Embedded

OK, so SPI is a bus protocol, although when you're talking that low level that's almost overselling it. With SPI you have a clock signal on one pin, a device select signal on another pin, and then two unidirectional signal pins, MOSI (master out, slave in) and MISO/SOMI (master in, slave out, this pin is often unused in SPI if the device being talked to is write only). The big advantage to this setup is that you can read and write from devices at the same time since the read and write pins are separate. The other tradeoff with SPI is that adding additional devices requires one extra pin per device (for the device select signal, usually identified as CS). The way you route signals to a particular device is by driving the CS pin low for the device you want to communicate with. So you need 4 pins for 1 device, 5 pins for 2 devices, 6 pins for 3, etc.

It's is entirely possible to implement SPI communication using software, you just manually flip the CLK and MOSI pins to send your messages (and read from the MISO pin). That's usually referred to as bit bashing. Most microcontrollers though also include special hardware for managing SPI for you where you can set a desired clock speed and write/read from buffers and the controller will handle flipping the CLK and MOSI pins for you and filling the buffers.

The other major bus protocol you'll typically run into for embedded devices is I2C (or IIC). I2C only uses 2 or 3 pins, a clock pin, a command pin, and sometimes a data pin. I2C operates by assigning each device on the bus a unique address, and as part of each message sent on the bus the target devices address is included. The downside to this design is that reading and writing is often significantly slower than what's possible with SPI, but the upside is that you can have a nearly unlimited number of devices all driven by just 2 or 3 pins. Just like with SPI many microcontrollers will include hardware specifically for managing I2C communications, but you can also just bit bash your messages across the bus as well.

No matter how you decide to configure things with the MAX7219 it's using SPI, it's just the difference of whether you're "manually" implementing SPI where there's code running on the main CPU that's sending the messages, or if the actual sending is being handled by some dedicated SPI hardware and the main CPU is just writing to buffers. The library itself is handling the SPI communications when you pass it the pins, so you don't need to actually implement SPI yourself, the library handles it for you, but it is going to end up being noticeably slower than if you initialize the SPI module on the rp-pico and let its dedicate SPI hardware manage it instead.

Looking at the SPI example there's this chunk of code:

    let clocks = hal::clocks::init_clocks_and_plls(
        rp_pico::XOSC_CRYSTAL_FREQ,
        pac.XOSC,
        pac.CLOCKS,
        pac.PLL_SYS,
        pac.PLL_USB,
        &mut pac.RESETS,
        &mut watchdog,
    )
    .ok()
    .unwrap();

That's initializing the system clocks in the rp-pico. You should be doing this no matter what and this doesn't really have anything to do with SPI except in that SPI uses the system clocks.

Then there's this chunk here:

    let _spi_sclk = pins.gpio2.into_mode::();
    let _spi_mosi = pins.gpio3.into_mode::();
    let _spi_miso = pins.gpio4.into_mode::();
    let spi_cs = pins.gpio5.into_push_pull_output();

That block is configuring a few pins to be usable with the hardware SPI driver and configuring one final pin as a CS pin.

Edit: N.B. this chunk is getting mangled by lemmy for some reason, it keeps removing the turbofish operators. You can follow this link to see what I'm talking about. Additionally you can reference this chart to see what pins on the rp-pico support which hardware functions. GPIO 0 through 7 and 16 to 19 all support the SPI0 hardware driver, and GPIO 8 through 15 support SPI1.

Lastly there's this chunk:

    let spi = spi::Spi::<_, _, 8>::new(pac.SPI0);
    let spi = spi.init(
        &mut pac.RESETS,
        clocks.peripheral_clock.freq(),
        400.kHz(), // card initialization happens at low baud rate
        &embedded_hal::spi::MODE_0,
    );

That's using the first hardware SPI unit (SPI0) and initializing it. It's setting it to run at 400kHz and in Mode 0 (SPI loosely defines a couple different ways communication can be implemented, usually referred to as mode 0 through 4). The clock speed and mode that need to be configured will vary based on the device. For the MAX7219 it looks like it wants to run at a max of 1mHz and mode 0.

Once you have the MAX7219 struct returned from the from_spi_cs function you can basically just ignore SPI entirely and just use the functions on the MAX7219 driver to send messages to the display.