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Writing a “bare metal” operating system for Raspberry Pi 4 (Part 7)

Getting Bluetooth up

Controlling the RPi4 solely via a UART-connected laptop is not much fun. Our Breakout game deserves a better controller than that - ideally a wireless one.

In this part, we set up a second UART to communicate with the RPi4’s onboard Bluetooth modem. There is nothing simple about Bluetooth, but it is at least simpler than USB, and that’s the reason I’ve chosen to pursue it.

The Broadcom firmware

The Bluetooth modem is a Broadcom chip (BCM43455), and it needs to be loaded with proprietary software before it’s useful to us. I have taken their BCM4345C0.hcd file from the Bluez repo.

As we don’t yet have any filesystem, we won’t be able to load this at runtime, so instead we’ll need to build it into our kernel. We can use objcopy to build a .o file that we can link. We add these lines to Makefile:

BCM4345C0.o : BCM4345C0.hcd
	$(GCCPATH)/aarch64-none-elf-objcopy -I binary -O elf64-littleaarch64 -B aarch64 $< $@

We also need to modify our kernel8.img dependencies to include our new .o file:

kernel8.img: boot.o $(OFILES) BCM4345C0.o
	$(GCCPATH)/aarch64-none-elf-ld -nostdlib -nostartfiles boot.o $(OFILES) BCM4345C0.o -T link.ld -o kernel8.elf
	$(GCCPATH)/aarch64-none-elf-objcopy -O binary kernel8.elf kernel8.img

If you build the kernel now, you’ll see that the new image is much bigger - because it contains the firmware bytes. You can run objdump -x kernel8.elf, and you’ll also see a few new symbols in there:

We’ll use these symbols later to reference the firmware from our C code.

Setting up the UART

Look at our new bt.c. We’ll focus on the // UART0 section for now. A lot of the techniques will be familiar to you as this hardware also uses the MMIO technique, and we’re implementing a lot of the same functions that we did to get our serial debug working.

To use Bluetooth at the same time as our serial debug, we’ll need to remap some GPIO pins. Pins 30, 31, 32 and 33 will all need to take on their alternate function 3 to give us access to CTS0, RTS0, TXD0 and RXD0. You can read all about this in Section 5.3 of the BCM2711 ARM Peripherals document.

We already have a function in io.c to do this, and so we add the function definition of gpio_useAsAlt3 to io.h to give us access to it from here. In bt_init(), we go ahead and remap the GPIO pins, then flush the receive buffer just to be safe. The MMIO writes which follow, set us up for 115200 baud and 8-N-1 communication - we know the Bluetooth modem can cope with this. Perhaps the most important line to get right was this one:

mmio_write(ARM_UART0_CR, 0xB01);

It enables the UART (bit 0 on), enables TX & RX (bits 8 & 9 on), and drives RTS0 low (bit 11 on) - very important as the Bluetooth modem will sit unresponsive until it sees this. I lost a lot of time figuring this one out.

We should now be able to talk to the Bluetooth modem over our new UART.

Talking to the Bluetooth modem

The Bluetooth spec is massive, and to implement a full driver would take a long while. I’ll settle for “proof of life” for now. We still have a way to travel though…

We talk to the Bluetooth modem using HCI commands. I enjoyed reading the TI HCI docs as an intro to this.

bt_reset() simply calls hci_Command, which in turn called hci_CommandBytes to write the bytes out to the UART that tell the Bluetooth chip to reset and await firmware. This is a vendor-specific call, so you won’t find it documented anywhere. I reverse-engineered the calls using the following files from the Raspberry Pi Linux distribution:

hci_CommandBytes then waits for a very specific response before it returns successfully - the “command complete” response.

Loading the firmware

Now the device is waiting. We need to send it the firmware bytes we included in our kernel:

void bt_loadfirmware()
{
    volatile unsigned char empty[] = {};
    if (hciCommand(OGF_VENDOR, COMMAND_LOAD_FIRMWARE, empty, 0)) uart_writeText("loadFirmware() failed\n");

    extern unsigned char _binary_BCM4345C0_hcd_start[];
    extern unsigned char _binary_BCM4345C0_hcd_size[];

    unsigned int c=0;
    unsigned int size = (long)&_binary_BCM4345C0_hcd_size;

    unsigned char opcodebytes[2];
    unsigned char length;
    unsigned char *data = &(_binary_BCM4345C0_hcd_start[0]);

    while (c < size) {
        opcodebytes[0] = *data;
        opcodebytes[1] = *(data+1);
        length =         *(data+2);
        data += 3;

        if (hciCommandBytes(opcodebytes, data, length)) {
           uart_writeText("Firmware data load failed\n");
           break;
        }

        data += length;
        c += 3 + length;
    }

    wait_msec(0x100000);
}

First, we send a command to tell the chip that we’re about send the firmware. You’ll see that we then reference our new symbols, which point us at our firmware bytes. We now know the size of the firmware, and so we iterate over it.

The firmware is simply a sequence of HCI commands following this format:

We check each HCI command succeeds as we go, wait a second and then return. If it runs without error then we’ve loaded our firmware and we’re ready to start some Bluetooth communications.

I’ve then chosen to implement bd_setbaud() and bt_setbdaddr(). This sets the speed at which the Bluetooth modem will talk (much like we did in our UART examples) and also its unique Bluetooth Device Address.

Building an Eddystone beacon

Perhaps the simplest Bluetooth device to build is a “beacon”. It simply advertises a small amount of data publicly, such that any passing receivers can view the data. A typical use case is to advertise a web URL for location-based marketing purposes.

Google defined the Eddystone format, which was reasonably widely adopted. We’ll implement this as our example. Here’s what we need to achieve:

Much of my learning was advanced by markfirmware’s code. Rather than just blindly copying code into my bt.c, I read this alongside relevant sections of the weighty Bluetooth specification. I recommend you do the same if you’re interested in truly understanding what’s going on here.

When constructing the advertising data, I also referred to PiMyLifeUp’s article on Eddystone.

To test the code, ensure run_eddystone() is uncommented in kernel.c instead of run_search() (we’ll talk about run_search() in more detail in part8-breakout-ble).

Once built and running, I used the eBeacon iPhone application to check that my Eddystone beacon was broadcasting. The screenshots below show my URL proudly advertised as intended:

Working Eddystone Beacon on RPi4

Go to part8-breakout-ble >