(1) https://raspberrypi.stackexchange.com/questions/97869/how-to-check-if-spi-is-enabled-and-functional-on-raspi-3b (2) https://raspberrypi.stackexchange.com/questions/99079/using-two-spi-ports-simultaneously-on-raspberry-pi-0-w (3) https://raspberrypi.stackexchange.com/questions/96225/why-is-spi-not-working-on-any-of-my-raspis