AT90PWM2/3/2B/3B
16.7.1
Frequency distribution
The frequency modulation is done by switching two frequencies in a 16 consecutive cycle frame.
These two frequencies are fb1 and fb2 where fb1 is the nearest base frequency above the wanted
frequency and fb2 is the nearest base frequency below the wanted frequency. The number of fb1
in the frame is (d-16) and the number of fb2 is d. The fb1 and fb2 frequencies are evenly distrib-
uted in the frame according to a predefined pattern. This pattern can be as given in the following
table or by any other implementation which give an equivallent evenly distribution.
Table 16-5. Distribution of fb2 in the modulated frame
Distribution of fb2 in the modulated frame
PWM - cycle
Fraction
al
Divider
(d)
0
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
0
1
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
2
X
X
X
X
X
X
X
X
X
X
X
X
X
X
3
X
X
X
X
X
X
X
X
X
X
X
X
X
4
X
X
X
X
X
X
X
X
X
X
X
X
5
X
X
X
X
X
X
X
X
X
X
X
6
X
X
X
X
X
X
X
X
X
X
7
X
X
X
X
X
X
X
X
X
8
X
X
X
X
X
X
X
X
9
X
X
X
X
X
X
X
10
11
12
13
14
15
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
While ‘X’ in the table, fb2 prime to fb1 in cycle corresponding cycle.
So for each row, a number of fb2 take place of fb1.
Figure 16-12. Resulting Frequency versus d.
f
f
b2
b1
0
fOP
d:
1
2
3
4
5
6
7
8
9 10 11 12 13 14 15
141
4317J–AVR–08/10
ATMEL [ ATMEL ]
