Don't use a diode unless your power supply is very stable. i've used that module with a external power supply regulated to 4.95V with a diode to down voltage to 4.25V. Then i replaced the power supply for a pack of batteries (5.08V), after that i got strange behavior, sometimes SIM800 didn´t send any messages. I figured that at 5.08V, the voltage on the SIM800 was 4.35V that leeds to OVERVOLTAGE warnings. Better is to use a LDO or some external power very stable, around 4.2V
It's not only about the Vcc power, pins need also a voltage divider. The serial port characteristic of the spec says 2.8 V max:see p. 31 http://www.seeedstudio.com/wiki/images/4/46/SIM800L_Hardware_Design_V1.00.pdf
Yes, it could be. But sometimes these modules came with a voltage "translator" and you can use a 5V signals, but you have to be really carerful with power voltage, not to burn it.
Maybe a diode is faster o slower than others? (i don't know about this kind of problem)Regards,
Are you sure you are powering at 3,9V? Measure it with a voltimeter when SIM800 is ON, i guess your batteries cannot mantain a steady 4,5VI had one working for a month and suddenly it stops working. I was powering it with 4V but RX/TX lines were connected directly to 5V arduino ports. I replaced by a spare one but this time add a level shiftter between arduino and RX to lower the signal to 2,8V. It is running now for 2 weeks without a fault.For my experience, i advise:1) Power SIM800 with a 4V LDO. Use a diode only if your input is very stable2) Keep RX/TX lines below 2,8V