Enabling an External UART on AutoPi TMU CM4 Devices

This guide is applicable to the following device/ devices:

TMU CM4

CAN-FD Pro

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Overview

A UART (Universal Asynchronous Receiver/Transmitter) is the simplest and most widely supported way to move serial data between two devices. On an AutoPi TMU CM4 you already have one UART occupied by the automotive interface, but many real‑world projects—GPS receivers, CAN bridges, industrial sensors or even another microcontroller—need a second serial interface.

Goal – Re‑purpose the built‑in UART 2 of the CM4 and break it out on the HAT so your add‑on hardware can talk to the AutoPi at 300 baud or 4 Mbit/s and everything in between.

By the end of this page you will have:

• Wired the right pins
• Edited the Raspberry Pi firmware so it exposes /dev/ttyAMA1, and
• Verified data can flow both ways with a quick loop‑back test.

When should I use an external UART?

Scenario	Why a second UART helps
Adding a GNSS (GPS) module	NMEA sentences at 9600 or 115200 baud stream continuously—best handled on a dedicated port.
Talking to an OBD‑II dongle while keeping the main AutoPi vehicular interface free	Avoids contention and keeps logs clean.
Bridging to RS‑232/RS‑485 industrial sensors	A 3‑wire TX/RX/GND connection keeps wiring simple.
Debugging another microcontroller’s boot output	Power up your MCU and capture console logs without a USB serial dongle.

Quick facts

• Signal names – TXD2 transmits from the Pi, RXD2 receives into the Pi.
• Pin numbers – Physical pins 27 & 28 on the 40‑pin header map to GPIO 0 & 1 (ID_SD and ID_SC on the HAT).
• Logic level – 3.3 V; never connect directly to 5 V RS‑232 without level shifting.

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Prerequisites

2. SSH access or Physical HDMI/USB console.
3. Micro‑jumper wires or a breakout header to reach pins 27/28.
4. A multimeter (optional but handy for continuity checks).

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Why GPIO 0/1?

The CM4 has six UART blocks but only GPIO 0 & 1 expose the pins for UART 2 (alt‑function 4). Your other free pins—GPIO22‑24, 27—can do many clever things (SPI, PCM, PWM) but not TX/RX. Hence we use the HAT‑EEPROM I²C pins (ID_SD, ID_SC) for serial use.

EEPROM probe vs. UART

Normally the Pi firmware tugs GPIO0/1 low during the first 200 ms of boot while it looks for a HAT EEPROM. The force_eeprom_read=0 flag in the next section disables that so your external device sees a clean idle‑high line from power‑up.

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1  Hardware wiring

HAT label	BCM GPIO	UART2 signal	Connect to
ID_SD	0	TXD2 (Pi → device)	RX of the external device
ID_SC	1	RXD2 (device → Pi)	TX of the external device
I2C1_SDA*	2	CTS2 (optional)	RTS of the device
I2C1_SCL*	3	RTS2 (optional)	CTS of the device
GND	—	—	Common ground

Wire CTS/RTS only if you need hardware flow control and intend to enable it in the dtoverlay.

       AutoPi TMU CM4       External Device
      ┌──────────────┐     ┌────────────┐
 ID_SD│27   TXD2  ➔──┼────►│ RX         │
 ID_SC│28   RXD2  ◄──┼─────│ TX         │
 SDA  │ 3   CTS2  ◄──┼─────│ RTS  (opt) │
 SCL  │ 5   RTS2  ➔──┼────►│ CTS  (opt) │
  GND │ 6   GND   ───┼─────│ GND        │
      └──────────────┘     └────────────┘

Level shifting

If the external device works at 5 V TTL or ±12 V RS‑232, you must level‑shift or use a proper line driver (e.g. MAX3232, SN65HVD230 for RS‑485). Direct 5 V into a CM4 pin can permanently damage the SoC.

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2  Enable UART2 in config.txt

Open the file with sudo nano /boot/config.txt and add:

# === External UART2 on GPIO0/1 ===
force_eeprom_read=0          # skip HAT‑EEPROM probe on GPIO0/1
dtoverlay=uart2              # TX=GPIO0, RX=GPIO1  => /dev/ttyAMA1
#dtoverlay=uart2,ctsrts      # remove # to enable flow control on GPIO2/3
enable_uart=1                # leave as‑is if already present

Save, exit, and sudo reboot.

Check dmesg after the reboot:

dmesg | grep -i ttyAMA1

You should see a line similar to ttyAMA1 at MMIO 0x..... confirming the driver is active.

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3  Verify with a loop‑back test

1. Jumper GPIO0 to GPIO1 directly on the HAT.

2. Open a terminal on the Pi and run:

   stty -F /dev/ttyAMA1 115200 raw -echo
   echo "hello" > /dev/ttyAMA1 &
   cat < /dev/ttyAMA1

3. The word hello should appear. Press Ctrl‑C, remove the jumper, and connect your real device.

Alternative: use minicom

sudo apt install -y minicom
minicom -b 115200 -D /dev/ttyAMA1

Minicom gives you scroll‑back, line‑ending control and macros—handy for debugging.

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4  Using the port in Python

import serial

ser = serial.Serial(
    port='/dev/ttyAMA1',
    baudrate=115200,
    bytesize=serial.EIGHTBITS,
    parity=serial.PARITY_NONE,
    stopbits=serial.STOPBITS_ONE,
    timeout=1
)

ser.write(b'AT\r')      # send a modem‑style command
print(ser.readline())   # read back a line
ser.close()

Python’s pyserial package makes scripting trivial—ideal for quick sensor polls.

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5  Troubleshooting

Symptom	Likely cause & fix
No /dev/ttyAMA1	Overlay typo or missing enable_uart=1. Re‑check /boot/config.txt.
Boot hang with garbage on console	Another service (e.g. serial‑getty@ttyAMA1.service) grabbed the port. Disable it with sudo systemctl disable --now serial-getty@ttyAMA1.service.
Lines held low for ~0.2 s after power‑up	force_eeprom_read=0 missing; firmware still probing EEPROM.
Flow‑control ignored	Verify you removed # in dtoverlay=uart2,ctsrts and wired GPIO2/3 correctly.
Overruns at >1 Mbit/s	Increase init_uart_clock in config.txt or use DMA mode via dwc_uart_fifo_enable=1.

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6  Advanced topics

Disable I²C on GPIO0/1 entirely

If another overlay or user‑space application tries to re‑enable I²C0, add dtparam=i2c_vc=off to config.txt.

Throughput tuning

Use core_freq_min=250 to keep the UART clock stable during idle throttling, and prefer 8N1 framing for max payload density.

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7  References

• Raspberry Pi CM4 Datasheet – Alternate‑function table
• Understanding Device Tree Overlays
• AutoPi TMU CM4 HAT Pinout