RM0082
Clock & reset system
Doc ID 018672 Rev 1
201/844
11
Clock & reset system
The Clock system block is able to generate all clocks necessary at the chip. The main
clocks, at default operative frequency, are:
●
CPU_CLK @ 333 MHz for the CPUs.
(1)
●
HCLK @ 166 MHz for AHB Bus and AHB peripherals.
(1)
●
PCLK @ 83 MHz for APB Bus and APB peripherals.
(1)
●
DDR_CLK @ 100-333 MHz for DDR memory interface.
(2)
●
Clock @ 12 MHz, 30 MHz, 48 MHz.
(3)
The above frequencies are the maximum allowed values. All these clocks are generated by
three PLLs.
PLL1 and PLL2 sources are fully programmable through dedicated registers.
See the sections from
Section 12.4.5: PLL 1/2_CTR registers
in the
Miscellaneous registers (Misc)
The PLLs input reference clocks can be chosen between (see
)
●
24 MHz oscillator or PL_CLK(4) pad for PLL1
●
24 MHz oscillator or PL_CLK(3) pad for PLL2
At reset the 24 MHz source is selected.
To reduce the electromagnetic emission both PLL1 and PLL2 can be programmed to work in
dithered mode.
When the dithered mode is enabled the PLL output clock is modulated and the frequency
assumes a triangular shape. In this way the clock power spectrum is spread on a small
range (programmable) of frequencies decreasing the emission power peak.
This method replaces the other traditional methods of E.M.I. reduction, as filtering, ferrite
beads, chokes, adding power layers and ground planes to PCBs, metal shielding etc.,
allowing sensible cost saving for customers.
PLL1 and PLL2 can work in three operating modes:
●
Normal Mode (Dither-Off Mode): the PLL behaves as a normal PLL
●
Fractional-N Synthesis Mode: with this mode it is possible to select VCO frequencies
that aren't integer multiples of the reference frequency.
●
Dither-On Mode (double side modulation): in this mode a the triangular wave is added
to the VCO frequency.
●
Dither-On Mode (single side modulation): it is similar to double side modulation but the
modulation is only subtracting from the main frequency.
For further information see PLL registers in
Chapter 12: Miscellaneous registers (Misc)
.
PLL3 is used to generate the USB controller clocks and it can't be configured through
registers.
1.
This frequency is based on the PLL1.
2.
This frequency is typically based on the PLL2.
3.
This frequency is typically based on the PLL3.