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105
8246B–AVR–09/11
ATtiny2313A/4313
an inverted PWM output can be generated by setting the COM1x1:0 to three (See
Table 12-4 onpage 112). The actual OC1Fx value will only be visible on the port pin if the data direction for the
port pin is set as output (DDR_OCF1x). The PWM waveform is generated by setting (or clearing)
the OCF1x Register at the compare match between OCR1x and TCNT1 when the counter incre-
ments, and clearing (or setting) the OCF1x Register at compare match between OCR1x 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 OCR1x Register represents special cases when generating a PWM
waveform output in the phase correct PWM mode. If the OCR1x is set equal to BOTTOM 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.
12.9
Timer/Counter Timing Diagrams
The Timer/Counter is a synchronous design and the timer clock (clk
T1) 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 OCR1x Register is updated with the OCR1x buffer value (only for
modes utilizing double buffering).
Figure 12-10 shows a timing diagram for the setting of OCF1x.
Figure 12-10. Timer/Counter Timing Diagram, Setting of OCF1x, no Prescaling
Figure 12-11 shows the same timing data, but with the prescaler enabled.
f
OCnxPFCPWM
fclk_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