STM32 Standard Peripheral Library (SPL) - Modify SPI Configuration

Question

I am using the STM32 SPL library with an STM32F103C8 microcontroller. As part of the project, I am using the SPI1 peripheral to interface with an SD card.

Initially the SPI speed must be kept under 400 kHz for the SD card. My SPI1 initialization code is

SPI_InitTypeDef spi;

//ENABLE THE CLOCK TO SPI1 PERIPHERAL
RCC_APB2PeriphClockCmd(RCC_APB2Periph_SPI1, ENABLE);

//SET SPI SPEED TO LOW SPEED MODE (~282KHZ)
spi.SPI_BaudRatePrescaler = SPI_BaudRatePrescaler_256;
spi.SPI_CPHA = SPI_CPHA_1Edge;
spi.SPI_CPOL = SPI_CPOL_Low;
spi.SPI_DataSize = SPI_DataSize_8b;
spi.SPI_Direction = SPI_Direction_2Lines_FullDuplex;
spi.SPI_FirstBit = SPI_FirstBit_MSB;
spi.SPI_Mode = SPI_Mode_Master;
spi.SPI_NSS = SPI_NSS_Soft;

SPI_Init(SPI1, &spi);
SPI_Cmd(SPI1, ENABLE);

Of course, once the initial initialization of the SD card is complete, I would like to speed up the SPI1 peripheral.

I could not find any function in the SPL that would return a structure of the type SPI_InitTypeDef containing the currently configured settings of an SPI peripheral.

Does that mean I have to create a new such structure (and repeat all the configuration values even if I only wish to change the speed and keep the rest the same as already configured) and then call SPI_Init() with this new structure?

Answer 1

If you find the SPI_CR1 register in the Reference Manual, you can see that the speed is set by the BR bits (bits 5:3 in SPI_CR1). You can adjust the baud rate on the fly by changing these bits, as long as there isn't a transmission currently in progress:

Conveniently, the SPI_BaudRatePrescaler defines provided by the Standard Peripheral Library map directly on top of the SPI_CR1 bits.

What I do is create the following #defines:

#define BAUD_RATE_BITMASK    0xFFC7                // Used to clear CR1->BR[2:0]
#define SPI_SPEED_LOW    SPI_BaudRatePrescaler_32  // 72/32 = 2.25MHz
#define SPI_SPEED_HIGH   SPI_BaudRatePrescaler_4   // 72/4 = 18MHz

Then I use something like this to adjust the speed (where NewSpeed is one of the SPL defines):

void spiSetSpeed(uint16_t NewSpeed)
{
    // Assert the new speed setting
    SPI1->CR1 &= BAUD_RATE_BITMASK;                // Clear the BR[2:0] bits
    SPI1->CR1 = (uint16_t)(SPI1->CR1 | NewSpeed);  // Set the bits
}