Datasheet

143
4317K–AVR–03/2013
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 f
b1
and f
b2
where f
b1
is the nearest base frequency above the wanted
frequency and f
b2
is the nearest base frequency below the wanted frequency. The number of f
b1
in the frame is (d-16) and the number of f
b2
is d. The f
b1
and f
b2
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 equivalent evenly distribution.
Table 16-5.
Distribution of f
b2
in the modulated frame
While ‘X’ in the table, f
b2
prime to f
b1
in cycle corresponding cycle.
So for each row, a number of fb2 take place of fb1.
Figure 16-12. Resulting Frequency versus d.
Distribution of fb2 in the modulated frame
PWM - cycle
Fraction
al
Divider
(d)
0123456789101112131415
0
1X
2X X
3XXX
4XXXX
5XXXXX
6X X X X X X
7XXXXXXX
8XXXXXXXX
9XXXXXXXXX
10 XXX X X XXX X X
11 XXX XXX XXX X X
12 XXX XXX XXX XXX
13 XXXXXXX XXX XXX
14 XXXXXXX XXXXXXX
15 XXXXXXXXXXXXXXX
f
b1
f
b2
d: 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
f
OP