Rainbow Electronics ATtiny43U - Manual (Page 84)

84
8048B–AVR–03/09
ATtiny43U
The design of the Output Compare pin logic allows initialization of the OCnx state before the out-
put is enabled. Note that some COMnx1:0 bit settings are reserved for certain modes of
operation, see
“Register Description” on page 90
12.6.1 Compare Output Mode and Waveform Generation
The Waveform Generator uses the COMnx1:0 bits differently in Normal, CTC, and PWM modes.
For all modes, setting the COMnx1:0 = 0 tells the Waveform Generator that no action on the
OCnx Register is to be performed on the next Compare Match. For compare output actions in
the non-PWM modes refer to
Table 12-2 on page 90 . For fast PWM mode, refer to Table 12-3 on
page 91 , and for phase correct PWM refer to Table 12-4 on page 91 .
A change of the COMnx1:0 bits state will have effect at the first Compare Match after the bits are
written. For non-PWM modes, the action can be forced to have immediate effect by using the
FOCnx strobe bits.
12.7 Modes of Operation
The mode of operation, i.e., the behavior of the Timer/Counter and the Output Compare pins, is
defined by the combination of the Waveform Generation mode (WGMn2:0) and Compare Output
mode (COMnx1:0) bits. The Compare Output mode bits do not affect the counting sequence,
while the Waveform Generation mode bits do. The COMnx1:0 bits control whether the PWM out-
put generated should be inverted or not (inverted or non-inverted PWM). For non-PWM modes
the COMnx1:0 bits control whether the output should be set, cleared, or toggled at a Compare
Match (See
“Modes of Operation” on page 84 ).
For detailed timing information refer to Figure 12-8 on page 89 , Figure 12-9 on page 89 , Figure
12-10 on page 89 and Figure 12-11 on page 90 in “Timer/Counter Timing Diagrams” on page
88 .
12.7.1 Normal Mode
The simplest mode of operation is the Normal mode (WGMn2:0 = 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 8-bit value (TOP = 0xFF) and then restarts from the bot-
tom (0x00). In normal operation the Timer/Counter Overflow Flag (TOVn) will be set in the same
timer clock cycle as the TCNTn becomes zero. The TOVn Flag in this case behaves like a ninth
bit, except that it is only set, not cleared. However, combined with the timer overflow interrupt
that automatically clears the TOVn Flag, the timer resolution can be increased by software.
There are no special cases to consider in the Normal mode, a new counter value can be written
anytime.
The Output Compare Unit can be used to generate interrupts at some given time. Using the Out-
put Compare to generate waveforms in Normal mode is not recommended, since this will
occupy too much of the CPU time.
12.7.2 Clear Timer on Compare Match (CTC) Mode
In Clear Timer on Compare or CTC mode (WGMn2:0 = 2), the OCRnA Register is used to
manipulate the counter resolution. In CTC mode the counter is cleared to zero when the counter
value (TCNTn) matches the OCRnA. The OCRnA defines the top value for the counter, hence
also its resolution. This mode allows greater control of the Compare Match output frequency. It
also simplifies the operation of counting external events.