Driver SPI

SPI Clock Frequency

Source: Mbed ST Wiki / SPI output clock frequency

Based on the source documentation, we are able to define the SPI clock frequency for the STM32F7 family of MCUs:

• from spi_api.c (diff is used to better highlight information)

// [...]

case SPI_6:
    /* SPI_1, SPI_4, SPI_5 and SPI_6. Source CLK is PCKL2 */
    spi_hz = HAL_RCC_GetPCLK2Freq();
    break;
case SPI_2:
case SPI_3:
    /* SPI_2 and SPI_3. Source CLK is PCKL1 */
    spi_hz = HAL_RCC_GetPCLK1Freq();
    break;

// [...]

• from system_clock.c (diff is used to better highlight information)

#if ((CLOCK_SOURCE) & USE_PLL_HSI) // <-- we are using USE_PLL_HSI

// [...]

RCC_ClkInitStruct.APB1CLKDivider = RCC_HCLK_DIV4;           //  54 MHz
RCC_ClkInitStruct.APB2CLKDivider = RCC_HCLK_DIV2;           // 108 MHz

// [...]

Clock	PCKL1	PCKL2
SPI	SPI_1, SPI_4, SPI_5, SPI_6	SPI_2, SPI_3
Base Clock	54 MHz	108 MHz
Scalar	PCKL1	PCKL2
2	27 MHz	54 MHz
4	13.5 MHz	27 MHz
8	6.75 MHz	13.5 MHz
16	3.38 MHz	6.75 MHz
32	1.69 MHz	3.38 MHz
64	843.75 kHz	1.69 MHz
128	421.88 kHz	843.75 kHz
256	210.94 kHz	421.88 kHz

Now if you want to have a SPI output frequency around 1.5 MHz, you will need to pass a value above 1.69 MHz and below 3.38 MHz when you call the SPI frequency method.

SPI mydevice(SPI_MOSI, SPI_MISO, SPI_SCK);
mydevice.frequency(1800000); // real value = 1.69 MHz