I think I may have found the reason why it is not possible to disable the ECC double bit error NMI.
In 7.9.10 of RM0481 (Rev 1) I found the following information:
[...]
When the ECCD the flag is raised, an NMI is generated, it can be masked in SBS registers (SBS flift ECC NMI mask register (SBS_ECCNMIR)) for data access (OTP, data area, RO data).
[...]
I am calculating the checksum in "normal" code space in flash, so I assume that's why setting this bit has no effect in my application. Maybe an ST employee can confirm that.
My solution was to handle this with 2 (static) global variables and some code in the NMI handler, similar to the suggestion of @Pavel A. :
volatile bool ignoreEccErr = false;
volatile bool eccDoubleBitErrOccurred = false;
/**
* NMI handler.
* Handles ECC errors during Bootloader checksum calculation.
* Returns from NMI if it is caused by an ECC double bit error and the 'Ignore ECC errors' flag is set.
* Never returns in all other cases.
*/
void __attribute__((used)) NMI_Handler(void)
{
/* Check if ECC double bit error occurred if ignoring ECC errors flag is set */
if (ignoreEccErr && (READ_BIT(FLASH->ECCDETR, FLASH_ECCR_ECCD) != 0U))
{
/* Reset the detection flag to enable detecting subsequent double bit ECC errors */
SET_BIT(FLASH->ECCDETR, FLASH_ECCR_ECCD);
eccDoubleBitErrOccurred = true;
return;
}
while (1)
{
}
}
The application verification function looks now like:
bool isValidApplication(void)
{
bool isValid;
ignoreEccErr = true;
/* CRC calculation and verification; reads from program flash memory */
isValid = isCrcCorrect();
ignoreEccErr = false;
if (eccDoubleBitErrOccurred)
{
isValid = false;
}
return (isValid);
}
In my testcase this solution seems to work. After the NMI returns, the function that read the bad memory, got the bad memory value, adds it to the checksum and continues with the next memory address. The checksum calculation finishes and because of the 'eccDoubleBitErrOccurred' flag, the checksum verification is set to invalid (just in case to avoid an accidently correct CRC with ECC errors in the application area).
