Administrivia

• “Lab 1: Bootstrapping Raspberry Pi” is due on Jan 27
• TA office hours
  • Fri at Coda C0915, 10-11 AM, Yechan Bae
  • Tue at Coda C0906, 05-06 PM, Sujin Park
  • Wed at Coda C1008, 11-12 PM, Mansour Alharthi

Q&A

• Q. + / - of 64-bit OS?
  • What do we mean by 64-bit?
  • Virtual address space??
  • Memory usage??
  • Performance??

Common answers:

It is important to support 64-bit by an operating system because a 64-bit processor can handle more data at once.

Q&A

• What about the RPi’s default kernel, Raspbian?

Q&A

• Q. Have you used Rpi before?
  • Vision tasks, in-person queuing system
  • VPN server
  • RPG game
  • IoT projects
  • Wake-on-book to manage desktop
  • Smart speaker (Echo alternative)

Q&A

• Q. Is there any problem of running a 32-bit OS on 64-bit processor?
• Q. Why Pi 3 and not Pi 4? Just curious?
• Q. What exactly is so special about Raspberry Pi? Why not Arduino?

Q&A

Raspberry Pi 3

Specification of Raspberry Pi 3

Specification of Raspberry Pi 3

About BCM2837

• BCM2836: Raspberry Pi 2 chip
• BCM2837: Raspberry Pi 3 chip

This is the Broadcom chip used in the Raspberry Pi 3, and in later models of the Raspberry Pi 2. The underlying architecture of the BCM2837 is identical to the BCM2836. The only significant difference is the replacement of the ARMv7 quad core cluster with a quad-core ARM Cortex A53 (ARMv8) cluster.

Ref. BCM2837

Specification of Raspberry Pi 3

Specification of Raspberry Pi 3

General-purpose input/output (GPIO)

• I/O pins whose roles can be configurable by software
• Alternative function: e.g., UART TX/RX, PWM, etc

Example: using a GPIO pin as output

• Using 3.3V power and ground pins (left)
• One GPIO pin as output, and drives a led (right)

How to configure and drive GPIO pins?

• GPIO is one of peripherals inside the BCM2837 package
• Configurable via Memory-mapped I/O (via the I/O peripheral map)

Ref. BCM2837 Data sheet

Note. pull-up/pull-down resistor

• Ensuring the known state: up (high) and down (low)

Peripherals

• BCM2837 packages a few peripherals inside SoC that are accessible by the ARM CPU
  • GPIO (Lab1)
  • UART (Lab2)
  • Timers (Lab1/Lab4)
  • Interrupt controller (Lab4)
  • USB (Lab5)
  • PWM (perhaps, final project? e.g., controlling a robot)
  • …

About VideoCore IV

• VideoCore is a low-power multimedia processor (i.e., GPU)
• Compare how much current is being used when idle and during the game play!

Ref. VideoCore

VideoCore IV has a role beyond GPU

• GPU boots first when the power is on (using software in the ROM)
• Loads bootcode.bin (see ext/firmware)
• Initialize peripherals and, in fact, ARM cores!

Firmware

• VideoCore loads the firmware to launch kernel8.img

$ ls ext/rpi3-uart
bootcode.bin
fixup.dat
start.elf
config.txt
kernel8.img

$ cat config.txt
arm_control=0x200        ; AArch64
kernel_address=0x4000000 ; Address for loading kernel8.img

Configuring peripherals via MMIO

• Accessing to 0x3f00_0000 (physical address) → 0x7e00_0000 (bus address)
• MMIO: str/ldr to access peripheral registers

Example: GPIO

• 54 GPIO in BCM2837, which can be configured by 41 registers (p 89)

Example: UART (Lab 2)

• Configuring mini UART via 0x3f21_5000 (physical address)

Accessing peripheral region in C

#define GPIO_BASE (0x3F000000 + 0x200000)

volatile unsigned *GPIO_FSEL1 = (volatile unsigned *)(GPIO_BASE + 0x04);

int kmain(void) {
  // reading
  unsigned int sel1 = *GPIO_FSEL1;
  
  // writing
  *GPIO_FSEL1 = sel1;
}

Accessing peripheral regions in Rust

const GPIO_BASE: usize = 0x3F000000 + 0x200000;

const GPIO_FSEL1: *mut u32 = (GPIO_BASE + 0x04) as *mut u32;

unsafe fn kmain() -> ! {
    // reading
    let mut sel1 =  GPIO_FSEL1.read_volatile();

    // writing
    GPIO_FSEL1.write_volatile(sel1);
}

Next lecture

• Next lecture: recitation on lab01!
• “Lab 1: Bootstrapping Raspberry Pi” is due on Jan 27

References

• Rpi GPIO
• GPIO pads control
• Raspberry Pi 3 Block Diagram