How do I know if the SPI is correctly working, if I can only check on the master's board? - embedded

Question

I have an STM32F429ZI Nucleo board (for SPI master and UART to check everything's working alright) and an EVB-LAN9252-SPI board(for SPI slave).
I have to check if the SPI is correctly working, but it seems that I can't debug or check on the slave's side.
Shown below is the test code that I worked on the STM32F429ZI Nucleo board to check if the SPI is working correctly. SPI1 and SPI4 is configured in one board.
while (k < 32)
{
HAL_UART_Transmit(&huart4, &SPI1_Buffer_Tx[k], 1, 100);
k++;
}
k = 0;
while (k < 32)
{
HAL_GPIO_WritePin(GPIOE, GPIO_PIN_9, GPIO_PIN_RESET); // this GPIO is connected to hardware NSS
HAL_SPI_Transmit(&hspi1, &SPI1_Buffer_Tx[k], 1, 100);
HAL_SPI_Receive(&hspi4, &SPI4_Buffer_Rx[k], 1, 100);
HAL_GPIO_WritePin(GPIOE, GPIO_PIN_9, GPIO_PIN_SET);
k++;
}
k = 0;
while (k < 32)
{
HAL_UART_Transmit(&huart4, &SPI1_Buffer_Tx[k], 1, 100);
k++;
}
k = 0;
while (k < 32)
{
HAL_UART_Transmit(&huart4, &SPI4_Buffer_Rx[k], 1, 100);
k++;
}
In this case the UART shows me such answer
abcdefghijklmnopqrstuvwxyzABCDEF //what was originally in the transmit buffer
bcdefghijklmnopqrstuvwxyzABCDEF //what was received in the receive buffer
Maybe this was possible because I could read on the slave's side, with such code
HAL_SPI_Receive(&hspi4, &SPI4_Buffer_Rx[k], 1, 100);
Now back to the original project.
At first I assumed that the data transmitted from the master should circulate in the slave somehow and transmit back to the master, so that if I read from the master I should get the original data, but in backwards.
so this was the code.
while (k < 32)
{
HAL_GPIO_WritePin(GPIOD, GPIO_PIN_14, GPIO_PIN_RESET);
HAL_SPI_Transmit(&hspi1, &SPI1_Buffer_Tx[k], 1, 100);
HAL_SPI_Receive(&hspi1, &SPI1_Buffer_Rx[k], 1, 100);
HAL_GPIO_WritePin(GPIOD, GPIO_PIN_14, GPIO_PIN_SET);
k++;
}
and what I received from the master is 32 0xFFs.
I'm not sure where I'm wrong about.
Does the data actually circulate in the slave's side and is it just me doing something wrong?
The data seems to be correctly transmitted, but the slave hasn't been ordered to transmit anything back to the master. That's why I can't receive meaningful data from the master.
2-1. If so, how do I know that the slave has received the data correctly?
2-2. How do I order the slave to transmit back to the master some meaningful data? I can only debug my code on the master's board.

Answer 1

Thanks for your kind comments!
It took a whole month for me to get this work done, and I'm sure there'll be newbies like me in the future, so..
As in the LAN9252 Datasheet page 302/329 and 303/329, you can send "0x0050" to receive "0x92520001", and "0x0064" to receive "0x87654321".
the way you send the data is kinda delicate, and that was the part I kept failing. I was (and am) not fully understanding the principles of SPI communication and you're gonna need an oscilloscope with multiple probes(It took me only 2 days after using the oscilloscope, because before then I couldn't check what I was doing).