Enabling an External UART on AutoPi TMU CM4 Devices

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