Day29 - UART and TTY Subsystem

🎯 Objective

• Understand how UART is exposed as TTY in Linux
• Learn how to configure serial port using termios / stty
• Create a clean UART device (ttyAMA2)
• Perform loopback test
• Understand TTY behavior differences

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🧠 Key Concepts

UART vs TTY

• UART = hardware interface (TX/RX)
• TTY = Linux abstraction layer

User App
  → /dev/ttyXXX
  → TTY core
  → line discipline
  → serial driver
  → UART hardware

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🧪 Lab 1 - Enable Additional UART

Edit config:

sudo nano /boot/firmware/config.txt

Add:

dtoverlay=uart2

Reboot:

sudo reboot

Verify:

ls -l /dev/ttyAMA*
dmesg | grep -i ttyAMA

Expected:

• /dev/ttyAMA10 → console
• /dev/ttyAMA2 → new UART

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🧪 Lab 2 - Verify Pinmux

pinctrl get 4
pinctrl get 5

Expected:

• GPIO4 = TXD2
• GPIO5 = RXD2

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🧪 Lab 3 - Loopback Wiring

Connect:

• Pin 7 (GPIO4 / TXD2)
• Pin 29 (GPIO5 / RXD2)

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🧪 Lab 4 - Configure TTY

stty -F /dev/ttyAMA2 115200 cs8 -cstopb -parenb -icanon -echo
stty -F /dev/ttyAMA2 -a

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🧪 Lab 5 - Shell Loopback Test

Terminal A:

dd if=/dev/ttyAMA2 bs=1 count=16 status=none | hexdump -C

Terminal B:

printf 'ABC' > /dev/ttyAMA2

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🧪 Lab 6 - Python Loopback Test

python3 uart_loopback_test.py /dev/ttyAMA2

Expected:

TX (...)
RX (...)
PASS

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⚠️ Observations

• cat is not reliable for UART debugging
• printf + cat may fail due to timing issues
• program-based testing is more stable

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🧠 Conclusion

• UART is accessed via /dev/ttyXXX
• termios controls TTY behavior
• clean UART must not be used by console
• loopback verifies full system path