Accessing an IoT Server via a SIM Module in Micropython

[tirejas]

Hi,
I'm trying to connect to an IoT server via a SIM800L module, but I'm not able to. I'm able to communicate with the GRSM network but not with the server. I always get error 6.
I'm attaching the code and the Thonny console messages.
Can you help me?

import time
import machine
from machine import UART
from umqtt.simple import MQTTClient

# Configuración de la interfaz UART (reemplazar con los pines apropiados para tu placa)
uart = UART(0, baudrate=9600, tx=16, rx=17)

# Función para enviar comandos AT y esperar respuesta
def send_at_command(command, timeout=5000):
    print("Enviando:", command)
    uart.write(command + '\r\n')
    time.sleep_ms(100)
    start_time = time.ticks_ms()
    response = ""
    while time.ticks_diff(time.ticks_ms(), start_time) < timeout:
        if uart.any():
            response += uart.read().decode('utf-8')
            if "OK" in response or "ERROR" in response:
                break
    print("Respuesta:", response)
    return response

# Inicializa el módulo SIM
def initialize_sim_module():
    send_at_command('AT')  # Test AT startup
    send_at_command('AT+CSQ')  # Signal quality test
    send_at_command('AT+CREG?')  # Network registration
    send_at_command('AT+CGATT?')  # GPRS attachment
    send_at_command('AT+CIPSHUT')  # Shut down previous connections
    send_at_command('AT+CIPSTATUS')  # Get connection status
    send_at_command('AT+CIPMUX=0')  # Configure for single connection
    send_at_command('AT+CSTT="APN","UUUU","PASSWORD"')  # Set APN, replace with your APN, user, and password
    time.sleep(2)  # Agrega un pequeño retraso
    send_at_command('AT+CIICR')  # Bring up wireless connection
    time.sleep(2)  # Agrega un pequeño retraso
    send_at_command('AT+CIFSR')  # Get local IP address
    time.sleep(2)  # Agrega un pequeño retraso
    response = send_at_command('AT+CIPSTART="TCP","io.adafruit.com",1883')  # Start TCP connection to MQTT server
    if "ERROR" in response:
        raise Exception("No se pudo establecer la conexión TCP")
    print("Conexión TCP establecida con éxito")

# Función principal
def main():
    try:
        initialize_sim_module()

        # Configuración del cliente MQTT
        client = MQTTClient("APN", "io.adafruit.com", port=1883, user="UUUU", password="PASSWORD")

        # Conectar al servidor MQTT
        client.connect()
        print("Conectado al servidor MQTT")

        # Función de callback para mensajes recibidos
        def sub_cb(topic, msg):
            print(f"Mensaje recibido en el topic {topic}: {msg}")

        # Suscribirse a un topic
        client.set_callback(sub_cb)
        client.subscribe(b"UUUU/feeds/rizq")
        print("Suscrito al topic: Tirejas/feeds/rizq")

        # Publicar mensajes en un topic
        while True:
            client.publish(b"UUUU/feeds/presionizq", b"12.3")
            print("Publicado en el topic: UUUU/feeds/presionizq, Mensaje: 12.3")
            time.sleep(10)  # Publicar cada 10 segundos

    except Exception as e:
        print("Error:", str(e))

if __name__ == '__main__':
    main()

>>> %Run -c $EDITOR_CONTENT
MPY: soft reboot
Enviando: AT
Respuesta: AT
OK
Enviando: AT+CSQ
Respuesta: AT+CSQ
+CSQ: 25,0
OK
Enviando: AT+CREG?
Respuesta: AT+CREG?
+CREG: 0,1
OK
Enviando: AT+CGATT?
Respuesta: AT+CGATT?
+CGATT: 1
OK
Enviando: AT+CIPSHUT
Respuesta: AT+CIPSHUT
SHUT OK
Enviando: AT+CIPSTATUS
Respuesta: AT+CIPSTATUS
OK
STATE: IP INITIAL
Enviando: AT+CIPMUX=0
Respuesta: AT+CIPMUX=0
OK
Enviando: AT+CSTT="telefonica.es","AAAA","BBB"
Respuesta: AT+CSTT="telefonica.es","AAAA","BBB"
OK
Enviando: AT+CIICR
Respuesta: AT+CIICR
OK
Enviando: AT+CIFSR
Respuesta: AT+CIFSR
10.127.27.201
Enviando: AT+CIPSTART="TCP","io.adafruit.com",1883
Respuesta: AT+CIPSTART="TCP","io.adafruit.com",1883
OK
Conexión TCP establecida con éxito
Error: -6
>>> 

[tirejas]

Thanks, Stewart, for the advice. Should I delate the previous post and publish it after it's been corrected? I'm really not sure how to proceed. Best regards. Luis Sanz De: Stewart Russell ***@***.***> Enviado el: lunes, 31 de marzo de 2025 3:05 Para: micropython/micropython ***@***.***> CC: tirejas ***@***.***>; Author ***@***.***> Asunto: Re: [micropython/micropython] Accessing an IoT Server via a SIM Module in Micropython (Discussion #17049) please read the Welcome (Please read before posting) · micropython · Discussion #9111 <#9111> post and format your code properly. We can't read it — Reply to this email directly, view it on GitHub <#17049 (comment)> , or unsubscribe <https://github.com/notifications/unsubscribe-auth/BHPU4PWSWSZGYC6VN2TGZAD2XCIFJAVCNFSM6AAAAAB2C7STFKVHI2DSMVQWIX3LMV43URDJONRXK43TNFXW4Q3PNVWWK3TUHMYTENRXGA3DCOA> . You are receiving this because you authored the thread.Message ID: ***@***.***>

[robert-hh]

You should be able to edit the first post and insert the missing lines with three backticks ``` as needed.

