The SPI output frequency can only be equal to some values. This is due because the SPI output frequency is divided by a prescaler which is equal to 2, 4, 8, 16, 32, 64, 128 or 256. It depends also on SPI input clock frequency.

To illustrate this, we take the NUCLEO_F103RB as example. Follow these steps to know the available SPI1 output frequencies:

1) Find in the device Reference Manual which input clock is used for SPI1.

You can also look in the spi_api.c file (function spi_get_clock_freq). You can see that PCLK2 clock is used for SPI1.

2) Check in system_clock.c file (function SetSysClock_PLL_HSE used per default for this board) which value has PCLK2 clock.

This is given by the line:

RCC_ClkInitStruct.APB2CLKDivider = RCC_HCLK_DIV1;           // 72 MHz

Now you know that SPI1 input clock is equal to 72 MHz.

3) Knowing the SPI input clock, you can only have these SPI output frequencies:

• 36 MHz (72 MHz / 2)
• 18 MHz (72 MHz / 4)
• 9 MHz (72 MHz / 8)
• 4.5 MHz (72 MHz / 16)
• 2.25 MHz (72 MHz / 32)
• 1.125 MHz (72 MHz / 64)
• 562 kHz (72 MHz / 128)
• 281 kHz (72 MHz / 256)

4) If you want to have a SPI output frequency around 1 MHz, you will need to pass a value above 1.125 MHz and below 2.25 MHz when you call the SPI frequency method.

SPI mydevice(SPI_MOSI, SPI_MISO, SPI_SCK);
mydevice.frequency(1200000); // real value = 1.125 MHz

5) If you want to have other output frequencies, you will have to modify the different clock dividers inside the system_clock.c file. But note that will also have an impact on other peripherals. Read the device reference manual for more information about clock tree.