topic Re: LPC802 SPI Code (lpcxpresso802_spi_interrupt_master) in LPC Microcontrollers

LPC802 SPI Code (lpcxpresso802_spi_interrupt_master)

Babycat — Wed, 30 Dec 2020 06:48:06 GMT

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.

Re: LPC802 SPI Code (lpcxpresso802_spi_interrupt_master)

Alice_Yang — Mon, 04 Jan 2021 07:01:48 GMT

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

Re: LPC802 SPI Code (lpcxpresso802_spi_interrupt_master)

Babycat — Mon, 04 Jan 2021 08:54:54 GMT

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

Re: LPC802 SPI Code (lpcxpresso802_spi_interrupt_master)

Alice_Yang — Mon, 04 Jan 2021 09:06:30 GMT

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

Re: LPC802 SPI Code (lpcxpresso802_spi_interrupt_master)

Babycat — Mon, 04 Jan 2021 15:08:37 GMT

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.

Re: LPC802 SPI Code (lpcxpresso802_spi_interrupt_master)

Alice_Yang — Tue, 05 Jan 2021 01:47:39 GMT

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

Re: LPC802 SPI Code (lpcxpresso802_spi_interrupt_master)

Babycat — Tue, 05 Jan 2021 05:52:27 GMT

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.