ATMEGA2561V ATMEL [ATMEL Corporation], ATMEGA2561V Datasheet - Page 156

• Manufacturer Part Number: ATMEGA2561V
• Description: 8-bit Microcontroller with 64K/128K/256K Bytes In-System Programmable Flash
• Manufacturer: ATMEL [ATMEL Corporation]

156
ATmega640/1280/1281/2560/2561
Figure 56. Phase and Frequency Correct PWM Mode, Timing Diagram
The Timer/Counter Overflow Flag (TOVn) is set at the same timer clock cycle as the
OCRnx Registers are updated with the double buffer value (at BOTTOM). When either
OCRnA or ICRn is used for defining the TOP value, the OCnA or ICFn Flag set when
TCNTn has reached TOP. The Interrupt Flags can then 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
TCNTn and the OCRnx.
As Figure 56 shows the output generated is, in contrast to the phase correct mode, sym-
metrical in all periods. Since the OCRnx Registers are updated at BOTTOM, the length
of the rising and the falling slopes will always be equal. This gives symmetrical output
pulses and is therefore frequency correct.
Using the ICRn Register for defining TOP works well when using fixed TOP values. By
using ICRn, the OCRnA Register is free to be used for generating a PWM output on
OCnA. However, if the base PWM frequency is actively changed by changing the TOP
value, using the OCRnA as TOP is clearly a better choice due to its double buffer
feature.
In phase and frequency correct PWM mode, the compare units allow generation of
PWM waveforms on the OCnx pins. Setting the COMnx1:0 bits to two will produce a
non-inverted PWM and an inverted PWM output can be generated by setting the
COMnx1:0 to three (See Table 85 on page 161). The actual OCnx value will only be vis-
ible on the port pin if the data direction for the port pin is set as output (DDR_OCnx). The
PWM waveform is generated by setting (or clearing) the OCnx Register at the compare
match between OCRnx and TCNTn when the counter increments, and clearing (or set-
ting) the OCnx Register at compare match between OCRnx and TCNTn when the
counter decrements. The PWM frequency for the output when using phase and fre-
quency correct PWM can be calculated by the following equation:
TCNTn
OCnx
OCnx
Period
1
f
OCnxPFCPWM
2
3
=
--------------------------- -
2 N TOP
⋅
f
clk_I/O
⋅
4
OCnA Interrupt Flag Set
or ICFn Interrupt Flag Set
(Interrupt on TOP)
(COMnx1:0 = 2)
(COMnx1:0 = 3)
OCRnx/TOP Updateand
TOVn Interrupt Flag Set
(Interrupt on Bottom)
2549K–AVR–01/07