# STM32: Busy flag is set after I2C initialization

For the reference: the same problem is described there, but the author's solution doesn't work for me - I2C busy flag strange behaviour

I used STM32CubeMX to generate project template with I2C peripherals initialization. Unfortunately it works somehow strange: after HAL_I2C_MspInit(I2C1) is being invoked, bus is considered permanently busy.

If I try to apply

__HAL_RCC_I2C1_FORCE_RESET();
HAL_Delay(1000);
__HAL_RCC_I2C1_RELEASE_RESET();


That resolves problem with BUSY flag, but causes problem - SB bit not being set after START is generated. According to debugger, I2C registers are cleared completely after the reset - I suspect this is the problem with that method.

I also confimed short voltage drop at SDA line during startup, that is probably the cause of the issue. I took a closer look at SDA/SCL pins initialization code generated by CubeMX:

void HAL_I2C_MspInit(I2C_HandleTypeDef* hi2c)
{

GPIO_InitTypeDef GPIO_InitStruct;
if(hi2c->Instance==I2C1)
{
/* USER CODE BEGIN I2C1_MspInit 0 */

/* USER CODE END I2C1_MspInit 0 */

/**I2C1 GPIO Configuration
PB6     ------> I2C1_SCL
PB7     ------> I2C1_SDA
*/
GPIO_InitStruct.Pin = GPIO_PIN_6|GPIO_PIN_7;
GPIO_InitStruct.Mode = GPIO_MODE_AF_OD;
GPIO_InitStruct.Speed = GPIO_SPEED_FREQ_LOW;
HAL_GPIO_Init(GPIOB, &GPIO_InitStruct);

/* Peripheral clock enable */
__HAL_RCC_I2C1_CLK_ENABLE();

/* USER CODE BEGIN I2C1_MspInit 1 */

/* USER CODE END I2C1_MspInit 1 */
}

}


I changed it to put clock enable before HAL_GPIO_Init() invocation and now my I2C communications works (at least I didn't noticed anything weird yet).

Finally, my question is - is there any better solution for this? CubeMX places clock enable code after the GPIO init method invocation. I can stay with two invocations of __HAL_RCC_I2C1_CLK_ENABLE(), but that's quite ugly in my opinion, so I am looking for any better solution, either software or hardware.

Device is STM32F100RB on STM32VLDiscovery board (with STLink v1), in case that matters.

• Note, the correct errata sheet is located at st.com/content/ccc/resource/technical/document/errata_sheet/a9/… Which is for the STM32F100x4/6/8/B which is what you have. Section 2.10.7 deals with the BUSY lock. For whatever reason people are posting errata from the 10xxC/D/E and the 10(1-3)8/B chips. Yes, sometimes all the chips inside a series will have the same errata, but you can not count on that. – GB - AE7OO Dec 6 '19 at 7:59

In my opinion STM32CubeMX code should not be considered as a ready to use code, but some as an example you can start with. With most of the microcontrollers it works, but there are some rare cases when it is not.

If you know it is not working and you have found the solution as well, you do not have to stick to the original code. In your case you can omit the __HAL_RCC_I2C1_CLK_ENABLE() call after the GPIO initialization, and leave the one before it. If it works, and you have said it works, then use the working way. Even ST's software can have bugs.

You are using an official board so the hardware should be OK, but you can check if the pull-up resistor values are correct. Or if a slave device does something during the initialization.

The best would be to run your code with everything disconnected from the Discovery (apart from the pull-ups), and check if it is still stuck in busy. If yes, it is fine if you replace that line in the generated code. It is not that big modification.

Unfortunately there is not any I2C exapmle in the STM32CubeF1 example package (this is not the code generator), under the STM32Cube_FW_F1_V1.4.0\Projects\STM32VL-Discovery\Examples. But if you check the MspInit functions of the UART or SPI. The clocks are enabled in both of them before the GPIO init.

void HAL_SPI_MspInit(SPI_HandleTypeDef *hspi)
{
GPIO_InitTypeDef  GPIO_InitStruct;

if (hspi->Instance == SPIx)
{
/*##-1- Enable peripherals and GPIO Clocks #################################*/
/* Enable GPIO TX/RX clock */
SPIx_SCK_GPIO_CLK_ENABLE();
SPIx_MISO_GPIO_CLK_ENABLE();
SPIx_MOSI_GPIO_CLK_ENABLE();
/* Enable SPI clock */
SPIx_CLK_ENABLE();

/*##-2- Configure peripheral GPIO ##########################################*/
/* SPI SCK GPIO pin configuration  */
GPIO_InitStruct.Pin       = SPIx_SCK_PIN;
GPIO_InitStruct.Mode      = GPIO_MODE_AF_PP;
GPIO_InitStruct.Pull      = GPIO_PULLDOWN;
GPIO_InitStruct.Speed     = GPIO_SPEED_FREQ_LOW;
HAL_GPIO_Init(SPIx_SCK_GPIO_PORT, &GPIO_InitStruct);


void HAL_UART_MspInit(UART_HandleTypeDef *huart)
{
GPIO_InitTypeDef  GPIO_InitStruct;

