Rainbow Electronics ATmega128RFA1 - Manual (Page 257)

257
8266A-MCU Wireless-12/09
ATmega128RFA1
Table 18-5. Waveform Generation Mode Bit Description
(1)
Mode WGMn3
WGMn2
(CTCn)
WGMn1
(PWMn1)
WGMn0)
(PWMn0)
Timer/Counter
Mode of Operation TOP
Update of
OCRnx at
TOVn Flag
Set on
0 0 0 0 0 Normal 0xFFFF Immediate MAX
1 0 0 0 1 PWM, Phase Correct, 8-bit 0x00FF TOP BOTTOM
2 0 0 1 0 PWM, Phase Correct, 9-bit 0x01FF TOP BOTTOM
3 0 0 1 1 PWM, Phase Correct, 10-bit 0x3FF TOP BOTTOM
4 0 1 0 0 CTC OCRnA Immediate MAX
5 0 1 0 1 Fast PWM, 8-bit 0x00FF BOTTOM TOP
6 0 1 1 0 Fast PWM, 9-bit 0x01FF BOTTOM TOP
7 0 1 1 1 Fast PWM, 10-bit 0x03FF BOTTOM TOP
8 1 0 0 0
PWM, Phase and Frequency
Correct
ICRn BOTTOM BOTTOM
9 1 0 0 1
PWM, Phase and Frequency
Correct
OCRnA BOTTOM BOTTOM
10 1 0 1 0 PWM, Phase Correct ICRn TOP BOTTOM
11 1 0 1 1 PWM, Phase Correct OCRnA TOP BOTTOM
12 1 1 0 0 CTC ICRn Immediate MAX
13 1 1 0 1 (Reserved) – – –
14 1 1 1 0 Fast PWM ICRn BOTTOM TOP
15 1 1 1 1 Fast PWM OCRnA BOTTOM TOP
Notes: 1. The CTCn and PWMn1:0 bit definition names are obsolete. Use the WGMn2:0 definitions. However, the functionality
and location of these bits are compatible with previous versions of the timer.
For detailed timing information refer to "Timer/Counter Timing Diagrams" on page 264 .
18.9.1 Normal Mode
The simplest mode of operation is the Normal mode (WGMn3:0 = 0). In this mode the
counting direction is always up (incrementing) and no counter clear is performed. The
counter simply overruns when it passes its maximum 16-bit value (MAX = 0xFFFF) and
then restarts from the BOTTOM (0x0000). In normal operation the Timer/Counter
Overflow Flag (TOVn) will be set in the same timer clock cycle as the TCNTn becomes
zero. In this case the TOVn Flag behaves like a 17
th
bit, except that it is only set and not
cleared. However the timer resolution can be increased by software when combined
with the timer overflow interrupt that automatically clears the TOVn Flag. There are no
special cases to consider in the Normal mode. A new counter value can be written
anytime.
The Input Capture unit is easy to use in Normal mode. However it is important to note
that the maximum interval between the external events must not exceed the resolution
of the counter. The timer overflow interrupt or the prescaler must be used to extend the
resolution for the capture unit if the intervals between events are too long.
The Output Compare units can be used to generate interrupts at some given time.
Using the Output Compare to generate waveforms in Normal mode is not
recommended because this will occupy too much CPU time.
18.9.2 Clear Timer on Compare Match (CTC) Mode
In Clear Timer on Compare (CTC) mode (WGMn3:0 = 4 or 12), the OCRnA or ICRn
Register are used to manipulate the counter resolution. In CTC mode the counter is