SPRUI30H November 2015 – May 2024 DRA745 , DRA746 , DRA750 , DRA756
Figure 26-12 is a simplified block diagram of the DPLL_USB_OTG_SS instance integration in the USB3_PHY clock generator subsystem.
The input clock CLKINP goes to a predivider N + 1. The entire loop runs on the REFCLK clock after this predivider. The value of N + 1 is controlled through the DPLLCTRL_USB_OTG_SS.PLL_CONFIGURATION1[8:1] PLL_REGN bit field.
The frequency ranges for the DPLL_USB_OTG_SS input clock - CLKINP and the DPLL internal reference clock, REFCLK = CLKINP/N + 1 are:
The output clock CLKDCOLDO is synthesized by digitally controlled oscillator (the DCO block), that automatically detects the frequency range. The CLKDCOLDO frequency can be given with CLKDCOLDO = CLKINP×M/(N + 1). For that purpose the feedback multiplier M must be configured through the DPLLCTRL_USB_OTG_SS.PLL_CONFIGURATION1[20:9] PLL_REGM bit field.
The DPLL_USB_OTG_SS module supports fractional synthesis (that is, the frequency multiplication factor M can be programmed as fractional). This is achieved by having a sigma delta feedback divider (M). A fractional value (Fractional M) of 18 bits is supported, thus enabling control for a better accuracy. Programming the 18-bit Fractional M value is done by setting the DPLLCTRL_USB_OTG_SS.PLL_CONFIGURATION4[17:0] PLL_REGM_F bit field (similar to REGM). To enable integer only division, Fractional M should be set to 000…0.
Fractional synthesis is not supported for M > 4093.
The module also supports SSC on its output clocks. SSC is used to spread the spectral peaking of the clock to reduce any electromagnetic interference (EMI). When SSC is enabled, the clock spectrum is spread by the amount of frequency spread, and the attenuation is given by the ratio of the frequency spread (df) and the modulation frequency (fm); that is, {10×log10(df/fm)} dB.
The SSC is performed by changing the feedback divider (M) in a triangular pattern, which means the frequency of the output clock varies in a triangular pattern. The frequency of the triangular pattern is modulation frequency (fm). The peak (dM) or the amplitude of the triangular pattern as a percent of M is equal to the percent of the frequency spread (df); that is, dM/M = df/FOUT.
Because this is in-band modulation for the DPLL_USB_OTG_SS, the modulation frequency must be within the loop bandwidth of the DPLL. A higher modulation frequency would result in less spreading in the output clock.
The SSC can be enabled and disabled by asserting the DPLLCTRL_USB_OTG_SS.PLL_SSC_CONFIGURATION1[0] EN_SSC bit. The acknowledge signal SSCACK, observed by the DPLLCTRL_USB_OTG_SS.PLL_STATUS[12] SSC_EN_ACK bit, notifies the exact start and end of SSC. When EN_SSC is deasserted, SSC is disabled only after completion of one full cycle of the triangular pattern given by the modulation frequency. This is done to maintain the average frequency.
The modulation frequency (fm) can be programmed as a ratio of REFCLK/4; that is, the value programmed in the DPLLCTRL_USB_OTG_SS.PLL_SSC_CONFIGURATION2[29:20] MODFREQDIVIDER bit field must be = REFCLK/(4×fm).The ModFreqDivider is split into Mantissa and 2Exponent (ModFreqDivider = ModFreqDividerMantissa × 2ModFreqDividerExponent).
Although the same value of ModFreqDivider could be obtained by different combinations of Mantissa and Exponent values, it is preferred to get the target ModFreqDivider by programming maximum Mantissa and minimum Exponent values.
To define the frequency spread (df), dM must be controlled as previously explained. To define dM, the step size of M for each REFCLK during the triangular pattern must be programmed.This is defined as follows:
DeltaMStep value is split into integer part and fractional part, as follows:
If the DPLLCTRL_USB_OTG_SS.PLL_SSC_CONFIGURATION1[2] DOWNSPREAD 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 (except for 20 percent overshot).