Rainbow Electronics ATmega8515L - Manual (Page 108)

108
ATmega8515(L)
2512A–AVR–04/02
Fast PWM Mode The fast Pulse Width Modulation or fast PWM mode (WGM13:0 = 5, 6, 7, 14, or 15) pro-
vides a high frequency PWM waveform generation option. The fast PWM differs from
the other PWM options by its single-slope operation. The counter counts from BOTTOM
to TOP then restarts from BOTTOM. In non-inverting Compare Output mode, the output
compare (OC1x) is set on the compare match between TCNT1 and OCR1x, and cleared
at TOP. In inverting Compare Output mode output is cleared on compare match and set
at TOP. 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, rectification, 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
ICR1 or OCR1A. The minimum resolution allowed is 2-bit (ICR1 or OCR1A set to
0x0003), and the maximum resolution is 16-bit (ICR1 or OCR1A 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 (WGM13:0 = 5, 6, or 7), the value in
ICR1 (WGM13:0 = 14), or the value in OCR1A (WGM13:0 = 15). The counter is then
cleared at the following timer clock cycle. The timing diagram for the fast PWM mode is
shown in Figure 52. The figure shows fast PWM mode when OCR1A or ICR1 is used to
define TOP. The TCNT1 value is in the timing diagram shown as a histogram for illus-
trating the single-slope operation. The diagram includes non-inverted and inverted PWM
outputs. The small horizontal line marks on the TCNT1 slopes represent compare
matches between OCR1x and TCNT1. The OC1x interrupt flag will be set when a com-
pare match occurs.
Figure 52. Fast PWM Mode, Timing Diagram
The Timer/Counter Overflow Flag (TOV1) is set each time the counter reaches TOP. In
addition the OC1A or ICF1 flag is set at the same timer clock cycle as TOV1 is set when
R
FPWM
TOP 1 +
( ) log
2 ( ) log
----------------------------------- =
TCNTn
1 7 Period 2 3 4 5 6 8
OCnx
OCnx
(COMnx1:0 = 2)
(COMnx1:0 = 3)
OCRnx/TOP Update and
TOVn Interrupt Flag Set and
OCnA Interrupt Flag Set
or ICFn Interrupt Flag Set
(Interrupt on TOP)