attiny13-20ss ATMEL Corporation, attiny13-20ss Datasheet - Page 64

• Manufacturer Part Number: attiny13-20ss
• Description: 8-bit Microcontroller With 1k Bytes In-system Programmable Flash - Atmel Corporation
• Manufacturer: ATMEL Corporation

Fast PWM Mode
64
ATtiny13
to OCR0A is lower than the current value of TCNT0, the counter will miss the Compare
Match. The counter will then have to count to its maximum value (0xFF) and wrap
around starting at 0x00 before the Compare Match can occur.
For generating a waveform output in CTC mode, the OC0A output can be set to toggle
its logical level on each Compare Match by setting the Compare Output mode bits to
toggle mode (COM0A1:0 = 1). The OC0A value will not be visible on the port pin unless
the data direction for the pin is set to output. The waveform generated will have a maxi-
mum frequency of f
frequency is defined by the following equation:
The N variable represents the prescale factor (1, 8, 64, 256, or 1024).
As for the Normal mode of operation, the TOV0 Flag is set in the same timer clock cycle
that the counter counts from MAX to 0x00.
The fast Pulse Width Modulation or fast PWM mode (WGM02:0 = 3 or 7) provides a high
frequency PWM waveform generation option. The fast PWM differs from the other PWM
option by its single-slope operation. The counter counts from BOTTOM to TOP then
restarts from BOTTOM. TOP is defined as 0xFF when WGM2:0 = 3, and OCR0A when
WGM2:0 = 7. In non-inverting Compare Output mode, the Output Compare (OC0x) is
cleared on the Compare Match between TCNT0 and OCR0x, 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 use dual-slope opera-
tion. This high frequency makes the fast PWM mode well suited for power regulation,
rectification, and DAC applications. High frequency allows physically small sized exter-
nal components (coils, capacitors), and therefore reduces total system cost.
In fast PWM mode, the counter is incremented until the counter value matches the TOP
value. The counter is then cleared at the following timer clock cycle. The timing diagram
for the fast PWM mode is shown in Figure 31. The TCNT0 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 TCNT0
slopes represent Compare Matches between OCR0x and TCNT0.
OC0
= f
clk_I/O
f
OCnx
/2 when OCR0A is set to zero (0x00). The waveform
=
------------------------------------------------- -
2 N
&
&
$
f
clk_I/O
1
+
OCRnx
%
2535G–AVR–01/07