Wrong SPI transfer in 16-bit mode on STM32F7xx #3445
Description
Description
- Type: Bug
- Priority: Blocker
Bug
Target
STM32F7xx
Toolchain:
GCC_ARM
mbed-cli version:
0.9.10
Expected behavior
The bug shows up when you try to send a single 16-bit 'word' over an SPI link which is configured in 16-bit mode:
// Configure the SPI in 16-bit mode
SPI spi(PA_7, PA_6, PA_5);
DigitalOut cs(PD_14);
cs = 1;
// Send regularly a single 16-bit 'word' over the link
while(true)
{
Thread::wait(40);
cs = 0;
int response = spi.write(0x8FA3); // Transfer one single 16-bit 'word'
cs = 1;
}
This is what you would expect to see on the oscilloscope:
Actual behavior
Unfortunately, the oscilloscope shows that after outputting the intended 16-bit word, the chip outputs a second one (equal to 0x0000) that should not be there:
Steps to reproduce
To reproduce the error, your main.cpp file should be the following code:
#include "mbed.h"
DigitalOut led1(LED1);
int main()
{
Serial logger(USBTX, USBRX);
logger.baud(115200);
logger.printf("Test application started\r\n");
SPI spi(PA_7, PA_6, PA_5);
DigitalOut cs(PD_14);
cs = 1;
spi.format(16, 0); // 16-bit mode, pol = 0, pha = 0
spi.frequency(2000000); // 2 Mhz SPI speed
int i = 0;
while (true) {
led1 = !led1;
Thread::wait(40);
cs = 0;
int response = spi.write(0x8FA3); // Transfer one single 16-bit 'word'
cs = 1;
}
}
Solution
I have traced the bug back to the file stm_spi_api.c . The function spi_master_write(..) is called to transfer the value. In turn, this function calls the ST HAL function HAL_SPI_TransmitReceive(..). One of the parameters handed over is << size >>.
Mbed-OS wrongly assumed that this parameter represents the "number of bytes" to be transmitted over the SPI link. However, this parameter actually represents the "number of words" to be transmitted. A single 16-bit value is just one "word" - assuming that your SPI is configured in 16-bit mode.
The function spi_master_write(..) should therefore be changed:
int spi_master_write(spi_t *obj, int value)
{
uint16_t size, Rx, ret;
struct spi_s *spiobj = SPI_S(obj);
SPI_HandleTypeDef *handle = &(spiobj->handle);
// ERROR: The << size >> parameter in the ST HAL_SPI_TransmitReceive(..)
// function does not represent the nr. of bytes but rather the
// number of words (a word is 1 byte in 8-bit mode and 2 bytes in
// 16-bit mode).
// size = (handle->Init.DataSize == SPI_DATASIZE_16BIT) ? 2 : 1;
size = 1;
/* Use 10ms timeout */
ret = HAL_SPI_TransmitReceive(handle,(uint8_t*)&value,(uint8_t*)&Rx,size,10);
if(ret == HAL_OK) {
return Rx;
} else {
DEBUG_PRINTF("SPI inst=0x%8X ERROR in write\r\n", (int)handle->Instance);
return -1;
}
}