ATmega324P datasheet(124/377 Pages) ATMEL

124 / 377 page

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7674F–AVR–09/09

ATmega164P/324P/644P

Due to the single-slope operation, the operating frequency of the fast PWM mode can be twice

as high as the phase correct and phase and frequency correct PWM modes that use dual-slope

operation. This high frequency makes the fast PWM mode well suited for power regulation, recti-

fication, and DAC applications. High frequency allows physically small sized external

components (coils, capacitors), hence reduces total system cost.

The PWM resolution for fast PWM can be fixed to 8-, 9-, or 10-bit, or defined by either ICRn or

OCRnA. The minimum resolution allowed is 2-bit (ICRn or OCRnA set to 0x0003), and the max-

imum resolution is 16-bit (ICRn or OCRnA set to MAX). The PWM resolution in bits can be

calculated by using the following equation:

In fast PWM mode the counter is incremented until the counter value matches either one of the

fixed values 0x00FF, 0x01FF, or 0x03FF (WGMn3:0 = 5, 6, or 7), the value in ICRn (WGMn3:0 =

14), or the value in OCRnA (WGMn3:0 = 15). The counter is then cleared at the following timer

clock cycle. The timing diagram for the fast PWM mode is shown in Figure 14-7. The figure

shows fast PWM mode when OCRnA or ICRn is used to define TOP. The TCNTn value is in the

timing diagram shown as a histogram for illustrating the single-slope operation. The diagram

includes non-inverted and inverted PWM outputs. The small horizontal line marks on the TCNTn

slopes represent compare matches between OCRnx and TCNTn. The OCnx Interrupt Flag will

be set when a compare match occurs.

Figure 14-7. Fast PWM Mode, Timing Diagram

The Timer/Counter Overflow Flag (TOVn) is set each time the counter reaches TOP. In addition

the OCnA or ICFn Flag is set at the same timer clock cycle as TOVn is set when either OCRnA

or ICRn is used for defining the TOP value. If one of the interrupts are enabled, the interrupt han-

dler routine can be used for updating the TOP and compare values.

When changing the TOP value the program must ensure that the new TOP value is higher or

equal to the value of all of the Compare Registers. If the TOP value is lower than any of the

Compare Registers, a compare match will never occur between the TCNTn and the OCRnx.

Note that when using fixed TOP values the unused bits are masked to zero when any of the

OCRnx Registers are written.

RFPWM

TOP 1

()

log

()

log

-----------------------------------

TCNTn

OCRnx/TOP Update and

TOVn Interrupt Flag Set and

OCnA Interrupt Flag Set

or ICFn Interrupt Flag Set

(Interrupt on TOP)

Period

OCnx

(COMnx1:0 = 2)

(COMnx1:0 = 3)

ATmega324P Datasheet(PDF) 124 Page - ATMEL Corporation

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