Incorrect STM32L4P5xx and STM32L4Q5xx RNG Initialisation in Low Level API

[badcf00d]

In a post here https://community.st.com/t5/stm32-mcus-security/why-am-i-getting-an-rng-failure-on-the-stm32l4p5/m-p/203250. It was recognised that the HAL was initialising the RNG peripheral incorrectly.

The HAL function HAL_RNG_Init seems to have been corrected to set the correct CONFIG[1,2,3] and HTCR.

However the LL version of this function doesn't include any of this, so the CONFIG[1,2,3] and HTCR will all contain the wrong values when using this function.

stm32l4xx_hal_driver/Src/stm32l4xx_ll_rng.c

Lines 118 to 135 in eb4b2d2

	ErrorStatus LL_RNG_Init(RNG_TypeDef *RNGx, LL_RNG_InitTypeDef *RNG_InitStruct)
	{
	/* Check the parameters */
	assert_param(IS_RNG_ALL_INSTANCE(RNGx));
	assert_param(IS_LL_RNG_CED(RNG_InitStruct->ClockErrorDetection));
	#if defined(RNG_CR_CONDRST)
	/* Clock Error Detection Configuration when CONDRT bit is set to 1 */
	MODIFY_REG(RNGx->CR, RNG_CR_CED | RNG_CR_CONDRST, RNG_InitStruct->ClockErrorDetection | RNG_CR_CONDRST);
	/* Writing bits CONDRST=0*/
	CLEAR_BIT(RNGx->CR, RNG_CR_CONDRST);
	#else
	/* Clock Error Detection configuration */
	MODIFY_REG(RNGx->CR, RNG_CR_CED, RNG_InitStruct->ClockErrorDetection);
	#endif /* RNG_CR_CONDRST */
	return (SUCCESS);
	}

Could this function be updated to set the correct values, as described in Table 225. RNG configurations
of RM0432?

[badcf00d]

Another suggestion, it would be useful if LL_RNG_SetHealthconfiguration and LL_RNG_GetHealthconfiguration was modified to write the magic bytes 0x17590ABC to unlock the HTCR register before reading / writing as specified in RM0432

stm32l4xx_hal_driver/Inc/stm32l4xx_ll_rng.h

Lines 661 to 675 in eb4b2d2

	__STATIC_INLINE void LL_RNG_SetHealthconfiguration(RNG_TypeDef *RNGx, uint32_t HTCFG)
	{
	WRITE_REG(RNGx->HTCR, HTCFG);
	}
	/**
	* @brief Get RNG Health Test Control
	* @rmtoll HTCR HTCFG LL_RNG_GetHealthconfiguration
	* @param RNGx RNG Instance
	* @retval Return 32-bit RNG Health Test configuration
	*/
	__STATIC_INLINE uint32_t LL_RNG_GetHealthconfiguration(RNG_TypeDef *RNGx)
	{
	return (uint32_t)READ_REG(RNGx->HTCR);
	}

Currently the write function will have no effect if the user just calls it with the value they want in the HTCR e.g.

LL_RNG_SetHealthconfiguration(0x00007274);

[RJMSTM]

Hello @badcf00d,

Thank you for your contribution. Your report will be forwarded to our development team for analysis.
We will get back to you as soon as we have an answer.

As a first answer, it's Cleary mentioned in Reference manual to unlock the HCR register before reading or writing in the register in this case you can call the function LL_RNG_SetHealthconfiguration two times firstly write the magic bytes 0x17590ABC to unlock the HTCR register and then you can read or write this register.

Thanks in advance for your patience and understanding.

With Regards,

[RJMSTM]

ST Internal Reference: 173093