/*##-1- Enable peripherals and GPIO Clocks #################################*/
/* Enable GPIO TX/RX clock */
USARTx_TX_GPIO_CLK_ENABLE();
USARTx_RX_GPIO_CLK_ENABLE();

/* Enable USARTx clock */
USARTx_CLK_ENABLE();

/*##-2- Configure peripheral GPIO ##########################################*/
/* UART TX GPIO pin configuration  */
GPIO_InitStruct.Pin       = USARTx_TX_PIN;
GPIO_InitStruct.Mode      = GPIO_MODE_AF_PP;
GPIO_InitStruct.Pull      = GPIO_PULLUP;
GPIO_InitStruct.Speed     = GPIO_SPEED_FREQ_HIGH;

HAL_GPIO_Init(USARTx_TX_GPIO_PORT, &GPIO_InitStruct);


So I think your solution is perfectly fine.

• I have checked that behavior with no other devices than Discovery board on SDA and SCL lines, still BUSY :( Your last argument is quite solid, really have no thing to object :) Thanks. – Alexey Malev Dec 1 '16 at 10:44
• I am not using ST's HAL library, but I run into the same trouble - the busy flag always ON. And I can confirm that initializing GPIO after I2C clock really helped. – klasyc Feb 14 '17 at 10:58
• Same here, line remains busy, on an stm32 f103 "blue pill". Fixed by initializing the clock first. – kalmiya Feb 21 '18 at 22:01

Here is some code that might help you out. Basically, it's a realization of the Errata sheet (section 2.14.7) mentioned in a previous answer. I'm using the HAL library, and there are some references to the IKS01A1 driver header definitions (my periferal with the problem was the gyro on that board).

/* USER CODE BEGIN 1 */
/**
1. Disable the I2C peripheral by clearing the PE bit in I2Cx_CR1 register.
2. Configure the SCL and SDA I/Os as General Purpose Output Open-Drain, High level
(Write 1 to GPIOx_ODR).
3. Check SCL and SDA High level in GPIOx_IDR.
4. Configure the SDA I/O as General Purpose Output Open-Drain, Low level (Write 0 to
GPIOx_ODR).
5. Check SDA Low level in GPIOx_IDR.
6. Configure the SCL I/O as General Purpose Output Open-Drain, Low level (Write 0 to
GPIOx_ODR).
7. Check SCL Low level in GPIOx_IDR.
8. Configure the SCL I/O as General Purpose Output Open-Drain, High level (Write 1 to
GPIOx_ODR).
9. Check SCL High level in GPIOx_IDR.
10. Configure the SDA I/O as General Purpose Output Open-Drain , High level (Write 1 to
GPIOx_ODR).
11. Check SDA High level in GPIOx_IDR.
12. Configure the SCL and SDA I/Os as Alternate function Open-Drain.
13. Set SWRST bit in I2Cx_CR1 register.
14. Clear SWRST bit in I2Cx_CR1 register.
15. Enable the I2C peripheral by setting the PE bit in I2Cx_CR1 register.
**/
void HAL_I2C_ClearBusyFlagErrata_2_14_7(I2C_HandleTypeDef *hi2c) {

static uint8_t resetTried = 0;
if (resetTried == 1) {
return ;
}
uint32_t SDA_PIN = NUCLEO_I2C_EXPBD_SDA_PIN;
uint32_t SCL_PIN = NUCLEO_I2C_EXPBD_SCL_PIN;
GPIO_InitTypeDef GPIO_InitStruct;

// 1
__HAL_I2C_DISABLE(hi2c);

// 2
GPIO_InitStruct.Pin = SDA_PIN|SCL_PIN;
GPIO_InitStruct.Mode = GPIO_MODE_OUTPUT_OD;
GPIO_InitStruct.Pull = GPIO_NOPULL;
GPIO_InitStruct.Speed = GPIO_SPEED_FAST;
HAL_GPIO_Init(GPIOB, &GPIO_InitStruct);

HAL_GPIO_WRITE_ODR(GPIOB, SDA_PIN);
HAL_GPIO_WRITE_ODR(GPIOB, SCL_PIN);

// 3
GPIO_PinState pinState;
if (HAL_GPIO_ReadPin(GPIOB, SDA_PIN) == GPIO_PIN_RESET) {
for(;;){}
}
if (HAL_GPIO_ReadPin(GPIOB, SCL_PIN) == GPIO_PIN_RESET) {
for(;;){}
}

