No output from /dev/serial0 (RPI 4)

Question

I am also doing similar project with raspberry pi 4 (bullseye version) and neo 6m module. I have followed all the steps given in https://sparklers-the-makers.github.io/blog/robotics/use-neo-6m-module-with-raspberry-pi/ but I do not see any output when I do sudo cat /dev/serial0. It simply hangs, does not crash pi. Ctrl+C brings the unix prompt back, The blue light on the antenna is blinking as suggested by the article.

I don't know which other settings I am missing. Here is the gist of the tutorial:

0. I am trying to read the GPS data by using Raspberry Pi 4 + Neo 6M GPS module
1. I edited /boot/config.txt and added dtparam=spi=on dtoverlay=pi3-disable-bt core_freq=250 enable_uart=1 force_turbo=1 at the bottom of the file
2. I created a backup of /boot/cmdline.txt and in the new cmdline.txt file, added dwc_otg.lpm_enable=0 console=tty1 root=/dev/mmcblk0p2 rootfstype=ext4 elevator=deadline fsck.repair=yes rootwait quiet splash plymouth.ignore-serial-consoles
3. After rebooting, sudo cat /dev/ttyAMA0 hangs. I checked that /dev/serial0 is linked to ttyAMA0. I also used raspi-config to disable login and lastly, I also performed sudo systemctl stop serial-getty@ttyAMA0.service sudo systemctl disable serial-getty@ttyAMA0.service but sudo cat /dev/ttyAMA0 or /dev/serial0 still hangs. Note that the blue light on the antenna is blinking which means the antenna is receiving data. Is it transmitting the data to pi? I don't know. Is there anyway to find it out other than sudo cat /dev/ttyAMA0?

Answer 1

I do not have a Neo 6 GPS, but on the RP 4B there are more UARTs to use instead of 0 and 1. I will provide a short example of how to use UART3 on the RP 4B which you may use to adapt to the GPS device.

#!/usr/bin/env python3

"""
EXAMPLE of UART communication on Raspberry Pi 4B using Python.

Multiple UARTs on Raspberry Pi 4B (CM4):
          TXD RXD CTS RTS     Board Pins
  uart0   14  15              8   10
  uart1   14  15              8   10
  uart2   0   1   2   3       27  28  (I2C0)
  uart3   4   5   6   7       7   29  (GPCLK0), (RTS 7 = SPI0-CE1)
  uart4   8   9   10  11      24  21  (SPI0)
  uart5   12  13  14  15      32  33  (gpio-fan)

Raspberry Pi 4B (CM4) setup:
  /boot/config.txt has dtoverlay=uart3, sudo reboot
  jumper from GPIO pin 4 to GPIO pin 5
  
https://www.electronicwings.com/raspberry-pi/raspberry-pi-uart-communication-using-python-and-c
https://pinout.xyz/pinout/uart
https://raspberrypi.stackexchange.com/questions/45570/how-do-i-make-serial-work-on-the-raspberry-pi3-pizerow-pi4-or-later-models/107780#107780
https://github.com/raspberrypi/documentation/blob/develop/documentation/asciidoc/computers/configuration/uart.adoc
https://pyserial.readthedocs.io/en/latest/pyserial_api.html
"""

import serial
from time import sleep

ser = serial.Serial('/dev/ttyAMA1', 2500000, 8, 'N', 1)   # Open port
print(ser.get_settings)
x = 6
while x:
    ser.write(b'Hello out there!')          # Write byarray
    received_data = ser.read()              # Read serial port
    sleep(0.03)
    data_left = ser.inWaiting()             # Check for remaining byte
    received_data += ser.read(data_left)
    print(received_data.decode('utf-8'))   # Print received data
    x -= 1