STM32F4 I2C SDA & SCL rise time

[gil23]

Hi,

I working with STM32F401RET6 64 pins Nucleo board with IAR.

According to the datasheet DM00086815 Section 6.3.19 SDA and SCL rise time = 300ns.

I open the project from “STM32Cube_FW_F4_V1.25.0�? STM32F410xx-Nucleo\Examples\I2C\I2C_TwoBoards_ComPolling.

I change the clock setting to fit my board to get: SYSCLK(Hz) = 84MHz & Fpclk1 = 42MHz.

I connect my probes direct to board pins PB6 & PB9.

Then I measured and I get that  SDA and SCL rise time = 1.2us.

Attached oscilloscope picture.

I tried to change configuration to  “GPIO_InitStruct.Speed    = GPIO_SPEED_HIGH�? but I didn’t see any change.

Please advise.

The configuration of the clock that I change :

 RCC_OscInitStruct.OscillatorType = RCC_OSCILLATORTYPE_HSI;

 RCC_OscInitStruct.HSIState = RCC_HSI_ON;

 RCC_OscInitStruct.HSICalibrationValue = 16;

 RCC_OscInitStruct.PLL.PLLState = RCC_PLL_ON;

 RCC_OscInitStruct.PLL.PLLSource = RCC_PLLSOURCE_HSI;

 RCC_OscInitStruct.PLL.PLLM = 16;

 RCC_OscInitStruct.PLL.PLLN = 336;

 RCC_OscInitStruct.PLL.PLLP = RCC_PLLP_DIV4;

 RCC_OscInitStruct.PLL.PLLQ = 4;

 HAL_RCC_OscConfig(&RCC_OscInitStruct);

 RCC_ClkInitStruct.ClockType = RCC_CLOCKTYPE_SYSCLK|RCC_CLOCKTYPE_PCLK1;

 RCC_ClkInitStruct.SYSCLKSource = RCC_SYSCLKSOURCE_PLLCLK;

 RCC_ClkInitStruct.AHBCLKDivider = RCC_SYSCLK_DIV1;

 RCC_ClkInitStruct.APB1CLKDivider = RCC_HCLK_DIV2;

 RCC_ClkInitStruct.APB2CLKDivider = RCC_HCLK_DIV1;

 HAL_RCC_ClockConfig(&RCC_ClkInitStruct, FLASH_LATENCY_2);

[waclawek.jan]

I2C signals are driven by Open-Drain transistors, and their rise time is given by pullup resistors you are supposed to add externally.

The internal pullup is nominally cca 40kOhm, which may be sufficient only if the load is of low capacitance.

JW

[TDK]

The datasheet measurements are taken with a 4.7kOhm pullup resistor. Guessing you don't have any.

[gil23]

Thanks, it's woks,

I connect 4.7K pullup and the rise time is 100ns

[gil23]

Hi,

Problem didn't solve completely,

I connect another STM32F401RET6 board, and I configured it with the same project only as a slave.

I connect 4.7K pullup between master board to slave board.

the rise time is now 550ns.

Please advise.

Attached oscilloscope picture.

[TDK]

(ignore)

[waclawek.jan]

The risetime is simply given by RC constant. 550ns/4k7=cca 110pF, not an entirely unreasonable value, especially if you connect two boards with lengthy cables.

You can decrease the pullup's value, taking into consideration the maximum allowable pin current.

JW