In short, SLAK bit won't reset when SLEEP bit is manually reset. In details :
I am trying to achieve a successful transmission in loopback mode before venturing into making a network. I had it working at a point after a lot of documentation reading, but now I have a new issue. (Sadly I do not remember what I changed, played with the timings maybe)
After setting the peripheral to loopback and providing coherent bit timing values (so I may have played with them but they are back to being ok), I generate the code with Cube. This implies that the flow should first exit the sleep mode, enter the init mode, do the settings, exit the init mode, and start normal mode. According to the reference manual :
If software requests entry to initialization mode by setting the INRQ bit while bxCAN is in Sleep mode, it must also clear the SLEEP bit. [...] After the SLEEP bit has been cleared, Sleep mode is exited once bxCAN has synchronized with the CAN bus [...]. The Sleep mode is exited once the SLAK bit has been cleared by hardware
and
To synchronize, bxCAN waits until the CAN bus is idle, this means 11 consecutive recessive bits have been monitored on CANRX.
According to wiki
A 0 data bit encodes a dominant state, while a 1 data bit encodes a recessive state
So
Checking the code generated by Cube this is exactly what is happening. I pasted here the essential part from stm32f4xx_hal_can.c :
HAL_StatusTypeDef HAL_CAN_Init(CAN_HandleTypeDef *hcan)
{
[...]
/* Exit from sleep mode */
CLEAR_BIT(hcan->Instance->MCR, CAN_MCR_SLEEP);
/* Get tick */
tickstart = HAL_GetTick();
/* Check Sleep mode leave acknowledge */
while ((hcan->Instance->MSR & CAN_MSR_SLAK) != 0U)
{
if ((HAL_GetTick() - tickstart) > CAN_TIMEOUT_VALUE)
{
[...]
/*Error*/
}
}
/* Request initialisation */
SET_BIT(hcan->Instance->MCR, CAN_MCR_INRQ);
/* Get tick */
tickstart = HAL_GetTick();
/* Wait initialisation acknowledge */
while ((hcan->Instance->MSR & CAN_MSR_INAK) == 0U)
{
if ((HAL_GetTick() - tickstart) > CAN_TIMEOUT_VALUE)
{
[...]
/*Error*/
}
The SLEEP bit of CAN_MSR is reset and waits for the SLAK bit from CAN_MSR to be reset by the hardware. CAN_TIMEOUT_VALUE is set to 10, basically giving time for the 11 recessive bits to settle in.
And this is where I am stuck. SLACK would not reset... I tried to remove if ((HAL_GetTick() - tickstart) > CAN_TIMEOUT_VALUE) so that the MCU waits indefinitely for a SLAK reset. Did not help.
Looking at the CAN_MSR RX register, giving the current value on RX, while waiting for SLACK to change, I noticed that it is always at 0. So I tried to set GPIOs as pull-up and pull-downs for RX and TX, but I think it has no effect since, in loopback mode, RX of bxCAN is isolated from GPIOs :) This meaning also, that the issue should not be on the hardware side (like wiring and stuff, external things, not internal hardware). Leading me to believe that something is wrong during the global HAL_Init() or MX_GPIO_Init() or other stuff, but since it is generated by Cube and I did not change anything, I don't see how it could have an effect on SLAK not going away.
My idea was maybe to do a software reset, on something, but I don't know where this path will lead me since powering off and on the chip do not resolve the issue...