FUJITSU MB91460 SERIES FR60 - Manual (Page 255)

239
Chapter 17 Clock Modulator
1.Overview
Chapter 17 Clock Modulator
This chapter provides an overview of the Clock Modulator and its features. It describes the reg-
ister structure and operation of the Clock Modulator.
1. Overview
The clock modulator is intended for the reduction of electromagnetic interference - EMI, by spreading the
spectrum of the clock signal over a wide range of frequencies.
The module is fed with an unmodulated reference clock with frequency F0, provided by the PLL circuit. This
reference clock is frequency modulated, controlled by a random signal.
The mean frequency of the modulated clock is equal to the reference clock frequency F0.
Figure 1-1 Frequency spectrum of the modulated clock (fundamentals only)
●
Modulation degree and frequency resolution in frequency modulation mode
Maximum and minimum frequencies (F
max
and F
min
) of the modulated clock are defined by the modulation
degree parameter. Furthermore the resolution of the modulation range is selectable in 7 steps from low (1) to high
(7). Higher resolution implies a finer granularity of discrete frequencies in the spectrum of the modulated clock but
less possible modulation degrees.
In general the highest possible frequency resolution combined with the highest possible modulation degree
results in the highest EMI reduction. But for some cases lower modulation degrees may result in a better EMI
behavior. Please refer to the table of possible settings in Table 2-4 Modulation Parameter settings .
frequency
F
0
F
min
F
max
modulation range