Datasheet

112
7734Q–AVR–02/12
AT90PWM81/161
Then the frequency resolution is divided by 16. In the example above, the resolution equals
25Hz.
According to the ramp mode and the enhanced resolution mode (defined by PBFMn1:0), the
average frequency deviation Df can take three different values:
These values are applied according to the running mode and the enhanced resolution mode as
per Table 12-5;
It must be noted that, in one and two ramps modes, it is possible to apply the FWM only on
pulse width while keeping a constant frequency.
Table 12-5. Frequency deviation with Flank Width Modulation.
Notes: 1. The modulation is on the pulse width.
12.7.1 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.
At the end of the 15th cycle (numbered 14 on Table 12-6 on page 113) an interrupt can be gen-
erated. This is the case if the bit PEOEPEn (PSC n End Of Enhanced Cycle Interrupt Enable) is
set. This allows:
• To modify the modulation only on a new enhanced cycle start
• To extend the enhanced modulation accuracy by software
PBFMn1:0
00 01 10 11
Running mode RB RB+RA SB SB+SA
Four Ramps Df1 Df2 Df1 Df2
Two Ramps Df1 Df2 0
(1)
0
One Ramp Df1 Df1 0 0
Center aligned Df2 Df2 Df2 Df2
f
AVERAGE
16 d–
16
---------------
f
PLL
k
----------
×
d
16
------
f
PLL
k 1+
------------
×+=
Δf average()0=
Δf1 average()f
PSC
d
16kk 1+()
--------------------------
×=
Δf2 average()f
PSC
d
8kk 2+()
-----------------------
×=