Rainbow Electronics ATmega128RFA1 - Manual (Page 261)

261
8266A-MCU Wireless-12/09
ATmega128RFA1
up-counting, and set on the compare match while down-counting. In inverting Output
Compare mode, the operation is inverted. The dual-slope operation has a lower
maximum operation frequency than single slope operation. However these modes are
preferred for motor control applications due to the symmetric feature of the dual-slope
PWM modes.
The PWM resolution for the phase correct PWM mode can be fixed to 8, 9 or 10 bit, or
be defined by either ICRn or OCRnA. The minimum resolution allowed is 2 bit (ICRn or
OCRnA set to 0x0003), and the maximum resolution is 16-bit (ICRn or OCRnA set to
MAX). The PWM resolution R
PCPWM
in bits can be calculated with the following equation:
) 2 log(
) 1 log( +
=
TOP
R
PCPWM
In phase correct PWM mode the counter is incremented until the counter value matches
either one of the fixed values 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 18-8 below . The figure shows phase correct PWM mode
when OCRnA or ICRn is used to define TOP. The TCNTn value is shown in the timing
diagram as a histogram 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 18-8. Phase Correct PWM Mode Timing Diagram
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
OCnx
(COMnx1:0 = 2)
(COMnx1:0 = 3)
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
generate 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