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FAQ: How can I make a software-based (bit-bang) SPI bus/I need 2 SPI devices/Can I use any 4 GPIOs as an SPI bus?


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    Software-based (bit-bang) SPI on the GPIOs is doable!

    The hardware SPI can only drive 2 devices, as there is only CS0 (already used by internal flash) and CS1.

    If you need more than 1 additional SPI device, you'll need to resort to software (GPIO based) SPI. (Also known as bit-bang SPI).

    Setting up a Software I2C Bus

    Install the required kernel modules

    opkg update
    opkg install kmod-spi-gpio-custom
    

    Then load the spi-gpio-custom kernel module:

    # configure software GPIO SPI
    # bus<Y>=<devid>,<SCK>,<MOSI>,<MISO>,<mode>,<speedInHz>,<CS>
    insmod spi-gpio-custom bus0=1,11,5,4,0,100000,18
    

    Where

    • Y just numbers the software SPI "buses" controlled by spi-gpio-custom. If you need multiple software SPI, you need to specify all of them on the same insmod command line, and specify parameters like bus0=... bus1=... bus2=...
    • devid is the number X you'll have in the /dev/spidevX.0 name
    • SCK, MOSI, MISO, CS are GPIO numbers for the respective signals
    • mode is the SPI mode, usually 0
    • speedinHz is the speed. Note that software SPI speed is limited, you'll not get the HW SPI top speed with it

    The example above will get you a /dev/spidev1.0 on pins 11,5,4 and 18, in mode 0 and with 100kHz speed.


    Credit for this FAQ goes to @luz from this thread



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