RE: Reading a second register over SPI seemingly overwrites value from first register read.

wagoo — Sun, 07 Sep 2025 01:42:32 GMT

I still don't quite understand why but it appears to have been fixed by:

1. Transmitting the register + a dummy byte in read_reg() and,

2. Setting MSb of reg (this is how read operations are defined for ADXL345)

Of course also change the length of tx_buffer on line 7, like so:

static int read_reg(uint8_t reg, uint8_t *data, uint8_t size)
{
	int err;

	/* STEP 4.1 - Set the transmit and receive buffers */
	uint8_t tx_buffer[] = {0x80 | reg, 0x00};
	struct spi_buf tx_spi_buf			= {.buf = (void *)&tx_buffer, .len = sizeof(tx_buffer)};
	struct spi_buf_set tx_spi_buf_set 	= {.buffers = &tx_spi_buf, .count = 1};
	struct spi_buf rx_spi_bufs 			= {.buf = data, .len = size};
	struct spi_buf_set rx_spi_buf_set	= {.buffers = &rx_spi_bufs, .count = 1};

	/* STEP 4.2 - Call the transceive function */
	err = spi_transceive_dt(&spi, &tx_spi_buf_set, &rx_spi_buf_set);
	if (err < 0) {
		LOG_ERR("spi_transceive_dt() failed, err: %d", err);
		return err;
	}

	return 0;
}

RE: Reading a second register over SPI seemingly overwrites value from first register read.

Benjamin — Fri, 05 Sep 2025 11:58:30 GMT

Hi,
This seems strange. You’re seeing an unexpected register value from the first read, even before the second read starts? Could you capture a logic analyzer trace of this?

Also, how are DEVID and INT_SOURCE defined? Could you upload the source file?

Regards,
Benjamin