “n” in the register and bit names indicates the device number (n = 1, 3, 4 for Timer/Counter 1, 3, 4), and the “x” indicates Output Compare unit (A/B).

The next figure shows the same timing data, but with the prescaler enabled.

Figure 18-10. Timer/Counter Timing Diagram, Setting of OCFnx, with Prescaler (fclk_I/O/8)
Note: The “n” in the register and bit names indicates the device number (n = 1, 3, 4 for Timer/Counter 1, 3, 4), and the “x” indicates Output Compare unit (A/B).

The next figure shows the count sequence close to TOP in various modes. When using phase and frequency correct PWM mode the OCRnx Register is updated at BOTTOM. The timing diagrams will be the same, but TOP should be replaced by BOTTOM, TOP-1 by BOTTOM+1 and so on. The same renaming applies for modes that set the TOVn Flag at BOTTOM.

Figure 18-11. Timer/Counter Timing Diagram, no Prescaling.
Note: The “n” in the register and bit names indicates the device number (n = 1, 3, 4 for Timer/Counter 1, 3, 4), and the “x” indicates Output Compare unit (A/B).

The next figure shows the same timing data, but with the prescaler enabled.

Figure 18-12. Timer/Counter Timing Diagram, with Prescaler (fclk_I/O/8)
Note: The “n” in the register and bit names indicates the device number (n = 1, 3, 4 for Timer/Counter 1, 3, 4), and the “x” indicates Output Compare unit (A/B).