ATmega162 datasheet(145/324 Pages) ATMEL

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145

2513K–AVR–07/09

ATmega162/V

Fast PWM Mode

The fast Pulse Width Modulation or fast PWM mode (WGM21:0 = 1) provides a high frequency

PWM waveform generation option. The fast PWM differs from the other PWM option by its sin-

gle-slope operation. The counter counts from BOTTOM to MAX then restarts from BOTTOM. In

non-inverting Compare Output mode, the Output Compare (OC2) is cleared on the Compare

Match between TCNT2 and OCR2, and set at BOTTOM. In inverting Compare Output mode, the

output is set on Compare Match and cleared at BOTTOM. Due to the single-slope operation, the

operating frequency of the fast PWM mode can be twice as high as the phase correct PWM

mode that uses dual-slope operation. This high frequency makes the fast PWM mode well suited

for power regulation, rectification, and DAC applications. High frequency allows physically small

sized external components (coils, capacitors), and therefore reduces total system cost.

In fast PWM mode, the counter is incremented until the counter value matches the MAX value.

The counter is then cleared at the following timer clock cycle. The timing diagram for the fast

PWM mode is shown in Figure 64. The TCNT2 value is in the timing diagram shown as a histo-

gram for illustrating the single-slope operation. The diagram includes non-inverted and inverted

PWM outputs. The small horizontal line marks on the TCNT2 slopes represent compare

matches between OCR2 and TCNT2.

Figure 64. Fast PWM Mode, Timing Diagram

The Timer/Counter Overflow Flag (TOV2) is set each time the counter reaches MAX. If the inter-

rupt is enabled, the interrupt handler routine can be used for updating the compare value.

In fast PWM mode, the compare unit allows generation of PWM waveforms on the OC2 pin. Set-

ting the COM21:0 bits to two will produce a non-inverted PWM and an inverted PWM output can

be generated by setting the COM21:0 to three (See Table 62 on page 150). The actual OC2

value will only be visible on the port pin if the data direction for the port pin is set as output. The

PWM waveform is generated by setting (or clearing) the OC2 Register at the Compare Match

between OCR2 and TCNT2, and clearing (or setting) the OC2 Register at the timer clock cycle

the counter is cleared (changes from MAX to BOTTOM).

The PWM frequency for the output can be calculated by the following equation:

The N variable represents the prescale factor (1, 8, 32, 64, 128, 256, or 1024).

TCNTn

OCRn Update and

TOVn Interrupt Flag Set

Period

OCn

(COMn1:0 = 2)

(COMn1:0 = 3)

OCRn Interrupt Flag Set

fOCnPWM

clk_I/O

N 256

⋅

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