How to change the CPU frequency

1. Purpose

This article explains how to change the CPU operating point (also known as OPP): an operating point corresponds to the frequency of the processor and the voltage that needs to be supplied to sustain it.

2. Hardware side

On STM32MP1 series, the Cortex-A7 core is:

• clocked by the PLL1 from the RCC internal peripheral: the PLL1P output frequency can be directly propagated to the core or it can go through an intermediate MPUDIV dividor,
• supplied with VDDcore voltage.

The part number tells which devices can be clocked up to 800 MHz, with associated usage conditions. Otherwise, the frequency must be kept below 650 MHz.

3. Software side

3.1. For ecosystem release ≥ v1.2.0

The ecosystem release ≥ v1.2.0 is backward compatible with the previous deliveries, so it is still possible to set the CPU frequency in TF-A device tree, as described in the paragraph below. If your part number supports up to 800 MHz, ensures that VDDcore minimum voltage is increased from 1.2V to 1.35V while running above 650 MHz: this is done in the regulators node of the board device tree.

But, by default, the ecosystem release ≥ v1.2.0 does not set any configuration for the PLL1 in the FSBL (TF-A) and instead, TF-A automatically selects the operating point that is suitable for the current part number according to the OPP table below (embedding frequency and voltage couples), defined in the SOC device tree fdts/stm32mp157c.dtsi .

	cpu0_opp_table: cpu0-opp-table {
		compatible = "operating-points-v2";
		opp-shared;

		opp-650000000 {
			opp-hz = /bits/ 64 <650000000>;
			opp-microvolt = <1200000>;
			opp-supported-hw = <0x1>;
		};
		opp-800000000 {
			opp-hz = /bits/ 64 <800000000>;
			opp-microvolt = <1350000>;
			opp-supported-hw = <0x2>;
		};
	};

Notes:

• The operating point supported by the devices supporting above 650 MHz (and up to 800 MHz) is identified by the opp-supported-hw property set to 0x2,
• This same OPP table must also be present in the Linux SOC device tree arch/arm/boot/dts/stm32mp157c.dtsi ,
• It is possible to do a mix, specifying the PLL1 configuration (without OPP table) in TF-A device tree whereas Linux will use the OPP table to increase the CPU frequency once started.
• This description is valid for cold boot, but also when coming back from Standby low power mode.
• During cold boot, TF-A computes and saves PLL1 settings for all operating points available in the device tree and compliant with hardware capabilities. Later on, those saved paramaters are used to increase the performance of the system states transitions.

3.2. For ecosystem release ≤ v1.1.0

The Cortex-A7 core frequency is selected at boot time, by the FSBL (TF-A), following the Clock device tree configuration - Bootloader specific syntax. The frequency is set to 650 MHz by default, as shown in STM32MP15 clock tree. This configuration is set in TF-A device tree, for instance fdts/stm32mp157a-dk1.dts file for STM32MP157C-DK1 board.

&rcc {
	...
	/* VCO = 1300.0 MHz => P = 650 (CPU) */
	pll1: st,pll@0 {
		cfg = < 2 80 0 0 0 PQR(1,0,0) >;
		frac = < 0x800 >;
	};
	...
};

The user can reduce this frequency by changing the above configuration, whether manually or via STM32CubeMX graphical user interface that allows to generate the corresponding device tree file.

Central processing unit

Operating performance point (link to voltage and frequency scaling)

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