Migrating a design from Raspberry pi Zero to nRF9160 DK - Need I2C and SPI interfaces

[RodWatt]

Hi,

I have developed a simple system based around a Raspberry Pi Zero. The system uses a couple of sensors to measure temperature and water pressure using a ADS51115 A/D and a MAX 31865 RTD to digital convertor. These connect to the Pi zero via the !2C bus and the SPI interface. I would like to replace the Pi zero with the nRF9160 dev board, can some one point me to some example hw and sw that will enable me to quickly convert. Thanks, Rod

[Jan Tore Guggedal]

Hi, RodWatt

In order to use SPI and I2C peripherals, the following configurations can be placed in `prj.conf` to enable them and select the nrfx driver:

CONFIG_I2C=y
CONFIG_I2C_NRFX=y
CONFIG_I2C_2=y
CONFIG_I2C_2_NRF_TWIM=y

CONFIG_SPI=y
CONFIG_SPI_NRFX=y
CONFIG_SPI_3=y
CONFIG_SPI_3_NRF_SPIM=y

This will enable I2C instance 2 and SPI instance 3, configure them to use nrfx driver. Note that these are the ones declared in the default secure boot to be available for a non-secure application:
https://github.com/NordicPlayground/fw-nrfconnect-nrf/blob/master/samples/nrf9160/secure_boot/src/main.c#L321

You can choose other instances as long as you also adjust which peripherals are set as non-secure.

You then need to defined the peripherals in an overlay file for the board where you can set the pin numbers and clock speed. Name it `nrf9160_pca10090.overlay` and place it in the application folder. The contents of the file may be as follows:

&i2c2 {
        status = "ok";
        sda-pin = <30>;
        scl-pin = <31>;
        clock-frequency = <I2C_BITRATE_FAST>;
};

&spi3 {
        status = "ok";
        sck-pin = <6>;
        mosi-pin = <7>;
        miso-pin = <8>;
        spi-max-frequency = <4000000>;
};

The devices get the labels "I2C_2" and "SPI_3" to be used in the `device_get_binding()` call, according to their instance numbers.

When you have the configuration set, you will be able to use the generic I2C and SPI drivers in Zephyr, and can build use those samples and the API documentation as reference:

I2C API:
https://docs.zephyrproject.org/latest/api/io_interfaces.html?#i2c-interface

SPI API:
https://docs.zephyrproject.org/latest/api/io_interfaces.html?#spi-interface

Samples:
https://github.com/zephyrproject-rtos/zephyr/tree/master/samples/drivers 

[RodWatt]

Jan, thank you for your reply. A couple  of follow on questions.

1. Am I correct in saying that I can assign any GPIO pins as SPI and I2C interfaces?

2. As part of my development process, I want to phase in the replacement of the Raspberry Pi. In the next version on the prototype, I would like to keep the Pi zero and add the nRF9160DK effectively as a peripheral. 

So, please can you advise how I can add the nRF9160DK as a peripheral to the Pi. Do I just connect over UART or can you suggest a better way?

[Jan Tore Guggedal]

1. In general, yes, any GPIO can be used. Some pins have dedicated functionality on the dev kit which is shown in a table on the back of the kit, so you might want to avoid them as a convenience, but they’re usable if you need them.
   
   
2. UART, I2C, SPI etc should work fine. Note that some peripherals share the same addresses, as shown by having the same peripheral ID in this table, so that needs to be kept in mind:
   https://www.nordicsemi.com/DocLib/Content/Product_Spec/nRF9160/latest/memory#topic

[Jeff Seaman]

Did you get the SPI to function on the nRF9160 I have done the setup and ried running a version of the sample code.  I do get a device pointer back, but when I try to write to the SPI device I do not see any activity on the pins, I am looking fro the clock signal as a first step the I would connect a AT25SF041-SSHD flash memory chip for testing.  Also I was not seeing in the sample code where the chip select lins was asserted.

[Martin Lesund]

Hi Jeff,

I see that you have made a new case regarding this, great!
We will follow up your issue there.