Sim800L call ready sms ready - Uploading images & Power supply connections


I would like to ask you for a bit of help. We are trying to build a fleet management system. As a GSM module we use SIM800L

We only need the GSM Module for Data Transfer, so we tested with RP3.
The problem is that it is very hectic/rare, sometimes it connects to the network sometimes it does not.
It is working very strange. My biggest problem is that it is giving me frequently the following message:
Why is it doing this?

We plugged/set it up as follow:
SIM800L - Rpi
vcc -> 5V
RX/TX -> Tx/Rx

We tried to separate the power supply of VCC and GND both from a different power by 4,2 V, but the outcome is the same.
Interesting that if we do not plug them directly to RX/TX pins, instead of this we use a cable and connect to the USB port it works perfectly.

Why do we experience this? What is the reason of this? How should we connect them to each other? Thank you very much for your help in advance

SMS READY & CALL READY are URC reported when module restarts and is ready to accept SMS & CALL commands. Its just a indication that the initialization is complete.

SIM800L board you bought, does it have onboard regulator?
If not , You cannot give 5V to it. Its maximum input voltage is 4.2V. If you give 5V it will get damaged.

The restart can also happen if the IO levels are not translated properly. if you connect UART TX RX lines of a MCU running at 5V it might lead to restart as module will go into over voltage shutdown mode.
Make sure you use appropriate level translation from 5V IO levels to 2.8V IO levels.

Dear Ravi,
I read about your writings on forums and blogs.
I am writing this email in orde to ask for your help.

We are an international transport company and we are building a fleet tracking system.
As a Base we use a Rapsberry Pi3, because we would like to connect many peripheries e.g.:

  • GPS datas
  • Temperature sendor datas
  • The vehicle’s CAN/FMS datas
  • We also would like to fit in a tablet to the truck in order to send messages/communicate to/with the driver
  • Take a picture inside the tralier and send itt o the office

Mainly we use these functions and they are working…
But we are stucked with the GSM modul. We have the following criterias with the GSM modul:

  • Continous data transfer, in order the have immediate message sending/receiving.
  • We do use it for SMS sending/receiving
  • We do not need it for phonecalls
    We need the smallest and most practical accesory, what I found which is the SIM800L.

Now I need your help, because we have promlems with it and damaged a couple as a test. :smiley:
Do you think that this would be OK for our project?

Would you be so kind and help me with a Noun in order to connect the shield to the RPI3? Do we have to use the RX/TX pins to connect it directly to the R-PI? RPI gives 3,3 OR 5V?!The power is sorted out by RPI, or should we use totally different power and power it up by 4V?

  • How can we sort out the Reset? Do we have to reset the tool? For example if the truck drive in to a long tunnel or get to the ferry and it has no signal for hours after this do we have to reset or it reconnecting to the network automatically?

Thank you very much for reading my message and waiting for your prompt reply

Dear Zsolt,
Using SIM800L which is a 2G module, you can expect to upload 100KB of data in 5-10 minutes with error check and retries in 2KB chunks.
You can send maximum 2KB data in HTTP requests reliably. So you have to divide data and keep sending.
You have to use 4V,2A capable supply to power the GSM module.
The TX RX lines from R-Pi should be 2.8V IO compatible or it will damage the UART lines of SIM800L.
GSM modules cannot be powered from USB as current is not enough. And again you have to reduce 5V from USB to 4V.
Where are you from?
You can use 3G module instead of 2G for faster data transfer.

Dear Ravi!

We based in Hungary Middle of Europe.
We think about 3G or 4G, but first we try the 2G. as it is available everywhere.
How we can restart the sim800L, with a Rpi and who we can connect a little batery or capacitors to the SIM800L?
Thanks for your reply

I would advise you use a development board for your proof of concept:
or with GPS
(Dont forget the main board)

2G is better coverage for EU at the moment. I have discussed with O2/Telefonica/Three that there are no plans to turn it off for at least 5 years. In fact 3g may switch off before 2G here, so if you need fast data go for 4g with 2g fallback.

Helo Patrick!
I think 2G is better right now, than the 3G, unfortunatelly I did not find a good shield with 4G or LTE. Can you advise me a useful 4G shield and may you link it here?!

Hy Ravi,

Please help me to connect the SIM800L to the RPI.
We would like to use the RPI + SIM800L + capacitator (or LiPO), but we do not use what is the best way to connect to each other in order to power it with 4V.
What is the situation with the RX/TX levels?
You would be able to save the world, :smiley: at least my world

The power supply connection is straight forward. It should work by connecting 4V battery + and - to VBATT and GND of module.
For TX and RX you need to use level translator chips like TXS0102 or TXS0104. Then you are safe to connect. You will get level translators made out of transistors also. You can use them too.


You could try this devkit:
Though I have not used it myself.

One thing I just noticed:

It is working very strange. My biggest problem is that it is giving me frequently the following message:
Why is it doing this?

Sounds like a power reset. The power supply is not able for the current demands or you need more capacitance


Have you figured out how to solve “SMS ready call ready” problem?
I am stuck with same

The problem is due to the current of supply . your supply must support high current for a short period of time . If you have a buck converter that supports more than 2A but still have the same problem just add a big capacitor to output of your converter (2000Uf or higher)