Sensors中的主题 Re: FXOS8700 SPI question

FXOS8700 SPI question

warr1 — Tue, 03 Feb 2026 21:22:32 GMT

I am want to use the FXOS8700 in SPI mode. I set the parameters of the SPI bus according to the FXOS8700 data sheet.

In order to get the FXOS into SPI mode, I tri-state the MISO pin before toggling the reset pin, and waiting the required time for the auto detection circuit to put the device into SPI mode.

void FXOS8700_GPIO_Init(void)
{
GPIO_InitTypeDef GPIO_InitStruct = {0};
__HAL_RCC_GPIOA_CLK_ENABLE();
GPIO_InitStruct.Pin = GPIO_PIN_MISO;
GPIO_InitStruct.Mode = GPIO_MODE_INPUT;
GPIO_InitStruct.Pull = GPIO_NOPULL;
GPIO_InitStruct.Speed = GPIO_SPEED_FREQ_LOW;
HAL_GPIO_Init(GPIOA, &GPIO_InitStruct);
HAL_Delay(10);
GPIO_InitStruct.Pin = GPIO_PIN_RST;
GPIO_InitStruct.Mode = GPIO_MODE_OUTPUT_PP;
GPIO_InitStruct.Pull = GPIO_NOPULL;
GPIO_InitStruct.Speed = GPIO_SPEED_FREQ_LOW;
HAL_GPIO_Init(GPIOA, &GPIO_InitStruct);
HAL_GPIO_WritePin(GPIOA, GPIO_PIN_RST, GPIO_PIN_SET);
HAL_Delay(1);
HAL_GPIO_WritePin(GPIOA, GPIO_PIN_RST, GPIO_PIN_RESET);
HAL_Delay(1);
}

Next, initialize the SPI bus.

void MX_SPI1_Init(void)
{
hspi1.Instance = SPI1;
hspi1.Init.Mode = SPI_MODE_MASTER;
hspi1.Init.Direction = SPI_DIRECTION_2LINES;
hspi1.Init.DataSize = SPI_DATASIZE_8BIT;
hspi1.Init.CLKPolarity = SPI_POLARITY_LOW;
hspi1.Init.CLKPhase = SPI_PHASE_1EDGE;
hspi1.Init.NSS = SPI_NSS_SOFT;
hspi1.Init.BaudRatePrescaler = SPI_BAUDRATEPRESCALER_128;
hspi1.Init.FirstBit = SPI_FIRSTBIT_MSB;
hspi1.Init.TIMode = SPI_TIMODE_DISABLE;
hspi1.Init.CRCCalculation = SPI_CRCCALCULATION_DISABLE;
hspi1.Init.CRCPolynomial = 10;
if (HAL_SPI_Init(&hspi1) != HAL_OK)
{
Error_Handler();
}
}

My STM32 only connects to the FXOS8700. Therefore, the cs_b of FXOS8700 is directly grounded. During the reading process, cs_b remains at a low voltage, so it is not operated in the code. I then attempt to read the WHO_AM_I register and get the value 0xff instead of 0xff which is what I would expect.

Below is the code I am using to reset the device and read the WHO_AM_I register:

unsigned char FXOS8700CQ_ReadRegister(uint8_t RegisterAddress)
{
uint8_t tx_data[3];
unsigned char rx_data[3];
tx_data[0] = (RegisterAddress & 0x7F);
tx_data[1] = (RegisterAddress & 0x80);
tx_data[2] = 0x00;
if (HAL_SPI_TransmitReceive(&hspi1, tx_data, rx_data, 3, HAL_MAX_DELAY) != HAL_OK)
{
return 0x01; 
}
return rx_data;
}

The final result is below：

rx_data[0] = 0xFF
rx_data[1] = 0xFF
rx_data[2] = 0xFF

Anybody else getting this problem?

Re: FXOS8700 SPI question

TomasVaverka — Thu, 19 Dec 2024 16:37:52 GMT

Hi,

First of all, please note that the FXOS8700CQ is EOL and not recommended for new designs.

Based on description, the first thing I would recommend to try is to toggle the CS_B pin between low and high to latch data during SPI communication. Grounding the CS_B pin directly may not work.

Next, as I am not familiar with STM MCUs, I cannot check your SPI initialization. Please double check that it matches the FXOS8700 requirements:
Clock Polarity (CPOL): 0 (idle low)
Clock Phase (CPHA): 0 (data captured on the leading edge of the clock)

Your read a register seems to be correct.

Do you have a logic analyzer or an oscilloscope to see what is going on the bus?

BRs, Tomas

Re: FXOS8700 SPI question

warr1 — Fri, 20 Dec 2024 03:21:19 GMT

Hi Tomas,I have revised CS_S according to your suggestion, and now it is completely controlled by the MCU. I still haven't received the correct signal yet. I captured the signal through a logic analyzer as follows:

Re: FXOS8700 SPI question

warr1 — Fri, 20 Dec 2024 03:35:24 GMT

During the reset process of FXOS8700, I set SA0 to a high impedance state. After resetting, int1 was detected and a reset signal can be detected. Is there any way to detect if FXOS8700 has switched to SPI mode?