Rainbow Electronics ATtiny861_V User Manual
Page 103
103
2588B–AVR–11/06
ATtiny261/461/861
Figure 16-11. Fast PWM Mode, Timing Diagram
The Timer/Counter Overflow Flag (TOV1) is set each time the counter reaches TOP. If the inter-
rupt is enabled, the interrupt handler routine can be used for updating the compare value.
In fast PWM mode, the compare unit allows generation of PWM waveforms on the OC1x pins.
Setting the COM1x1:0 bits to two will produce a non-inverted PWM and setting the COM1x1:0 to
three will produce an inverted PWM output. Setting the COM1x1:0 bits to one will enable com-
plementary Compare Output mode and produce both the non-inverted (OC1x) and inverted
output (OC1x). The actual 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 Waveforn
Output (OCW1x) at the Compare Match between OCR1x and TCNT1, and clearing (or setting)
the Waveform Output 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 number of steps in single-slope operation. The value of N equals
either to the TOP value.
The extreme values for the OCR1C Register represents special cases when generating a PWM
waveform output in the fast PWM mode. If the OCR1C is set equal to BOTTOM, the output will
be a narrow spike for each MAX+1 timer clock cycle. Setting the OCR1C equal to MAX will result
in a constantly high or low output (depending on the polarity of the output set by the COM1x1:0
bits.)
A frequency (with 50% duty cycle) waveform output in fast PWM mode can be achieved by set-
ting the Waveform Output (OCW1x) to toggle its logical level on each Compare Match
(COM1x1:0 = 1). The waveform generated will have a maximum frequency of f
OC1
= f
clkT1
/4 when
OCR1C is set to three.
The general I/O port function is overridden by the Output Compare value (OC1x / OC1x) from
the Dead Time Generator, if either of the COM1x1:0 bits are set and the Data Direction Register
bits for the OC1X and OC1X pins are set as an output. If the COM1x1:0 bits are cleared, the
actual value from the port register will be visible on the port pin. The Output Compare Pin config-
urations are described in
Table 16-3
.
TCNTn
OCRnx Update and
TOVn Interrupt Flag Set
1
Period
2
3
OCWnx
OCWnx
(COMnx1:0 = 2)
(COMnx1:0 = 3)
OCRnx Interrupt Flag Set
4
5
6
7
f
OCnxPWM
f
clkT1
N
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