The default bootloader in system memory conveniently provides several ways to load a program into an STM32. In most models with USB, the bootloader incorporates also an option to use USB DFU bootloader.

However, as USB requires a higher clock precision than that provided by the internal RC oscillators of STM32 (HSI, MSI), in models which don't support "crystal-less USB", the DFU bootloader requires a crystal to be connected to the HSE pins. Frequency of the crystal cannot be arbitrary, and AN2606 describes the particular requirements on crystal frequencies for individual STM32 models.

For the 'F4/'F7 families, the crystal has to have frequency which is integer multiple of 1MHz, between 4MHz and 26MHz. The exact mechanism how bootloader detects, which of the possible crystals is present, is the following:

• use HSI with PLL to set system frequency to 24MHz
• start HSE, set the HSE-to-RTC prescaler to 31 (maximum value of RCC_CFGR.RTCPRE)
• enable TIM11 clock in RCC (APB dividers are 1 so timer clock is 24MHz), in TIM11_OR, set TIM11_CH1 to be connected to HSE-RTC
• set it to run freely and capture CH1 with capture prescaler set to 8 (maximum)
• capture two consecutive HSE/31/8 edges, calculate difference of captured values D
• calculate 24'000'000/D*31*8, that's the measured HSE frequency
• divide by 100'000, add 5, divide by 10, use result as M in setting up PLL to generate 60MHz system clock and 48MHz USB clock from HSE

This method involves several rounding and granularity errors. As the basis of measurement is the ratio between HSE/HSI, the errors get more pronounced as this ratio decreases i.e. HSE frequency increases. As long as HSI is within the 1% precision given by factory calibration, this method correctly measures all HSE values between 4MHz and 26MHz. However, the 1% factory calibration is guaranteed only for temperature of 25°C. HSI frequency varies quite significantly with temperature (which is the temperature of the silicon die, and that may be surprisingly higher from ambient by quite a couple of degrees very quickly).

At around 2% of HSI error, measurements of the highest frequencies (25MHz, 26MHz) start to fail. The consequence is, that the bootloader sets up PLL incorrectly, thus USB module is not clocked at 48MHz and the difference is too big for USB to work successfully.

ST recognizes this problem, as in AN2606, the descriptions for 'F4 and 'F7 bootloaders contain the following footnote:

Due to HSI deviation and since HSI is used to detect HSE value, the user must use low frequency rather than high frequency HSE crystal values (low frequency values are better detected due to larger error margin). For example, it is better to use 8 MHz instead of 25 MHz.

Indeed, the measurement for 8MHz HSE is good up to cca 6.5% error of HSI. For the maxumum error of HSI given by datasheet as -8% at the highest temperature range, only 4MHz and 5MHz are "safe" values.