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
// [...]
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
#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