Hello, we have a project which uses LPC55S28 MCU. We are trying to do an SPI communication where LPC55S28 is the slave and a custom hardware is master.
The communication is like this: total communication will last 2 bytes. Master sends 1 byte. Then slave will respond with 1 byte.
We are using the Flexcomm 8 which is the high speed SPI.
My SPI setup function looks like this:
void setup_spi() {
spi_slave_config_t slave_config;
SPI_SlaveGetDefaultConfig(&slave_config);
SPI_SlaveInit(SPI8, &slave_config);
NVIC_SetPriority(SPI_SLAVE_IRQ, 0U);
EnableIRQ(SPI_SLAVE_IRQ);
SPI_EnableInterrupts(SPI8, kSPI_RxLvlIrq);
}
This sets up so that we enter the interrupt every 1 byte.
And my interrupt function looks like this:
void SPI_SLAVE_IRQHandler() {
if (!(SPI_GetStatusFlags(SPI8) & kSPI_RxNotEmptyFlag)) {
return;
}
// Currently I am not doing anything with the incoming message.
uint16_t message = (uint16_t)(SPI_ReadData(SPI8) & 0xFFFF);
// This response will appear not 1 byte later, but 2 bytes later
SPI_WriteData(SPI8, 0xAB, 0);
SDK_ISR_EXIT_BARRIER;
}
The behavior I observe is different than what I expect: when I start a 2 bytes communication on master, I can observe that the interrupt is called 2 times, which is what I expect, but I read 0xFFFF from the slave. The next times I start another 2 bytes communication, I receive 0xABAB because the TX FIFO is already filled with two 0xAB bytes.
What I expect is to receive something like 0xFFAB. So half of the response would be empty, because slave still didn't get into the interrupt yet and TX FIFO is empty, but after receiving one byte, it goes into the interrupt, 0xAB is written to the TX FIFO, so that slave sends 0xAB when reading the second byte.
As a bonus, I started a 4 byte communication from master (by lowering chip select line for 4 bytes). I can see that the interrupt is entered 4 times. But the response I get is 0xFFFFABAB. Instead of 0xFFABFFAB.
It is as if SPI_WriteData is not fast enough. Or maybe one of my settings is not correct? What do you think is the problem here?
