LPC802 SPI Code (lpcxpresso802_spi_interrupt_master)

[Babycat]

Hello Everyone

I am playing around with SPI on LPCXpresso802 board. Everything seems OK but there is an unclear code that I try to understand but I could not configure out. 

It is in example: lpcxpresso802_spi_interrupt_master.

As the Data transfer Length is setup with 8 bits width, so RXDAT and TXDAT will be handle each single byte. but why does it need casting type (uint16_t) as below? 

I have connected to external SPI chip (AS3933), if no casting type, then read command sometime did not get right value.

Here is the origial interrupt code

void SPI_MASTER_IRQHandler(void)
{
/* Check if data is ready in RX register. */
if ((SPI_GetStatusFlags(EXAMPLE_SPI_MASTER) & kSPI_RxReadyFlag))
{
rxBuffer[BUFFER_SIZE - rxIndex] = SPI_ReadData(EXAMPLE_SPI_MASTER);
rxIndex--;
}
/* Write data to TX regsiter if TX register is ready. */
if ((SPI_GetStatusFlags(EXAMPLE_SPI_MASTER) & kSPI_TxReadyFlag) && (txIndex != 0U))
{
/* If this is the last byte to send. */
if (1U == txIndex)
{
/* Add end of transfer configuration. */
SPI_WriteConfigFlags(EXAMPLE_SPI_MASTER, kSPI_EndOfTransfer);
SPI_WriteData(EXAMPLE_SPI_MASTER, txBuffer[BUFFER_SIZE - txIndex]);
}
else
{
SPI_WriteData(EXAMPLE_SPI_MASTER, (uint16_t)(txBuffer[BUFFER_SIZE - txIndex]));
}
txIndex--;
}
/* If no data to be transferred, disable the interrupt. */
if ((txIndex == 0U) && (rxIndex == 0U))
{
slaveFinished = true;
SPI_DisableInterrupts(EXAMPLE_SPI_MASTER, kSPI_RxReadyInterruptEnable);
}
__DSB();
}

Thank you.

[Alice_Yang]

Hello Babycat,

Because the DATA in SPI transmitter data register is 16 bits, so no matter the

data transfer length is, we need (uint16_t) as you said.

Regards,

Alice

[Babycat]

Hi Yang

Thanks for your reply, 

then the question is, why dont we cast it in the if clause?

if (1U == txIndex)
{
/* Add end of transfer configuration. */
SPI_WriteConfigFlags(EXAMPLE_SPI_MASTER, kSPI_EndOfTransfer);
SPI_WriteData(EXAMPLE_SPI_MASTER, txBuffer[BUFFER_SIZE - txIndex]);
}
else
{
SPI_WriteData(EXAMPLE_SPI_MASTER, (uint16_t)(txBuffer[BUFFER_SIZE - txIndex]));
}

Regards

[Alice_Yang]

Hello,

I think it will be ok if we also cast it here  (uint16_t)( txBuffer[BUFFER_SIZE - txIndex]). 

This is the last byte to send, so there should no error even though not cast it.

Regards,

Alice

[Babycat]

Hi Alice

TXDAT is a 32bit register, 

what could be the difference btw: 

TXDAT = 0xFF

and

TXDAT = (uint16_t)(0xFF) = 0x00FF

I am not so clear why casting does its job there? I might be missing something...

Thank you.

[Alice_Yang · Accepted solution]

Hello Babycat,

- TXDAT is 32 bits register, while the 31:16 are reserved, only 15:0 is  DATA part.

-"what could be the difference btw: 

TXDAT = 0xFF

and

TXDAT = (uint16_t)(0xFF) = 0x00FF

"

To some machine, when there is no casting, maybe add random data in 15:8, for example 0x10FF, not 0X00FF.

So for rigorous，all the data put to DATA need casting to 16 bits.

Regards,

Alice

[Babycat]

 Hi Alice

Oh, I understood. I did not know about that, I thought C complier will auto add that extra 00 as its job supposed to.

by the way, in case TXDAT = 0x10FF. If I configured Len = 8 bits, MSB first.

Which byte will be sent out, 10 or FF? 

Thank you.