Product Description
This module is designed with 5V/3.3V microcontroller logic conversion circuit, because GSM/GPRS core is 2.8V, and the logic level of ordinary microcontroller 3.3V or 5V is different. Many other development boards do not design this part of the conversion circuit, which can cause damage to the microcontroller or GSM module over a long period of time. Some manufacturers simply connect two resistors in series in order to save costs, which is also undesirable. In order to have a stable control system, logic level conversion is indispensable! The VTTL pin is not used to supply power to the GSM module, but only to let the GSM module know whether the TTL level it wants to output is to be converted to 3.3V or 5V.
Special note
1. The power supply voltage cannot exceed 4.4V, otherwise, the module may be damaged
2. Reserve IPEX antenna ports for self-matching antennas
3.When the PWX is grounded, the module starts automatically when it is powered on.Before use,the PWX and GND should be short-connected -- the module starts automatically when it is started
4. Do not connect the positive and negative terminals of the power supply inversely, otherwise, the module may be damaged

Product Features
SIM800C module is a high-performance and cost-effective industrial-grade GSM/GPRS module (development board). This module uses SIMCOM's industrial grade quad-band 850/900/1800/1900MHz SIM800 chip, which can realize the transmission of voice, SMS, data and fax information with low power consumption.SIM80OC uses the high configuration 32M version with Bluetooth/TTS/DTMF, which is the high configuration among all SIM80OC versions.G800C is SIM800C compatible chip, only GSM/GPRS function, the command is compatible with SIM800C, you can choose this version if you have requirement for cost.
GPRS:
Downlink transmission maximum 85.6kbps
The maximum uplink transmission is 85.6kbps
Encoding format: CS-1, CS-2, CS-3,CS-4
Supports PAP(Password Authentication Protocol), which is commonly used for PPP connections
Embedded TCP/IP protocol
Support packet Broadcast Control Channel (PBCCH)
Support for unstructured supplementary Data Services (USSD)
A serial port
One standard fully functional serial port is supported by default
You can send AT commands and data through the serial port
Supports RTS/CTS hardware flow control and can be turned on or off by software
 
Pin Description
5V: power supply pin, the only input DC5V, used to power the board.
V_TTL: access control board microcontroller core target voltage of 5V / 3.3V (according to its own microcontroller is much to distinguish – kernel v), this pin is used to convert the GSM module board TXD and RXD for the corresponding TTL logic.
GND: power supply ground
TXD: send pin serial port module, TTL level (not directly connected to RS232 level)
RXD: receive pin serial port module, TTL level (not directly connected to RS232 level)
DTR: Data Terminal Ready
SPKP: Core Audio output pin
SPKN: Core Audio output pin
MICN: Core Audio input
MICP: Core Audio input
RI: Ring core pin tips
VRTC: RTC pin external battery
GND: power supply ground
PWX: This pin can turn down or turn off the module
GND: power supply ground
VBAT: lithium battery input pin, 3.3v-4.4v
 
Resources:
1. Send/Receive SMS with STM32F103C8 and SIM800C GSM Module