MEGA128CAN Datasheet, PDF (128/413 Pages) ATMEL Corporation – Microcontroller WITH 128K BYTES OF ISP FLASH AND CAN CONTROLLER

Timer/Counter Timing
Diagrams
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 on page 132). The actual OCnx value will only be visible
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:
f_OCnxPFCPWM = ₂-----_â‹-^f--^c_N---^l-^k--_â‹^_---^I_T-^/-^O--_O----_P--
The N variable represents the prescaler divider (1, 8, 64, 256, or 1024).
The extreme values for the OCRnx Register represents special cases when generating
a PWM waveform output in the phase correct PWM mode. If the OCRnx 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.
The Timer/Counter is a synchronous design and the timer clock (clk_Tn) is therefore
shown as a clock enable signal in the following figures. The figures include information
on when interrupt flags are set, and when the OCRnx Register is updated with the
OCRnx buffer value (only for modes utilizing double buffering). Figure 57 shows a timing
diagram for the setting of OCFnx.
Figure 57. Timer/Counter Timing Diagram, Setting of OCFnx, no Prescaling
clk_I/O
clk_Tn
(clk_I/O/1)
TCNTn
OCRnx - 1
OCRnx
OCRnx + 1
OCRnx + 2
OCRnx
OCRnx Value
OCFnx
Figure 58 shows the same timing data, but with the prescaler enabled.
128 AT90CAN128
4250E–CAN–12/04