Key Reasons for the Frequency Difference

KeyReasonsfortheFrequencyDifference.md

Key Reasons for the Frequency Difference Silicon Revision (Die Revision): Earlier revisions of the STM32H7 series, specifically revision "Y" devices, had a maximum qualified frequency of 400 MHz. STMicroelectronics later introduced revision "V" (and subsequent ones, often associated with production codes like 9S or 9T, though the code itself isn't the direct limitation but rather indicates the chip version) that was officially rated and tested for 480 MHz operation. Voltage Scaling (VOS Modes): Reaching 480 MHz requires the lowest voltage scaling mode, VOS0 (or VOS1 in some older documentation that was later updated). The older revision "Y" chips did not have VOS0 mode available, limiting them to 400 MHz. Proper software configuration is required to set the chip to the necessary voltage level for the higher speed. Power Supply Configuration: The STM32H7 line can use either an internal Low-Drop-Out (LDO) regulator or an external Switched-Mode Power Supply (SMPS) for core voltage regulation. Some development boards might default to a configuration that limits the maximum stable frequency to 400 MHz (e.g., using the SMPS without the necessary settings for the higher speed). Peripheral Limitations: While the core can reach 480 MHz, some peripherals or external memory interfaces (like SDRAM) might have maximum clock speeds of their own (e.g., 100 MHz for SDRAM). Running the system at 480 MHz might require specific prescaler settings to ensure these other components operate within their limits, which can sometimes be complex to configure correctly in software tools like STM32CubeMX. Heat Dissipation: Running at 480 MHz generates more heat than at 400 MHz. In small packages (like LQFP100), the thermal constraints might make it difficult to run at the full speed without a heatsink, leading some developers to stick to the cooler, more stable 400 MHz. In summary, the 400 MHz limit is often a result of using an older chip revision or a board/software configuration that doesn't properly enable the necessary voltage scaling and power settings for 480 MHz operation. The chip is specified for 480 MHz operation when all conditions (silicon revision, voltage, thermal, and configuration) are met.