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.
- voltage:
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.
-
current
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. -
baud rate
my chip works on 115200 baud rate defaut, try that rate if you see nothing on the right voltage. -
pin connection
make sure pins are well welded on the board. this simple sentence costed me a night. -
Soft Serial
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
SIM900/SIM800 not working - Possible reasons - Tips n Tricks - Embedded World
sorry for my bad english, hope this can save anybody's time