atmega323l ATMEL Corporation, atmega323l Datasheet - Page 50

• Manufacturer Part Number: atmega323l
• Description: Atmega323 8-bit Avr Microcontroller With 32k Bytes Of In-system Programmable Flash
• Manufacturer: ATMEL Corporation

PWM Modes (Up/Down and
Overflow)
50
ATmega323(L)
The two different PWM modes are selected by the CTC0 or CTC2 bit in the
Timer/Counter Control Registers –TCCR0 or TCCR2 respectively.
If CTC0/CTC2 is cleared and PWM mode is selected, the Timer/Counter acts as an
up/down counter, counting up from $00 to $FF, where it turns and counts down again to
zero before the cycle is repeated. When the counter value matches the contents of the
Output Compare Register, the PB3(OC0/PWM0) or PD7(OC2/PWM2) pin is set or
cleared according to the settings of the COMn1/COMn0 bits in the Timer/Counter Con-
trol Registers TCCR0 or TCCR2.
If CTC0/CTC2 is set and PWM mode is selected, the Timer/Counters will wrap and start
counting from $00 after reaching $FF. The PB3(OC0/PWM0) or PD7(OC2/PWM2) pin
will be set or cleared according to the settings of COMn1/COMn0 on a Timer/Counter
overflow or when the counter value matches the contents of the Output Compare Regis-
ter. Refer to Table 15 for details.
Table 15. Compare Mode Select in PWM Mode
Note:
Note that in PWM mode, the value to be written to the Output Compare Register is first
transferred to a temporary location, and then latched into the OCR when the
Timer/Counter reaches $FF. This prevents the occurrence of odd-length PWM pulses
(glitches) in the event of an unsynchronized OCR0 or OCR2 write. See Figure 34 and
Figure 35 for examples.
CTCn
0
0
0
0
1
1
1
1
1. n = 0 or 2
COMn1
0
0
1
1
0
0
1
1
COMn0
0
1
0
1
0
1
0
1
Effect on Compare Pin
Not connected
Not connected
Cleared on Compare Match, up-counting. Set on
Compare Match, down-counting (non-inverted
PWM)
Cleared on Compare Match, down-counting. Set
on Compare Match, up-counting (inverted PWM)
Not connected
Not connected
Cleared on Compare Match, set on overflow
Set on Compare Match, cleared on overflow
(1)
1457G–AVR–09/03
Frequency
f
f
f
f
TCK0/2
TCK0/2
TCK0/2
TCK0/2
/510
/510
/256
/256