ATmega88PA Automotive Atmel Corporation, ATmega88PA Automotive Datasheet - Page 124

• Manufacturer Part Number: ATmega88PA Automotive
• Manufacturer: Atmel Corporation

124
Atmel ATmega48PA/88PA/168PA [Preliminary]
The timing diagram for the CTC mode is shown in
increases until a compare match occurs with either OCR1A or ICR1, and then counter
(TCNT1) is cleared.
Figure 16-6. CTC Mode, Timing Diagram
An interrupt can be generated at each time the counter value reaches the TOP value by either
using the OCF1A or ICF1 Flag according to the register used to define the TOP value. If the
interrupt is enabled, the interrupt handler routine can be used for updating the TOP value.
However, changing the TOP to a value close to BOTTOM when the counter is running with
none or a low prescaler value must be done with care since the CTC mode does not have the
double buffering feature. If the new value written to OCR1A or ICR1 is lower than the current
value of TCNT1, the counter will miss the compare match. The counter will then have to count
to its maximum value (0xFFFF) and wrap around starting at 0x0000 before the compare match
can occur. In many cases this feature is not desirable. An alternative will then be to use the
fast PWM mode using OCR1A for defining TOP (WGM13:0 = 15) since the OCR1A then will
be double buffered.
For generating a waveform output in CTC mode, the OC1A output can be set to toggle its log-
ical level on each compare match by setting the Compare Output mode bits to toggle mode
(COM1A1:0 = 1). The OC1A value will not be visible on the port pin unless the data direction
for the pin is set to output (DDR_OC1A = 1). The waveform generated will have a maximum
frequency of f
defined by the following equation:
The N variable represents the prescaler factor (1, 8, 64, 256, or 1024).
As for the Normal mode of operation, the TOV1 Flag is set in the same timer clock cycle that
the counter counts from MAX to 0x0000.
f
OCnA
TCNTn
OCnA
(Toggle)
Period
=
-------------------------------------------------------- -
2
N
OC
1
A
f
clk_I/O
= f
1
1
+
clk_I/O
OCRnA
/2 when OCR1A is set to zero (0x0000). The waveform frequency is
2
3
Figure
4
16-6. The counter value (TCNT1)
OCnA Interrupt Flag Set
or ICFn Interrupt Flag Set
(Interrupt on TOP)
(COMnA1:0 = 1)
9223B–AVR–09/11