Atmel ATmega328P Xplained Mini MEGA328P-XMINI MEGA328P-XMINI Data Sheet
Product codes
MEGA328P-XMINI
125
ATmega48A/PA/88A/PA/168A/PA/328/P [DATASHEET]
Atmel-8271H-AVR- ATmega-Datasheet_08/2014
The PWM resolution for fast PWM can be fixed to 8-, 9-, or 10-bit, or defined by either ICR1 or OCR1A. The
minimum resolution allowed is 2-bit (ICR1 or OCR1A set to 0x0003), and the maximum resolution is 16-bit
(ICR1 or OCR1A set to MAX). The PWM resolution in bits can be calculated by using the following equation:
minimum resolution allowed is 2-bit (ICR1 or OCR1A set to 0x0003), and the maximum resolution is 16-bit
(ICR1 or OCR1A set to MAX). The PWM resolution in bits can be calculated by using the following equation:
In fast PWM mode the counter is incremented until the counter value matches either one of the fixed values
0x00FF, 0x01FF, or 0x03FF (WGM13:0 = 5, 6, or 7), the value in ICR1 (WGM13:0 = 14), or the value in OCR1A
(WGM13:0 = 15). The counter is then cleared at the following timer clock cycle. The timing diagram for the fast
PWM mode is shown in
0x00FF, 0x01FF, or 0x03FF (WGM13:0 = 5, 6, or 7), the value in ICR1 (WGM13:0 = 14), or the value in OCR1A
(WGM13:0 = 15). The counter is then cleared at the following timer clock cycle. The timing diagram for the fast
PWM mode is shown in
. The figure shows fast PWM mode when OCR1A or ICR1 is used to define
TOP. The TCNT1 value is in the timing diagram shown as a histogram for illustrating the single-slope operation.
The diagram includes non-inverted and inverted PWM outputs. The small horizontal line marks on the TCNT1
slopes represent compare matches between OCR1x and TCNT1. The OC1x Interrupt Flag will be set when a
compare match occurs.
The diagram includes non-inverted and inverted PWM outputs. The small horizontal line marks on the TCNT1
slopes represent compare matches between OCR1x and TCNT1. The OC1x Interrupt Flag will be set when a
compare match occurs.
Figure 16-7.
Fast PWM Mode, Timing Diagram
The Timer/Counter Overflow Flag (TOV1) is set each time the counter reaches TOP. In addition the OC1A 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.
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.
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.
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
Register is free to be used for generating a PWM output on OC1A. However, if the base PWM frequency is
R
FPWM
TOP
1
+
log
2
log
-----------------------------------
=
TCNTn
OCRnx/TOP Update and
TOVn Interrupt Flag Set and
OCnA Interrupt Flag Set
or ICFn Interrupt Flag Set
(Interrupt on TOP)
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
(COMnx1:0 = 2)
(COMnx1:0 = 3)