SPI bytes swapped

[Bret Foreman]

I'm using Zephyr tool chain version 2.3.0. My chip is the nRF52832.

I'm writing two bytes out on the SPI bus. Byte[1] = 0X3b and Byte[0]=0x28

Here's what my spi_config looks like.

struct spi_cs_control spim_cs = {
	.gpio = SPI_CS_GPIOS_DT_SPEC_GET(DT_NODELABEL(reg_my_spi_master)),
	.delay = 0,
};
static const struct spi_config spi_cfg = {
	.operation = SPI_WORD_SET(8) | SPI_TRANSFER_MSB |
				 SPI_MODE_CPOL | SPI_MODE_CPHA,
	.frequency = 1000000,
	.slave = 0,
	.cs = &spim_cs,
};

But the bytes are coming out swapped with byte[0] first and then byte[1]. Here it is on the logic analyzer.

How can I fix this so that the bytes come out most-significant first, the same way the bits come out? I believe this is what is expected of the SPI bus in its normal operation so that you can shift out multi-byte words without have to scramble and unscramble the bytes.

Regards,

Bret

[Jørgen Holmefjord]

Hi,

The bytes are clocked out from the buffer to the SPI bus in the order that you have placed them in the buffer, starting with the byte at index 0. If you want to clock out 0X3b first, you need to place this at index 0 in the buffer, and place 0x28 at index 1 in the buffer.

Best regards,
Jørgen