attiny13-20ss ATMEL Corporation, attiny13-20ss Datasheet - Page 65

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

2535G–AVR–01/07
Figure 31. Fast PWM Mode, Timing Diagram
The Timer/Counter Overflow Flag (TOV0) is set each time the counter reaches TOP. If
the interrupt is enabled, the interrupt handler routine can be used for updating the com-
pare value.
In fast PWM mode, the compare unit allows generation of PWM waveforms on the
OC0x pins. Setting the COM0x1:0 bits to two will produce a non-inverted PWM and an
inverted PWM output can be generated by setting the COM0x1:0 to three: Setting the
COM0A1:0 bits to one allows the AC0A pin to toggle on Compare Matches if the
WGM02 bit is set. This option is not available for the OC0B pin (See Table 27 on page
69). The actual OC0x 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
OC0x Register at the Compare Match between OCR0x and TCNT0, and clearing (or
setting) the OC0x Register at the timer clock cycle the counter is cleared (changes from
TOP to BOTTOM).
The PWM frequency for the output can be calculated by the following equation:
The N variable represents the prescale factor (1, 8, 64, 256, or 1024).
The extreme values for the OCR0A Register represents special cases when generating
a PWM waveform output in the fast PWM mode. If the OCR0A is set equal to BOTTOM,
the output will be a narrow spike for each MAX+1 timer clock cycle. Setting the OCR0A
equal to MAX will result in a constantly high or low output (depending on the polarity of
the output set by the COM0A1:0 bits.)
A frequency (with 50% duty cycle) waveform output in fast PWM mode can be achieved
by setting OC0x to toggle its logical level on each Compare Match (COM0x1:0 = 1). The
waveform generated will have a maximum frequency of f
set to zero. This feature is similar to the OC0A toggle in CTC mode, except the double
buffer feature of the Output Compare unit is enabled in the fast PWM mode.
TCNTn
OCn
OCn
Period
1
2
3
f
OCnxPWM
4
5
=
----------------- -
N 256
f
clk_I/O
&
6
OC0
7
= f
clk_I/O
/2 when OCR0A is
OCRnx Interrupt Flag Set
OCRnx Update and
TOVn Interrupt Flag Set
(COMnx1:0 = 2)
(COMnx1:0 = 3)
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