TTGO T-Call + SD-Card

[tadejales]

Hello,

I have a specific problem with my system configuration. I have a TTGO T-Call module with an integrated SIM800L module. I want to add an external SD card module (3.3V) to the system as I want to store data there long-term. The expected size is in the area of hundreds of MB, so I can't just store it in flash memory.

What I want to achieve is to send data over GPRS to the cloud via HTTP POST requests and store the data on the SD card as well for archiving in case transmission fails. Initially, I was just using flash memory to store data as I thought storing them on an SD card later would be trivial. Everything worked fine. Then I added an external SD card, and now I have a problem with transmission. If I initialize the SIM module first, I am able to obtain an IP address and connection, but later I cannot transmit data. It fails even to get the signal strength.

Then I tried initializing the SD card first and the SIM module after that, and the SIM module initialization fails. Based on this, I assume I am having a problem with power. I am using an external power supply of 24V@1A and then stepping down the voltage with a voltage converter:

Is there a way to solve the issue without changing the power supply, perhaps by adding some inductors and/or capacitors to stabilize the circuit?

[davefes]

That buck converter is rated at 3A, which you should be able to get from your 24V 1A supply. Actually, with 100uH about 2.5A. I would be probing the 5V with an oscilloscope before I changed the power setup. If you see voltage dips maybe try a few more 1000uF low-ESR caps on the output.

[davefes]

Maybe, try SoftSPI and see if that changes behaviour.

Good luck!

[tadejales]

So I have taken my time to create MVP model with GSM and SD card module to test the functionality.

from lib.sim800l import Modem, Response
import sdcard 
from machine import  SPI, Pin
import utime as time
import json
import os

class Gsm():

    def __init__(self) -> None:
        self.config = {"username": "", "password": "", "apn": "internet.t-2.net"}

        self.modem = Modem(MODEM_POWER_ON_PIN=23, MODEM_PWKEY_PIN=4, MODEM_RST_PIN=5, MODEM_TX_PIN=26, MODEM_RX_PIN=27) 
        # Initialize the modem
        self.modem.initialize()

        timeout = 10
        try:
            self.modem.connect(apn=self.config["apn"], user=self.config["username"], pwd=self.config["password"])
            for _ in range(timeout):
                if self.modem.get_ip_addr():
                    print(f"GPRS connected: {self.modem.get_ip_addr()}")
                    break
                elif _ == timeout-1:
                    print(f"TIMEOUT: Unable to obtain GPRS connection")
                else:
                    time.sleep(1)

        except Exception as e:
            print(f"Error during GPRS connection establishment: {e}")

    def http_request(self, url:str, mode: str='GET', data: any=None, content_type: str='application/json', auth_token: str=None) -> Response:
        return self.modem.http_request(url=url, mode=mode, data=data, content_type=content_type, auth_token=auth_token)

def sd_init():
    print("Trying to mount SD card")
    spi = SPI(1, sck=Pin(18), mosi=Pin(23), miso=Pin(19))
    db = sdcard.SDCard(spi, Pin(5))
    os.mount(db, "/db")

def test_write_read():

    # Test write
    print("Testing write")
    with open("/db/test.txt", "w") as f:
        f.write("TEST")

    # Test read
    print("Testing read")
    with open("/db/test.txt", "r") as f:
        print(f"File contents: {f.read()}")

def boot():
    # Start modem
    gsm = Gsm()

    # Test SD card interaction
    sd_init()
    test_write_read()

    # Verify if modem connection still live
    print("Verifying modem connection")
    if gsm.modem.get_ip_addr():
        print(f"Modem connected with IP: {gsm.modem.get_ip_addr()}")
    else:
        print("Modem not connected")

# RUN
boot()

So here are my findings:

1. If I comment out sd_init() and test_write_read() in boot() modem connects and verifys the connection

GPRS connected: 10.20.186.123
Verifying modem connection
Modem connected with IP: 10.20.186.123

2. If I init GSM module first it establishes the connection. After initing SD card and write/read the modem loses connection. Output bellow:

GPRS connected: 10.20.201.231
Trying to mount SD card
Testing write
Testing read
File contents: TEST
Verifying modem connection
Traceback (most recent call last):
  File "boot.py", line 68, in <module>
  File "boot.py", line 62, in boot
  File "lib/sim800l.py", line 287, in get_ip_addr
  File "lib/sim800l.py", line 183, in execute_at_command
Exception: Timeout for command "getbear" (timeout=3)

3. If I put sd_test() and test_write_read() above gsm = Gsm() and init SD card before GSM module, gsm gets connected afterwards. Output bellow:

Trying to mount SD card
Testing write
Testing read
File contents: TEST
GPRS connected: 10.40.216.190
Verifying modem connection
Modem connected with IP: 10.40.216.190

4. If I put sd_test() and test_write_read() above gsm = Gsm() and init SD card before GSM module, gsm gets connected afterwards. If I then add test_write_read() after IP check SD card interface fails. Output bellow:

Trying to mount SD card
Testing write
Testing read
File contents: TEST
GPRS connected: 10.20.226.10
Verifying modem connection
Modem connected with IP: 10.20.226.10
Testing write
Traceback (most recent call last):
  File "boot.py", line 79, in <module>
  File "boot.py", line 68, in boot
  File "boot.py", line 45, in test_write_read
  File "/lib/sdcard.py", line 238, in readblocks
OSError: [Errno 5] EIO

I hope you can take a bit of time if you see any concerning parts here or give me any suggestion what to do. I will take my time ang go thrue the threads that you have sugested looking at.

From what I see the problem might be in pins 5 and 23 as they are used in both instances.

[davefes]

There are a few SPI experts around. I didn't pickup on the duplication of pins. I think for SPI you need at least a separate reset pin for each device. If you have some spare GPIO I would try a SoftSPI on them.

Edit:
In codebase.zip

# Pin SD SPI
MOSI_PIN = const(23)
MISO_PIN = const(19)
SCK_PIN = const(18)
CS_PIN = const(5)

# SIM800L
MODEM_TX_PIN = const(26)
MODEM_RX_PIN = const(27)
MODEM_PWKEY_PIN = const(4)
MODEM_RST_PIN = const(5)
MODEM_POWER_ON_PIN = const(23)
`
maybe it is that pin 23.  I see where I got confused ... the names for the SIM800L pins suggested that it was driven via a UART, not SPI.

[robert-hh]

a separate reset pin for each device

a separate chip select pin for each device. And you may have to adapt the baud rates for each device.

[davefes]

Be aware that there are only 2 hardware SPI buses on specific pins.
https://docs.micropython.org/en/latest/esp32/quickref.html

[jonnor]

Have you checked the data lines to/from the the SIM800 with an oscilloscope - ensuring that the signal integrity is high, both generally and not too much affected by SD card communication? Also, be sure to verify the timings on the serial connection - if they are borderline, corruptions can trigger from slight temperature/humidty/voltage changes.

SIM800L, and GSM generally, is a power hungry beast. Major PITA compared to 4G LTE modems. Would not recommend for new design.

[tadejales]

I do not have oscilloscope neither do I have the knowlage about the things you are explaining to me. The problem was that some pins were shared. Now I am using different pins for GSM and SD card and it seems to work fine.

Another question would be; Are LTE modules easier to use and simpler to implement, hardware and software wise? I can still alter the design. I am just using TTGO T-call v1.3 because I got it from the friend and it was the only one that had everything onboard. Now I was looking at the TTGO site and v1.4 has option with LTE module. Would you recommed using that?