making a summing up on the solution here after get the sim800l chip going working through 3 night.
hope this can help anybody.
check the following may get your chip going and save your time.
the chip only works exactly between 3.4V ~ 4.4V, < 3.4V you will get a “UNDER-VOLTAGE POWER DOWN” from serial and > 4.4V you will get a “OVER-VOLTAGE POWER DOWN” error message, and you may not have the chance to see this because under unsuitable voltage the chip is just a bad ass.
a quick work around on power supply is to find a cellphone battery, which will most likely give you a stable 3.7V power(at least in China).
if you see a green led keep blinking once per sec nonstop, good for you, that means your chip is good to go.
if you see the green led went out after blinking for 4 or 5 times, that may mean the chip is up but voltage supply is wrong(mostly too high), the chip will be shutdown after the blinking, you may see the error message after a big while if your baud rate is right.
if you see nothing, that means your input voltage is too low, can’t drive the chip.
make sure the chip can get > 1amps. i used a 5V1.5A power source, and measured out an current around 100mA when get an “OK” response on “AT” command.
my chip works on 115200 baud rate defaut, try that rate if you see nothing on the right voltage.
make sure pins are well welded on the board. this simple sentence costed me a night.
if you still cant get it going, try to use a hardware serial instead of soft one.
if you still have any problem with it, carefully read exactly every word here, really helps a lot
GitHub - stephaneAG/SIM800L: LIttle repo containing code & docs for the SIM800L module
sorry for my bad english, hope this can save anybody’s time