[tirejas]

Thanks robert-hh
Done.
Can you help me with this isue?

[jonnor]

SIM800L and similar modules are quite complex/finnicky. I do not recommend writing a driver from scratch. I have done so before, and it took many weeks of my life. They are also very finnicky from a hardware perspective, the peak power requirements are crazy.

A much better way is to use a LTE cat-m (or NB-IoT) module. And use the PPP driver in for example esp-idf as the driver. It is exposed in MicroPython, I think.

[tirejas]

Thanks, Jon.
Yes, you're right, it's proving very complicated and taking me a long time. The problem could be the 2G network. Where I live, the 2G network is still operational, but I'm afraid it's only for voice calls and text messages. No one has told me it's valid for IoT. I just bought a 3G SIM5320E modem. As soon as I receive it and test it, I'll let you know the results.

[CrabbyPete]

I made this https://github.com/CrabbyPete/watchible. Works great. I had issues when I tried the SIM800L

[tirejas]

hanks, Crabby Pete, for your help. I'm afraid my electronics and programming skills are very poor, so I can't keep up.
In any case, if you'd be so kind as to take a look at the program I'm sending and give me some hints as to why I'm not able to connect, I'd be very grateful.

[CrabbyPete]

tirejas,

I was playing around with the 800L and I used these commands, after CREG came back connected to the network.
commands = [
'cmee=2',
'sapbr=3,1,"Contype","GPRS"',
f'sapbr=3,1,APN,"{NETWORK}"',
'sapbr=1,1',
'sapbr=2,1',
'httpinit',
'httpssl=1',
'httppara="CID",1',
f'httppara=url,{URL}',
'httppara="REDIR",1',
'httpaction=0',
'httpread'
]

[tirejas]

Thank you very much. I'll try it and let you know how it goes.

[tirejas]

I entered the series of commands you suggested and the result was positive, thank you very much. However, when I try to connect to the server, I always get an error.
I've changed servers, now without a password, but it doesn't matter.
I don't know if the problem is the SIM module. I've tried other SIM7020E modules, with the same result, or perhaps the mqtt.simple library doesn't work with SIM modules.
I'm attaching the latest program and the console message when running it.

import machine
import time
from umqtt.simple import MQTTClient

# Configuración del UART
uart = machine.UART(0, baudrate=9600, tx=0, rx=1)

def send_command(command, delay=2):
    uart.write(command + '\r\n')
    time.sleep(delay)
    response = uart.read()
    print(response)
    return response

# Secuencia de comandos
commands = [
    'AT+CMEE=2',
    'AT+SAPBR=3,1,"Contype","GPRS"',
    'AT+SAPBR=3,1,"APN","telefonica.es"',
    'AT+SAPBR=1,1',
    'AT+SAPBR=2,1'
]

for command in commands:
    response = send_command(command)
    if 'ERROR' in response:
        print(f"Error en el comando: {command}")
        break

# Verificar si la conexión GPRS está activa
response = send_command('AT+SAPBR=2,1')
if '1,1' in response:
    print("Conexión GPRS establecida correctamente.")
else:
    print("Error en la conexión GPRS.")

# Continuar con los comandos HTTP si la conexión GPRS está activa
http_commands = [
    'AT+HTTPINIT',
    'AT+HTTPSSL=1',
    'AT+HTTPPARA="CID",1',
    'AT+HTTPPARA="URL","broker.emqx.io"',
    'AT+HTTPPARA="REDIR",1',
    'AT+HTTPACTION=0',
    'AT+HTTPREAD'
]

for command in http_commands:
    send_command(command)

# Publicar el valor en el topic MQTT
mqtt_server = "broker.emqx.io"
client_id = "mqttx_765f3c8f"
topic = "Tirejas/feeds/presionizq"
message = "33.3"

def connect_and_publish():
    client = MQTTClient(client_id, mqtt_server)
    time.sleep(2)
    client.connect()
    client.publish(topic, message)
    client.disconnect()

connect_and_publish()

