Peripheral Clock Drift Tolerance on STM32F303ZE (USART, USB, SPI, I2C, ADC, CAN)

[Umesh_Kumar278]

Hi ST community,

I'm using the STM32F303ZE in a design and need to ensure that the clocks for various peripherals stay within acceptable tolerances (in ppm or %) — especially across the full temperature range (–40°C to +105°C). I plan to use an external HSE crystal oscillator, and I’m trying to validate whether my chosen crystal (in terms of stability and accuracy) will meet the needs of the following peripherals.

Could you please help clarify the maximum allowable clock frequency drift for each of these peripherals:

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USART (1–5)

• What is the maximum clock drift (in % or ppm) tolerated between the transmitter and receiver over temperature?

• How do oversampling modes (8x or 16x) and word length (M bits) affect this tolerance?

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USB FS

• I understand it requires 48 MHz ±0.25%. Is this tolerance strictly enforced by the USB hardware?

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SPI (1–3)

• For slave mode, is there a specified maximum allowable clock skew or drift from the external SPI master?

• Are there any special considerations when using HSE as the SPI clock source, especially over temperature?

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I2C (1–3)

• For Fast-mode Plus (1 MHz), what is the acceptable clock inaccuracy or timing deviation for compliance?

• Are there any temperature-related constraints defined by ST for reliable I2C operation?

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CAN

• What is the maximum oscillator tolerance (in % or ppm) per CAN node to ensure stable communication?

• Is ±1% oscillator tolerance required per ISO 11898-1, or does ST recommend a more conservative value?

[FBL]

Hi @Umesh_Kumar278 

First, external clock signal has to respect the I/O characteristics in Section 6.3.14 in datasheet

ADC clock does not have strict tolerance requirements like USB peripheral especially for audio class, but stable clocking is important for accurate conversions.

For USB FS, the dedicated 48 MHz clock is generated from the internal main PLL (the clock source must use a HSE crystal oscillator). 

To comply with USB standard specifications, the tolerance for the HSE crystal oscillator used to generate the dedicated 48 MHz clock for USB peripherals is specified as you mentioned ±2500ppm.

[mƎALLEm · Accepted solution]

Hello @Umesh_Kumar278 ,

I think +/-50PPM crystal is quiet sufficient for all these interfaces. You can refer to any ST Eval boards as reference (look for Evals having CAN and USB functions).

[Umesh_Kumar278]

Hi @FBL  and @mƎALLEm,
Thanks for the information.

I plan to use a ±30 ppm oscillator module instead of a crystal on the STM32F303ZE. This will provide even greater stability and should be well within the required tolerances over the temperature for all peripherals.

 this approach is acceptable right?

[mƎALLEm]

No matter the type of the external clock source, XTAL or XO. The most important thing is the tolerence of that source. So what you proposed is perfect.