3 timing example – Rainbow Electronics ATmega128RFA1 User Manual
Page 308
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8266A-MCU Wireless-12/09
ATmega128RFA1
When the modulator is enabled the type of modulation (logical AND or OR) can be
selected by the PORTB7 Register. Note that the DDRB7 controls the direction of the
port independent of the COMnx1:0 bit setting.
20.3 Timing Example
illustrates the modulator in action. In this example the
Timer/Counter1 is set to operate in fast PWM mode (non-inverted) and Timer/Counter0
uses CTC waveform mode with toggle Compare Output mode (COMnx1:0 = 1).
Figure 20-3. Output Compare Modulator, Timing Diagram
1
2
OC0A
(CTC Mode)
OC1C
(FPWM Mode)
PB7
(PORTB7 = 0)
PB7
(PORTB7 = 1)
(Period)
3
clk
I/O
In this example Timer/Counter2 provides the carrier while the modulating signal is
generated by the Output Compare unit C of the Timer/Counter1.
The resolution of the PWM signal (OC1C) is reduced by the modulation. The reduction
factor is equal to the number of system clock cycles of one period of the carrier (OC0A).
In this example the resolution is reduced by a factor of two. The reason for the
reduction is illustrated in
at the second and third period of the PB7
output when PORTB7 equals zero. The period 2 high time is one cycle longer than the
period 3 high time, but the result on the PB7 output is equal in both periods.