# Cierra la conexión HTTP
time.sleep(2)
send_command('AT+HTTPTERM')


Console:
>>> %Run -c $EDITOR_CONTENT

MPY: soft reboot
b'AT+CMEE=2\r\r\nOK\r\n'
b'AT+SAPBR=3,1,"Contype","GPRS"\r\r\nOK\r\n'
b'AT+SAPBR=3,1,"APN","telefonica.es"\r\r\nOK\r\n'
b'AT+SAPBR=1,1\r\r\nOK\r\n'
b'AT+SAPBR=2,1\r\r\n+SAPBR: 1,1,"10.111.133.184"\r\n\r\nOK\r\n'
b'AT+SAPBR=2,1\r\r\n+SAPBR: 1,1,"10.111.133.184"\r\n\r\nOK\r\n'
Conexión GPRS establecida correctamente.
b'AT+HTTPINIT\r\r\nOK\r\n'
b'AT+HTTPSSL=1\r\r\nOK\r\n'
b'AT+HTTPPARA="CID",1\r\r\nOK\r\n'
b'AT+HTTPPARA="URL","broker.emqx.io"\r\r\nOK\r\n'
b'AT+HTTPPARA="REDIR",1\r\r\nOK\r\n'
b'AT+HTTPACTION=0\r\r\nOK\r\n'
b'AT+HTTPREAD\r\r\nOK\r\n'
Traceback (most recent call last):
  File "<stdin>", line 64, in <module>
  File "<stdin>", line 60, in connect_and_publish
  File "/lib/umqtt/simple.py", line 66, in connect
OSError: -6
>>> ```

[jonnor]

MQTT is not HTTP - so that cannot really work. Furthermore, the umqtt library uses the TCP/IP stack in the microcontroller. Whereas, the command you use are for using the TCP/IP stack in the modem. Those cannot be combined either.

As mentioned before, you should use PPP. That way the TCP/IP stack is on microcontroller, and the modem is relatively dumb transport.
See https://docs.micropython.org/en/latest/library/network.PPP.html

[tirejas]

Thank you very much, Jonnor, for focusing my attention.
I understand that I need to configure UART communication with the modem.
Initialize the modem with PPP-specific commands.
Initialize PPP and connect.
Configure MQTT and connect, publish topics.
Is this correct?

[krismc]

There's an mqqt example for the sim7020 at https://www.waveshare.com/wiki/Pico-SIM7020E-NB-IoT that might help?

[tirejas]

Thank you very much

[CrabbyPete]

Here are my commands for connecting with a Quectel BC66. Here I am using AWS and have certs for it. The SIM800L does not have any MQTT commands, but according to this You should be able to connect with TCP, which MQTT uses as a low level protocol. The issue will be how you provide the security, whether its a password or a cert. You also have to do the low level work that MQTT provides by hand.

"b'at+qsslcfg=0,0,"sslversion",4\r\r\n'"
"b'OK\r\n'"
"b'at+qsslcfg=0,0,"seclevel",2\r\r\n'"
"b'OK\r\n'"
"b'at+qsslcfg=0,0,"cacert"\r\r\n'"
"b'>\r\n'"
"b'-----BEGIN CERTIFICATE-----\n'"
"b'MIIDQTCCAimgAwIBAgITBmyfz5m/jAo54vB4ikPmljZbyjANBgkqhkiG9w0BAQsF\n'"
...
"b'rqXRfboQnoZsG4q5WTP468SQvvG5\n'"
"b'-----END CERTIFICATE-----\r\n'"
"b'+QSSLCFG: 0,0,"cacert",1187\r\n'"
"b'\r\n'"
"b'OK\r\n'"
"b'at+qsslcfg=0,0,"clientkey"\r\r\n'"
"b'>\r\n'"
"b'-----BEGIN RSA PRIVATE KEY-----\n'"
"b'MIIEpAIBAAKCAQEAyKmX5WCR5wxOMQFwYNDLrFh26bskm00PSJ7fB3cDsdV1dAtv\n'"
...
"b'c5N1BqiLoYx1zT7aCSYnSddnp9lGdVKbSaP5wWtzUSWLeaalHTp1Ew==\n'"
"b'-----END RSA PRIVATE KEY-----\n'"
"b'\r\n'"
"b'+QSSLCFG: 0,0,"clientkey",1679\r\n'"
"b'\r\n'"
"b'OK\r\n'"
"b'at+qsslcfg=0,0,"clientcert"\r\r\n'"
"b'>\r\n'"
"b'-----BEGIN CERTIFICATE-----\n'"
"b'MIIDWjCCAkKgAwIBAgIVAPmUhW4BqJMaJTvX/cgwXIOjUTYHMA0GCSqGSIb3DQEB\n'"
...
"b'ZsoFgDqRWr4jI4i3inKIN0LTSQKTATPoS/X+vhov/RsQYzVQdDfmol/c3kRKzA==\n'"
"b'-----END CERTIFICATE-----\n'"
"b'\r\n'"
"b'+QSSLCFG: 0,0,"clientcert",1224\r\n'"
"b'\r\n'"
"b'OK\r\n'"
"b'at+qmtcfg="ssl",1,1,0,0\r\r\n'"
"b'OK\r\n'"
"b'at+qmtopen=1,"-ats.iot.us-east-1.amazonaws.com",8883\r\r\n'"
"b'OK\r\n'"
"b'\r\n'"
"b'+QMTOPEN: 1,0\r\n'"
Opened MQTT

[tirejas]

Thank you very much