Datasheet

130
7728G–AVR–06/10
ATtiny87/ATtiny167
In phase and frequency correct PWM mode, the compare units allow generation of PWM
waveforms on the OC1A/B pins. Setting the COM1A/B1:0 bits to two will produce a
non-inverted PWM and an inverted PWM output can be generated by setting the COM1A/B1:0
to three (See Table on page 133). The actual OC1A/B value will only be visible on the port pin
if the data direction for the port pin is set as output (DDR_OC1A/B) and OC1A/Bi is set. The
PWM waveform is generated by setting (or clearing) the OC1A/B Register at the compare
match between OCR1A/B and TCNT1 when the counter increments, and clearing (or setting)
the OC1A/B Register at compare match between OCR1A/B and TCNT1 when the counter
decrements. The PWM frequency for the output when using phase and frequency correct
PWM can be calculated by the following equation:
The N variable represents the prescaler divider (1, 8, 64, 256, or 1024).
The extreme values for the OCR1A/B Register represents special cases when generating a
PWM waveform output in the phase correct PWM mode. If the OCR1A/B is set equal to BOT-
TOM the output will be continuously low and if set equal to TOP the output will be set to high
for non-inverted PWM mode. For inverted PWM the output will have the opposite logic values.
12.10 Timer/Counter Timing Diagrams
The Timer/Counter is a synchronous design and the timer clock (clk
T
1) is therefore shown as a
clock enable signal in the following figures. The figures include information on when interrupt
flags are set, and when the OCR1A/B Register is updated with the OCR1A/B buffer value
(only for modes utilizing double buffering). Figure 12-11 shows a timing diagram for the setting
of OCF1A/B.
Figure 12-11. Timer/Counter Timing Diagram, Setting of OCF1A/B, no Prescaling
Figure 12-12 shows the same timing data, but with the prescaler enabled.
f
OCnxPFCPWM
f
clk_I/O
2 NTOP⋅⋅
---------------------------------=
clk
Tn
(clk
I/O
/1)
OCFnx
clk
I/O
OCRnx
TCNTn
OCRnx Value
OCRnx - 1 OCRnx OCRnx + 1 OCRnx + 2