112
ATmega16M1/32M1/64M1 [DATASHEET]
8209E–AVR–11/2012
TOM (0x0000) to TOP and then from TOP to BOTTOM. In non-inverting Compare Output mode, the Output
Compare (OCnx) is cleared on the compare match between TCNTn and OCRnx while upcounting, and set on the
compare match while downcounting. In inverting Output Compare mode, the operation is inverted. The dual-slope
operation has lower maximum operation frequency than single slope operation. However, due to the symmetric
feature of the dual-slope PWM modes, these modes are preferred for motor control applications.
The PWM resolution for the phase correct PWM mode can be fixed to 8-bit, 9-bit, or 10-bit, or defined by either
ICRn or OCRnA. The minimum resolution allowed is 2-bit (ICRn or OCRnA set to 0x0003), and the maximum res-
olution is 16-bit (ICRn or OCRnA set to MAX). The PWM resolution in bits can be calculated by using the following
equation:
In phase correct PWM mode the counter is incremented until the counter value matches either one of the fixed val-
ues 0x00FF, 0x01FF, or 0x03FF (WGMn3:0 = 1, 2, or 3), the value in ICRn (WGMn3:0 = 10), or the value in
OCRnA (WGMn3:0 = 11). The counter has then reached the TOP and changes the count direction. The TCNTn
value will be equal to TOP for one timer clock cycle. The timing diagram for the phase correct PWM mode is shown
on
Figure 15-8. The figure shows phase correct PWM mode when OCRnA or ICRn is used to define TOP. The
TCNTn value is in the timing diagram shown as a histogram for illustrating the dual-slope operation. The diagram
includes non-inverted and inverted PWM outputs. The small horizontal line marks on the TCNTn slopes represent
compare matches between OCRnx and TCNTn. The OCnx Interrupt Flag will be set when a compare match
occurs.
Figure 15-8. Phase correct PWM mode, timing diagram.
The Timer/Counter Overflow Flag (TOVn) is set each time the counter reaches BOTTOM. When either OCRnA or
ICRn is used for defining the TOP value, the OCnA or ICFn Flag is set accordingly at the same timer clock cycle as
the OCRnx Registers are updated with the double buffer value (at TOP). The Interrupt Flags can be used to gener-
ate an interrupt each time the counter reaches the TOP or BOTTOM value.
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 TCNTn and the OCRnx. Note that when using fixed TOP values, the unused bits are
R
PCPWM
TOP
1
+
log
2
log
-----------------------------------
=
OCRnx/TOP update and
OCnA interrupt flag set
or ICFn interrupt flag set
(interrupt on TOP)
1
2
3
4
TOVn interrupt flag set
(interrupt on bottom)
TCNTn
Period
OCnx
(COMnx1:0 = 2)
(COMnx1:0 = 3)