Do any STM32 devices support SPI communication with variable data frame length?

[Todor Todorov]

Hi everyone,

I have a SPI eeprom on my hands, which has a weird communication protocol. Data is 8 bits wide, except for the 1st frame, which contains 1 start bit, 2 op-code bits, and 9 address bits - so 12 in total. It does not accept this first frame as 2x 8bit transfers - start bit and op codes need to be the MSB part of a 12 clock pulses frame.

While looking at various F0xx and F4xx data sheets, I noticed that there is NO explicit note about changing the DS bits in the SPI CR2 register only when SPI is disabled. So I decided to see if changing this parameter after the first frame would work... Needless to say, it does not do what I expected.

I tested on a STM32F070F6 Cortex-M0 MCU.

Here is my SPI initialization:

static void MX_SPI1_Init(void)
{
 /* SPI1 parameter configuration*/
 hspi1.Instance = SPI1;
 hspi1.Init.Mode = SPI_MODE_MASTER;
 hspi1.Init.Direction = SPI_DIRECTION_2LINES;
 hspi1.Init.DataSize = SPI_DATASIZE_8BIT;
 hspi1.Init.CLKPolarity = SPI_POLARITY_LOW;
 hspi1.Init.CLKPhase = SPI_PHASE_1EDGE;
 hspi1.Init.NSS = SPI_NSS_SOFT;
 hspi1.Init.BaudRatePrescaler = SPI_BAUDRATEPRESCALER_16;
 hspi1.Init.FirstBit = SPI_FIRSTBIT_MSB;
 hspi1.Init.TIMode = SPI_TIMODE_DISABLE;
 hspi1.Init.CRCCalculation = SPI_CRCCALCULATION_DISABLE;
 hspi1.Init.CRCPolynomial = 7;
 hspi1.Init.CRCLength = SPI_CRC_LENGTH_DATASIZE;
 hspi1.Init.NSSPMode = SPI_NSS_PULSE_ENABLE;
 if (HAL_SPI_Init(&hspi1) != HAL_OK)
 {
   Error_Handler();
 }
}

This was produced by CubeMX. The next snippet is my code to constantly send some test data, so I can capture it with a logic analyzer:

uint8_t data[8] = { 0xaa, 0x55, 0xaa, 0x55, 0xaa, 0x55, 0xaa, 0x55 };
uint16_t spiReg = hspi1.Instance->CR2;
uint8_t cmd[2] = { 0x0c, 0x00 }; // start bit and op-code, address shall remain 0
while (1)
 {
    HAL_GPIO_WritePin(SPI1_NSS_GPIO_Port, SPI1_NSS_Pin, 0); // chip select low
    hspi1.Init.DataSize = SPI_DATASIZE_12BIT;
    spiReg = (spiReg & ~SPI_DATASIZE_16BIT) | SPI_DATASIZE_12BIT;
    hspi1.Instance->CR2 = spiReg;
    HAL_SPI_Transmit(&hspi1, cmd, 2, 5000);
 
    HAL_Delay(2);
 
    spiReg = (spiReg & ~SPI_DATASIZE_16BIT) | SPI_DATASIZE_8BIT;
    hspi1.Init.DataSize = SPI_DATASIZE_8BIT;
    hspi1.Instance->CR2 = spiReg;
    HAL_SPI_Transmit(&hspi1, data, sizeof(data), 5000);
    HAL_GPIO_WritePin(SPI1_NSS_GPIO_Port, SPI1_NSS_Pin, 1); // chip select high
 
    HAL_Delay(2000);
 }

As you can see, the peripheral is initialized with 8bit data frame. When I attempt the switch to 12 bit frames "on the fly", the switching works, but the peripheral is sending the command bytes as 2 separate 12bit frames - my expectation was to see 1 frame with last 4 bits of the first byte and the 8 bits of the second byte.

And then when I attempt to switch back to 8 bit frames, it does not work. It continues using 12bit frames.

Is my code wrong somehow? Or is what I need not possible with STM32 HAL? Would it be possible if I use the registers directly?

Any help would be valued and appreciated! Thx!

[S.Ma]

I used dynamic bit length, you can change it when the spi is idle, no need to be overconservative for prototyping.

[TDK]

> uint16_t spiReg = hspi1.Instance->CR2;

> spiReg = (spiReg & ~SPI_DATASIZE_16BIT) | SPI_DATASIZE_12BIT;

This isn't doing anything except changing a variable in memory. It's not modifying an SPI register or changing the data width.

Perhaps you want this:

volatile uint16_t * spiReg = hspi1.Instance->CR2;
 
*spiReg = (spiReg & ~SPI_DATASIZE_16BIT) | SPI_DATASIZE_12BIT;

Or perhaps a little more readable and less prone to typos:

MODIFY_REG(hspi1.Instance->CR2, SPI_CR2_DS, SPI_DATASIZE_12BIT);

Also:

> uint8_t cmd[2] = { 0x0c, 0x00 }; // start bit and op-code, address shall remain 0

> HAL_SPI_Transmit(&hspi1, cmd, 2, 5000);

This transmits data past the end of the buffer. cmd should be uint16_t if it contains 12-bit values..

[S.Ma]

HAL does not provide access to all HW capabilities.... you need to dig down to hw register manipulation.

[TDK]

While true in general, in this case it absolutely does provide access to 9 bit word size.