UART

Rpi3 has 2 UARTs, mini UART and PL011 UART. We provide an overview about how to access them.

For details, please refer to https://cs140e.sergio.bz/docs/BCM2837-ARM-Peripherals.pdf

Background

MMIO

Rpi3 accesses its peripherals through memory mapped input output(MMIO). When a CPU loads/stores value at a specific physical address, it gets/sets a peripheral’s register. Therefore, you can access different peripherals from different memory addresses.

Note

There is a VideoCore/ARM MMU translating physical addresses to bus addresses. The MMU maps physical address 0x3f000000 to bus address 0x7e000000. In your code, you should use physical addresses instead of bus addresses. However, the reference uses bus addresses. You should translate them into physical one.

GPIO

Rpi3 has several GPIO lines for basic input-output devices such as LED or button. Besides, some GPIO lines provide alternate functions such as UART and SPI. Before using UART, you should configure GPIO pin to the corresponding mode.

GPIO 14, 15 can be both used for mini UART and PL011 UART. However, mini UART should set ALT5 and PL011 UART should set ALT0. You need to configure GPFSELn register to change alternate function.

Next, you need to configure pull up/down register to disable GPIO pull up/down. It’s because these GPIO pins use alternate functions, not basic input-output. Please refer to the description of GPPUD and GPPUDCLKn registers for a detailed setup.

Mini UART

Mini UART is provided by rpi3’s auxiliary peripherals. It supports limited functions of UART.

Initialization

  1. Set AUXENB register to enable mini UART. Then mini UART register can be accessed.

  2. Set AUX_MU_CNTL_REG to 0. Disable transmitter and receiver during configuration.

  3. Set AUX_MU_IER_REG to 0. Disable interrupt because currently you don’t need interrupt.

  4. Set AUX_MU_LCR_REG to 3. Set the data size to 8 bit.

  5. Set AUX_MU_MCR_REG to 0. Don’t need auto flow control.

  6. Set AUX_MU_BAUD to 270. Set baud rate to 115200

    After booting, the system clock is 250 MHz.

\[\text{baud rate} = \frac{\text{systemx clock freq}}{8\times(\text{AUX_MU_BAUD}+1)}\]

  1. Set AUX_MU_IIR_REG to 6. No FIFO.

  2. Set AUX_MU_CNTL_REG to 3. Enable the transmitter and receiver.

Read data

  1. Check AUX_MU_LSR_REG’s data ready field.

  2. If set, read from AUX_MU_IO_REG

Write data

  1. Check AUX_MU_LSR_REG’s Transmitter empty field.

  2. If set, write to AUX_MU_IO_REG

Interrupt

  • AUX_MU_IER_REG: enable tx/rx interrupt

  • AUX_MU_IIR_REG: check interrupt cause

  • Interrupt enable register1(page 116 of manual): set 29 bit to enable. (AUX interrupt enable)

Note

By default, QEMU uses UART0 (PL011 UART) as serial io. If you want to use UART1 (mini UART) use flag -serial null -serial stdio

PL011 UART

To use PL011 UART, you should set up the clock for it first. It’s configured by Mailbox.

Besides the clock configuration, it’s similar to mini UART.

Initialization

  1. Configure the UART clock frequency by mailbox.

  2. Enable GPIO (almost same as mini UART).

  3. Set IBRD and FBRD to configure baud rate.

  4. Set LCRH to configure line control.

  5. Set CR to enable UART.

Read data

  1. Check FR

  2. Read from DR

Write data

  1. Check FR

  2. Write to DR

Interrupt

  • IMSC: enable tx/rx interrupt

  • MIS: check interrupt cause

  • ICR: clear interrupt (read or write DR will automatically clear)

  • Interrupt enable register2(page 117 of manual): set 25 bit to enable. (UART interrupt enable)