ATMEGA8515L8AU ATMEL Corporation, ATMEGA8515L8AU Datasheet - Page 116



Manufacturer Part Number
ATMEL Corporation

Specifications of ATMEGA8515L8AU

The Timer/Counter Overflow Flag (TOV1) is set at the same timer clock cycle as the
OCR1x Registers are updated with the double buffer value (at BOTTOM). When either
OCR1A or ICR1 is used for defining the TOP value, the OC1A or ICF1 Flag set when
TCNT1 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
TCNT1 and the OCR1x.
As Figure 55 shows the output generated is, in contrast to the phase correct mode, sym-
metrical in all periods. Since the OCR1x 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 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 actively changed by changing the TOP
value, using the OCR1A as TOP is clearly a better choice due to its double buffer
In phase and frequency correct PWM mode, the compare units allow generation of
PWM waveforms on the OC1x pins. Setting the COM1x1:0 bits to 2 will produce a non-
inverted PWM and an inverted PWM output can be generated by setting the COM1x1:0
to 3 (See Table 1 on page 120). The actual OC1x value will only be visible on the port
pin if the data direction for the port pin is set as output (DDR_OC1x). The PWM wave-
form is generated by setting (or clearing) the OC1x Register at the Compare Match
between OCR1x and TCNT1 when the counter increments, and clearing (or setting) the
OC1x Register at Compare Match between OCR1x and TCNT1 when the counter dec-
rements. The PWM frequency for the output when using phase and frequency correct
PWM 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 phase correct PWM mode. If the OCR1x is set equal to
BOTTOM the output will be continuously low and if set equal to TOP the output will be
set to high for non-inverted PWM mode. For inverted PWM the output will have the
opposite logic values. If OCR1A is used to define the TOP value (WGM1 = 9) and
COM1A1:0 = 1, the OC1A output will toggle with a 50% duty cycle.
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