SPRUIL1D May 2019 – December 2024 DRA829J , DRA829J-Q1 , DRA829V , DRA829V-Q1 , TDA4VM , TDA4VM-Q1
Spread Spectrum Modulation (SSMOD) modules on the device reduces Electro Magnetic Interference (EMI). It is a fully-digital circuit, used to modulate the frequency of the selected fractional PLLs. Programming options include selection of center spread or down spread, modulation depth, and modulation shape. The default modulation profile is triangular.
All PLLs in this family of device have SSMOD module, except for PLL24, which is the only PLL of PLLTS16FFCLVDESKEWC type.
SSMOD in the PLL is performed by changing the feedback divider (FRAC and FBDIV) in a triangular pattern. This varies the frequency of the output clock in a triangular pattern. The frequency of this pattern is the modulation frequency (fm). It is programmed as a ratio of the CLKIN/4.
Figure 5-44 shows a diagram of the spreading generation block.
fc is the original output clock frequency.
fm is the spreading frequency.
This additional block generates the required waveform used to reduce EMI. This waveform is then modulated with the initial signal to add some controlled deviation to the clock signal frequency, which spreads the energy of the clock and its harmonics into a band of frequencies, and then reduces EMI. SSMOD_DOWNSPREAD can control the position of the generated signal. It is controlled by the <PLL_name>_CTRL[4] DOWNSPREAD_EN bit of the corresponding registers. If the DOWNSPREAD_EN bit is set to 1, the frequency spread on the lower side is twice the programmed value. The frequency spread on the higher side is 0.
The value of SSMOD_FREQ controls the rate of the generated signal. It can be programmed as a ratio of the reference clock: fref / 4. The value that must be programmed is calculated as follows:
ModFreqDivider = fref / (4 × fm), where fm < fref / 70, fref = finp / (1+N), and finp is the input clock for the PLL.