Transmitting 16 bit at once over SPI on nRF51-DK

[stephbur]

Forgive me if this is a stupid question, but I am new to programming microcontrollers other than Arduino and SPI in general.

I have a DAC that has a 16-bit register and I need to write to it using SPI. If I try to transfer 16 bits, only the second byte is transferred. For example, if I transfer 0xAAFF, 0xFF will be transferred and the second byte transferred is always 0x05, so the payload read off my oscilloscope is 0xFF05. Even if I transfer 0x0000, the total payload transferred will be 0x0005.

Everything works as expected if I set the buffer length to 1 and only send one byte.

How can I transmit 16 bit without this 0x05 coming up?

Here is my code

uint32_t err_code;
static const nrf_drv_spi_t m_spi_master_0 = NRF_DRV_SPI_INSTANCE(0);
nrf_drv_spi_config_t spi_config = NRF_DRV_SPI_DEFAULT_CONFIG;
spi_config.ss_pin = slave_select;
spi_config.miso_pin = -1;
spi_config.mosi_pin = sdo;
spi_config.sck_pin  = serial_clock;
nrf_gpio_pin_write(slave_select, LOW);
spi_config.frequency = NRF_DRV_SPI_FREQ_500K;
spi_config.mode = NRF_DRV_SPI_MODE_0;
err_code = nrf_drv_spi_init(&m_spi_master_0, &spi_config, NULL);
uint8_t test_payload = 43775; //  0xAAFF
err_code =  nrf_drv_spi_transfer(&m_spi_master_0, &test_payload, 2, NULL, 2); 
// results in 0xFF05 and err_code is NRF_SUCCESS

[RK]

uint8_t test_payload = 43775; 

that's not going to work very well is it, 43775 doesn't fit into a uint8_t. So you're getting one byte of real data and one byte of random garbage off the stack.

If you want to send 16 bit then use a uint16_t to store it and cast the pointer on send.

[stephbur]

Thank you for your answer, that could be indeed the problem. But apparently the transfer function only accepts uint8_t. Do i have to rewrite transfer in the SDK to accept 16bit integers?

[RK]

.. and let me repeat the last line of my answer which already covered this ..

If you want to send 16 bit then use a uint16_t to store it and cast the pointer on send.

it's just a byte pointer, it has a length, I so wish Nordic had used void* here but they didn't.

[Jakub Rzeszutko]

This driver has been written mainly for nrf52832 where when using SPI with EasyDMA you are limited to 8bit buffer. As nrf52810 and nrf52840 have different buffer size we plan to rework this soon.