Rainbow Electronics ATmega32L - Manual (Page 123)

123
ATmega32(L)
2503C–AVR–10/02
Note: 1. The CTC2 and PWM2 bit definition names are now obsolete. Use the WGM21:0 def-
initions. However, the functionality and location of these bits are compatible with
previous versions of the timer.
• Bit 5:4 – COM21:0: Compare Match Output Mode
These bits control the Output Compare pin (OC2) behavior. If one or both of the
COM21:0 bits are set, the OC2 output overrides the normal port functionality of the I/O
pin it is connected to. However, note that the Data Direction Register (DDR) bit corre-
sponding to OC2 pin must be set in order to enable the output driver.
When OC2 is connected to the pin, the function of the COM21:0 bits depends on the
WGM21:0 bit setting. Table 51 shows the COM21:0 bit functionality when the WGM21:0
bits are set to a normal or CTC mode (non-PWM).
Table 52 shows the COM21:0 bit functionality when the WGM21:0 bits are set to fast
PWM mode.
Note: 1. A special case occurs when OCR2 equals TOP and COM21 is set. In this case, the
compare match is ignored, but the set or clear is done at TOP. See “Fast PWM Mode”
on page 118 for more details.
Table 53 shows the COM21:0 bit functionality when the WGM21:0 bits are set to phase
correct PWM mode
Table 50. Waveform Generation Mode Bit Description (1)
Mode
WGM21
(CTC2)
WGM20
(PWM2) Timer/Counter Mode of
Operation
TOP
Update of
OCR2 TOV2 Flag
Set on
0 0 0 Normal 0xFF Immediate MAX
1 0 1 PWM, Phase Correct 0xFF TOP BOTTOM
2 1 0 CTC OCR2 Immediate MAX
3 1 1 Fast PWM 0xFF TOP MAX
Table 51. Compare Output Mode, non-PWM Mode
COM21 COM20 Description
0 0 Normal port operation, OC2 disconnected.
0 1 Toggle OC2 on compare match
1 0 Clear OC2 on compare match
1 1 Set OC2 on compare match
Table 52. Compare Output Mode, Fast PWM Mode (1)
COM21 COM20 Description
0 0 Normal port operation, OC2 disconnected.
0 1 Reserved
1 0 Clear OC2 on compare match, set OC2 at TOP
1 1 Set OC2 on compare match, clear OC2 at TOP