Datasheet

Figure 21-7. Fast PWM Mode, Timing Diagram
TCNTn
OCRnx/TOP Update and
TOVn Interrupt Flag Set and
OCnA Interrupt Flag Set
or ICFn Interrupt Flag Set
(Interrupt on TOP)
1 7
Period
2 3 4 5 6 8
OCnx
OCnx
(COMnx[1:0] = 0x2)
(COMnx[1:0] = 0x3)
The Timer/Counter Overflow Flag (TOV1) is set each time the counter reaches TOP. In addition the
OCF1A or ICF1 Flag is set at the same timer clock cycle as TOV1 is set when either OCR1A or ICR1 is
used for defining the TOP value. If one of the interrupts are enabled, the interrupt handler routine can be
used for updating the TOP and compare values.
When changing the TOP value the program must ensure that the new TOP value is higher or equal to the
value of all of the Compare Registers. If the TOP value is lower than any of the Compare Registers, a
Compare Match will never occur between the TCNT1 and the OCR1x. Note that when using fixed TOP
values the unused bits are masked to zero when any of the OCR1x Registers are written.
The procedure for updating ICR1 differs from updating OCR1A when used for defining the TOP value.
The ICR1 Register is not double buffered. This means that if ICR1 is changed to a low value when the
counter is running with none or a low prescaler value, there is a risk that the new ICR1 value written is
lower than the current value of TCNT1. The result will then be that the counter will miss the Compare
Match at the TOP value. The counter will then have to count to the MAX value (0xFFFF) and wrap around
starting at 0x0000 before the Compare Match can occur. The OCR1A Register, however, is double
buffered. This feature allows the OCR1A I/O location to be written anytime. When the OCR1A I/O location
is written the value written will be put into the OCR1A Buffer Register. The OCR1A Compare Register will
then be updated with the value in the Buffer Register at the next timer clock cycle the TCNT1 matches
TOP. The update is done at the same timer clock cycle as the TCNT1 is cleared and the TOV1 Flag is
set.
Using the ICR1 Register for defining TOP works well when using fixed TOP values. By using ICR1, the
OCR1A Register is free to be used for generating a PWM output on OC1A. However, if the base PWM
frequency is actively changed (by changing the TOP value), using the OCR1A as TOP is clearly a better
choice due to its double buffer feature.
In fast PWM mode, the compare units allow generation of PWM waveforms on the OC1x pins. Setting the
COM1x1:0 bits to 2 will produce a non-inverted PWM and an inverted PWM output can be generated by
setting the COM1x1:0 to 3. Refer to table Table 21-3. The actual OC1x value will only be visible on the
port pin if the data direction for the port pin is set as output (DDR_OC1x). The PWM waveform is
generated by setting (or clearing) the OC1x Register at the Compare Match between OCR1x and TCNT1,
and clearing (or setting) the OC1x 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:
AVR 8-Bit Microcontroller
16-bit Timer/Counter1
© 2017 Microchip Technology Inc.
Datasheet Complete
40001974A-page 128