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?

Lazar Demin

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).

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.

Lower Bus Speed. Since this is a software-based bus, it will not be as fast as a hardware SPI bus.

Credit for this FAQ goes to @luz from this thread