RM0456: 10.5.4 unclear restrictions for STM32U59x/5Ax/5Fx/5Gx devices

[Uwe Bonnes]

RM 0456 10.5.4 Dynamic voltage scaling management writes under "System frequency steps on STM32U59x/5Ax/5Fx/5Gx device"

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On STM32U59x/5Ax/5Fx/5Gx devices only, the maximum system frequency increase or
decrease in the VOS range 1 is 80 MHz.

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This is unclear to me. On first reading I thought that 80 .. 160 Mhz in range 1 is not possible! Or does this imply a special handling needed when in Range 1 and target frequency is 1..80 MHz while with target frequency 80 .. 160 Mhz the handling explained before is needed?

[CMYL]

Hello @Uwe Bonnes ,

The maximum frequencies is shown below (in table 114 of RM0456).

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On STM32U59x/5Ax/5Fx/5Gx devices only, the maximum system frequency increase or
decrease in the VOS range 1 is 80 MHz.

===

This sentence implies the special handling needed when we are in VOS range1. The 80 Mhz is a key frequency to consider in the sequences of frequency increase or decrease in case of VOS range1:

• When running at lower frequencies than 80 MHz, If we need to increase the frequency in the VOS range 1 above 80 MHz:

1. Divide the system clock by two, using the AHB prescaler (HPRE = 0b1000
in RCC_CFGR2).
2. Configure and enable the PLL1 if needed.
3. Select PLL1 as system clock source (SW = 0b11 in RCC_CFGR1).
4. Wait for 5 μs.
5. Set the AHB prescaler to 1 (HPRE = 0b0000 in RCC_CFGR2).

• When running at higher frequencies than 80 MHz and we need to decrease the frequency below 80 Mhz

1. Divide the system clock by two using the AHB prescaler (HPRE = 0b1000
in RCC_CFGR2).
2. Wait for 5 μs.
3. Define the lower speed clock as system clock source.
4. Set the AHB prescaler back to 1 (HPRE = 0b0000 in RCC_CFGR2).

Table 114 of RM0456 summarizes the Busses maximum frequencies:

Best Regards,

Younes

[CMYL]

Hi @Uwe Bonnes,

I will check internally your request for more clarifications in the RM !!

Best Regards