ATmega88PA Automotive Atmel Corporation, ATmega88PA Automotive Datasheet - Page 126

• Manufacturer Part Number: ATmega88PA Automotive
• Manufacturer: Atmel Corporation

126
Atmel ATmega48PA/88PA/168PA [Preliminary]
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. How-
ever, 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 two will produce a inverted PWM and an non-inverted PWM out-
put can be generated by setting the COM1x1:0 to three (see
OC1x value will only be visible on the port pin if the data direction for the port pin is set as out-
put (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 Regis-
ter 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:
The N variable represents the prescaler divider (1, 8, 64, 256, or 1024).
The extreme values for the OCR1x Register represents special cases when generating a
PWM waveform output in the fast PWM mode. If the OCR1x is set equal to BOTTOM (0x0000)
the output will be a narrow spike for each TOP+1 timer clock cycle. Setting the OCR1x equal
to TOP will result in a constant high or low output (depending on the polarity of the output set
by the COM1x1:0 bits.)
A frequency (with 50% duty cycle) waveform output in fast PWM mode can be achieved by
setting OC1A to toggle its logical level on each compare match (COM1A1:0 = 1). This applies
only if OCR1A is used to define the TOP value (WGM13:0 = 15). The waveform generated will
have a maximum frequency of f
ture is similar to the OC1A toggle in CTC mode, except the double buffer feature of the Output
Compare unit is enabled in the fast PWM mode.
f
OCnxPWM
=
------------------------------------- -
N
f
clk_I/O
1
+
TOP
OC
1
A
= f
clk_I/O
/2 when OCR1A is set to zero (0x0000). This fea-
Table on page
133). The actual
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