// 4
GPIO_InitStruct.Pin = SDA_PIN;
HAL_GPIO_Init(GPIOB, &GPIO_InitStruct);

HAL_GPIO_TogglePin(GPIOB, SDA_PIN);

// 5
if (HAL_GPIO_ReadPin(GPIOB, SDA_PIN) == GPIO_PIN_SET) {
for(;;){}
}

// 6
GPIO_InitStruct.Pin = SCL_PIN;
HAL_GPIO_Init(GPIOB, &GPIO_InitStruct);

HAL_GPIO_TogglePin(GPIOB, SCL_PIN);

// 7
if (HAL_GPIO_ReadPin(GPIOB, SCL_PIN) == GPIO_PIN_SET) {
for(;;){}
}

// 8
GPIO_InitStruct.Pin = SDA_PIN;
HAL_GPIO_Init(GPIOB, &GPIO_InitStruct);

HAL_GPIO_WRITE_ODR(GPIOB, SDA_PIN);

// 9
if (HAL_GPIO_ReadPin(GPIOB, SDA_PIN) == GPIO_PIN_RESET) {
for(;;){}
}

// 10
GPIO_InitStruct.Pin = SCL_PIN;
HAL_GPIO_Init(GPIOB, &GPIO_InitStruct);

HAL_GPIO_WRITE_ODR(GPIOB, SCL_PIN);

// 11
if (HAL_GPIO_ReadPin(GPIOB, SCL_PIN) == GPIO_PIN_RESET) {
for(;;){}
}

// 12
GPIO_InitStruct.Pin = SDA_PIN|SCL_PIN;
GPIO_InitStruct.Mode = GPIO_MODE_AF_OD;
GPIO_InitStruct.Alternate = NUCLEO_I2C_EXPBD_SCL_SDA_AF;
HAL_GPIO_Init(GPIOB, &GPIO_InitStruct);

// 13
hi2c->Instance->CR1 |= I2C_CR1_SWRST;

// 14
hi2c->Instance->CR1 ^= I2C_CR1_SWRST;

// 15
__HAL_I2C_ENABLE(hi2c);

resetTried = 1;
}

void HAL_GPIO_WRITE_ODR(GPIO_TypeDef* GPIOx, uint16_t GPIO_Pin)
{
/* Check the parameters */
assert_param(IS_GPIO_PIN(GPIO_Pin));

GPIOx->ODR |= GPIO_Pin;
}


Just another thing to consider: In this ERRATA document (page 24) you can find there is a glitch in I2C analog filter which can cause BUSY flag hanging. There is also a workaround you can try - it works for me.

See eratta sheet: eratta sheet

Workaround : The SCL and SDA analog filter output is updated after a transition occurs on the SCL and SDA line respectively. The SCL and SDA transition can be forced by software configuring the I2C I/Os in output mode. Then, once the analog filters are unlocked and output the SCL and SDA lines level, the BUSY flag can be reset with a software reset, and the I2C can enter master mode. Therefore, the following sequence must be applied:

1. Disable the I2C peripheral by clearing the PE bit in I2Cx_CR1 register.
2. Configure the SCL and SDA I/Os as General Purpose Output Open-Drain, High level (Write 1 to GPIOx_ODR).

3. Check SCL and SDA High level in GPIOx_IDR.

4. Configure the SDA I/O as General Purpose Output Open-Drain, Low level (Write 0 to GPIOx_ODR).
5. Check SDA Low level in GPIOx_IDR.
6. Configure the SCL I/O as General Purpose Output Open-Drain, Low level (Write 0 to GPIOx_ODR).
7. Check SCL Low level in GPIOx_IDR.
8. Configure the SCL I/O as General Purpose Output Open-Drain, High level (Write 1 to GPIOx_ODR).
9. Check SCL High level in GPIOx_IDR.
10. Configure the SDA I/O as General Purpose Output Open-Drain , High level (Write 1 to GPIOx_ODR).
11. Check SDA High level in GPIOx_IDR.
12. Configure the SCL and SDA I/Os as Alternate function Open-Drain.
13. Set SWRST bit in I2Cx_CR1 register.
14. Clear SWRST bit in I2Cx_CR1 register.
15. Enable the I2C periphe ral by setting the PE bit in I2Cx_CR1 register.

I got the same problem on STM32F429, using cube V1.15.0.

Still I noticed that on soft reset (when debugging for example), SCL goes LOW just after initializing the HAL_GPIO_Init() call.

I tried a bus reset by sending 16 clock at init, according to i2c-bus.org recommandation.

But it did not help. Adding you "reset" code resolved the trick :

__HAL_RCC_I2C1_FORCE_RESET();
HAL_Delay(2);
__HAL_RCC_I2C1_RELEASE_RESET();


After some tests I found out that a 2 ms delay is enough. I kept the manual clock reinitialization because a transmission can hang when resetting the CPU.