8.1.5.1. Build Configuration Options#

The following options can be used in the BSP section of the waf configuration INI file. The waf defaults can be used to inspect the values.

CONSOLE_USE_INTERRUPTS

Use interrupt driven mode for console devices (enabled by default).

GPU_CORE_CLOCK_RATE

The GPU processor core frequency in Hz (default is 500000000), The value of this option should be the same as the value of option core_freq in config.txt. See the Raspberry Pi documentation for details.

BSP_SPI_USE_INTERRUPTS

Use interrupt mode in the SPI driver (enabled by default).

BSP_CLOCK_USE_SYSTEMTIMER

Use the System Timer in the clock driver (disable by default).

BSP_CONSOLE_PORT

Default UART port for the console device (default is UART0). The optional ports are UART0, UART2, UART3, UART4, UART5.

BSP_PL011_CLOCK_FREQ

PL011 UART clock frequency in Hz (default is 48000000). The value of this option should be the same as the value of option init_uart_clock in config.txt. See the Raspberry Pi documentation for details.

8.1.5.2. Clock Driver#

Raspberry pi 4B has two timers.

The System Timer and The ARM Generic Timer.

The clock from the ARM timer is derived from the system clock. This clock can change dynamically e.g. if the system goes into reduced power or in low power mode. Thus the clock speed adapts to the overall system performance capabilities. For accurate timing it is recommended to use the system timers.

The clock driver uses the ARM Generic Timer by default. Set BSP_CLOCK_USE_SYSTEMTIMER = True in the Build Configuration Options to enable the System Timer.

8.1.5.3. Console Driver#

Raspberry pi 4B has 2 types of UARTs, ARM PL011 and Mini-uart. The PL011 is a capable, broadly 16550-compatible UART, while the mini UART has a reduced feature set. The console driver supports the default Qemu emulated ARM PL011 PrimeCell UART as well as the physical ARM PL011 PrimeCell UART in the raspberrypi hardware. Mini-uart is not supported.

Set BSP_CONSOLE_PORT in the Build Configuration Options to set the default UART port for the console device.

Initialize gpio of UART and install UART to the dev directory by raspberrypi_uart_init() function.

#include <assert.h>
#include <bsp/console.h>

void uart_init(void)
{
  int rv;

  /* The optional devices are UART0, UART2, UART3, UART4, UART5. */
  rv =  raspberrypi_uart_init(UART0);
  assert(rv == 0);
}

8.1.5.4. GPIO Driver#

The GPIO of Raspberry pi 4B can be controlled by the following functions: raspberrypi_gpio_set_function() raspberrypi_gpio_set_pin() raspberrypi_gpio_clear_pin() raspberrypi_gpio_set_pull()

void gpio(void)
{
  /* Define the operation of the general-purpose I/O pins. Each of the 58
   * GPIO pins has at least two alternative functions as defined.
   * The optional functions are GPIO_INPUT, GPIO_OUTPUT, GPIO_AF5, GPIO_AF4,
   * GPIO_AF0, GPIO_AF1, GPIO_AF2, GPIO_AF3
   */
  raspberrypi_gpio_set_function(8, GPIO_AF0);

  /* Control the actuation of the internal pull-up/down resistors.
   * The optional value are GPIO_PULL_NONE, GPIO_PULL_UP, GPIO_PULL_DOWN
   */
  raspberrypi_gpio_set_pull(8, GPIO_PULL_NONE);

  /* Clear a GPIO pin. */
  raspberrypi_gpio_clear_pin(8);

  /* Set a GPIO pin. */
  raspberrypi_gpio_set_pin(8);
}

8.1.5.5. SPI Driver#

The BCM2711 device has five SPI interfaces of this type: SPI0, SPI3, SPI4, SPI5 & SPI6. It has two additional mini SPI interfaces (SPI1 and SPI2). The SPI driver supports SPI0, SPI3, SPI4, SPI5 & SPI6. The mini SPI is not supported.

SPI drivers are registered by the raspberrypi_spi_init() function. The driver has no DMA support, but has interrupt support.

#include <assert.h>
#include <bsp/raspberrypi-spi.h>

void spi_init(void)
{
  int rv;

  /*
   * The optional devices are raspberrypi_SPI0, raspberrypi_SPI3,
   * raspberrypi_SPI4, raspberrypi_SPI5, raspberrypi_SPI6.
   */
  rv =  raspberrypi_spi_init(raspberrypi_SPI0);
  assert(rv == 0);
}

8.1.5.6. Watchdog Driver#

void raspberrypi_watchdog_example()
{
  raspberrypi_watchdog_init();
  raspberrypi_watchdog_start(15000);

  raspberrypi_watchdog_reload();
  /* ... */
  raspberrypi_watchdog_reload();

  raspberrypi_watchdog_stop();
}

8.1.5.7. Preparing to boot#

Raspberry Pi uses a different mechanism to boot when compared with any ARM SoC. First the GPU initializes, loads the bootloader (Raspberry pi firmware) and then looks for the kernel img. This whole process is done by the GPU (VideoCore IV) till the kernel is loaded. More information can be found on the Raspberry pi documentation page. By default the arm64 mode looks for the kernel8.img. Any other kernel can be loaded by adding kernel=<img_name> to the config.txt file.

The Firmware files are required in order to boot RTEMS. The latest firmware can be downloaded from the Raspberry Pi Firmware Repository. USB boot is supported. All the files (Firmwares and kernel) must be place in the FAT32 partition only. Add arm_64bit=1 in the config.txt file in order to boot the BSP in 64bit kernel mode (it is default).

$ aarch64-rtems7-objcopy -Obinary hello.exe kernel8.img

8.1.5.8. JTAG Setup#

The Raspberry Pi 4 doesn’t have dedicated JTAG pins. Instead, you must configure the GPIO pins (GPIO22-GPIO27) to activate the JTAG functionality. The RPi 4 documentation refers to this as Alt4 functions of those pins. Alt5 does exist too, which goes from GPIO4, 5, 6, 12 and 13. you can check this out from pinout.xyz or eLinux

One more thing to note on JTAG with Raspberry pi 4B is that, by default, All the GPIO pins are pulled down, according to the BCM2711 documentation. This wasn’t the case in the earlier models. So in order to let the data flow freely, we will have to disable them.

# Disable pull downs
gpio=22-27=np

# Enable jtag pins (i.e. GPIO22-GPIO27)
enable_jtag_gpio=1

8.1.5.9. Running Executables on QEMU#

Executables generated by these BSPs can be run using the following command:

qemu-system-aarch64 -M raspi4b -serial mon:stdio -nographic \
 -kernel kernel8.img