nRF5340 SPI DAC7311

[nrf5340]

Hello

I am going to use nRF5340 with DAC7311 DAC. It is a time critical application where nRF5340 has to send 16-bit data to DAC7311 exactly at 6us continuously.

Is it possible to run on nRF5340?

Can anyone give us advice and/or provide a sample code?

The nRF5340 and DAC7311 connection is as below:

nRF5340 SPI CS pin connects to DAC7311 /SYNC pin

nRF5340 SPI SCK pin connects to DAC7311 SCLK pin

nRF5340 SPI MOSI pin connects to DAC7311 DIN pin

Thank you

[Kenneth]

Hello,

I actually think this is possible.

Some pointers:

You would need to use nrfx api directly. Setup a timer every 6us that trigger a ppi and start a spi transfer. A good starting point can be the gppi example that show how a timer can toggle a gpio:
https://github.com/zephyrproject-rtos/hal_nordic/tree/master/nrfx/samples/src/nrfx_gppi/one_to_one

Instead of toggling a gpio, you would have to prepare the spi transfer using nrfx_spim_xfer. A good starting point would be:
https://github.com/zephyrproject-rtos/hal_nordic/tree/master/nrfx/samples/src/nrfx_spim/non_blocking

Note that when calling nrfx_spim_xfer() you should set the NRFX_SPIM_FLAG_REPEATED_XFER flag, then read out the start spi address using nrfx_spim_start_task_address_get().

Then you can look at the first example with the timer, and connect the ppi from the timer event to the spi start task(instead of toggle gpio).

Make sure you are using SPI master 4 peripheral that have hardware control of CSN, and can support up to 32MHz:
https://infocenter.nordicsemi.com/topic/ps_nrf5340/spim.html#topic 

Kenneth

[nrf5340]

Hello Kenneth

Thanks for your support. It works.

Now we have to write m_tx_buffer and read m_rx_buffer (in nrfx_spim/non_blocking sample code) once spim4 sent 16-bit data, however, the timer_handler and/or spim_handler is invoked more than 6us. Consequently, the m_tx_buffer is not updated in every 6us.

Is there any advice of real-time handler example such that we can update the m_tx_buffer and queue the m_rx_buffer?

Thank you

[Kenneth]

Hello again,

As you experience, you will likely not be able to handle this for every 6us no. However there is a feature called array list:
https://infocenter.nordicsemi.com/topic/ps_nrf5340/easydma.html#arraylist 
https://infocenter.nordicsemi.com/topic/ps_nrf5340/spim.html#register.RXD.LIST 
https://infocenter.nordicsemi.com/topic/ps_nrf5340/spim.html#register.TXD.LIST 

Make sure that you periodically read/write out the spi data from RAM, but also periodically update the RX.PTR and TX.PTR to ensure they don't write to the end of the RAM.

Kenneth

[nrf5340]

Hello Kenneth

Thanks for your support. The EasyDMA list is a good suggestion on buffering tx/rx data.

We are going to update the RXD.LIST and TXD.LIST periodically

We would like to make sure that the AMOUNT is valid, i.e. before START or after END transmission. 

With refer to the below graph, the timer_handler is invoked during SPI TX/RX sometimes. 

Could you advise any method to check the SPI transmission status and implement an atomic operation on updating PTR/LIST as well as reading AMOUNT?

Thank you

[Kenneth]

nrf5340 said:

Could you advise any method to check the SPI transmission status and implement an atomic operation on updating PTR/LIST as well as reading AMOUNT?

In this specific case, since you write so often (every 6us) I think it not will be possible, but instead my suggestion is to have two large buffers (e.g each that can handle several tens of readings), and simply switch between the buffers by updating the PTR frequently enough to ensure that you don't overwrite any of the buffers. Then you simply go through the non-active buffer to check how much is there (e.g. you can fill the buffer with memset or similiar first to understand how far the buffers have been filled).

Kenneth