Familiarize with SIM800C TTL Modem

SIM800C TTL UART modem, the latest addition to rhydoLABZ GPS/GPRS modem, helps to add wireless connectivity to your project using TTL UART interface. It is built with Quad Band GSM/GPRS engine – SIM800C, that works on frequencies 850/ 900/ 1800/ 1900 MHz. SIM800C comes in  SMT package with 42 pads and physical dimensions of 17.6*15.7*2.3mm.

The modem comes with selectable interfacing voltage which allows you to connect 5V & 3V3 microcontroller directly without any additional level conversion chips. SIM800C module is having internal TCP/IP stack to enable you to connect with internet via GPRS (supports the GPRS coding schemes CS-1, CS-2, CS-3 and CS-4). It is suitable for SMS, voice as well as data transfer application in M2M interface.

Hardware Overview

  • K1 -Micro SIM card socket

Due to the increasing popularity of micro SIM cards, micro SIM card socket has been integrated with the modem. The SIM card can be conveniently pushed in and pulled out as indicated on the socket.

  • K3Audio Interface

To make / answer a voice call with this modem, additional interface of headset/handset is required. The audio interface port allows you to directly make this addition. The port has 4 pins

SPKP / SPKN – The speaker of headset/handset can be tied directly to these two pins
MICP / MICN – The mic of headset/handset can be tied directly to these two pins

  • K4Debug / Firmware updation

SIM800C supports firmware updation through full modem serial port or USB port (SIMCOM recommends to use USB port)

  • K6 – Power and Communication port


  • Vcc – Power Supply +ve [Input range : 5-12V, 2A]
  • GND – Ground
  • VINT – Interface Voltage level. The voltage on this pin determines the communication level of the modem i.e, if 5V is supplied to this pin, the modem communicates in 5V level and if if 3.3V is supplied,  the modem communicates in 3.3V level.     CAUTION! : Maximum voltage possible is 5.5V
  • TXD – Transmitter pin of modem
  • RXD – Receiver pin of modem
  • PKEY – This pin can be used to power ON/OFF GSM module with external controller. To manually turn ON the module, remove resistor R6 [shown below], power the modem, keep this pin HIGH for sometime and make it low. The module alternately turns ON/OFF for each pulse given.

  • K7 – Signal port

  • STS – STATUS pin of the modem taken out with inverted output. Turns LOW when the modem is powered and remains HIGH when the module is shutdown
  • NLIGHT - NETLIGHT pin of the modem taken out with inverted output. Turns LOW when the modem is powered and remains HIGH when the module is shutdown
  • CTS/RI – This pin gives the CTS/RI pins of the modem in the interface voltage level of the modem as selected by jumper J3
  • RTS/RFSYNC - This pin gives the RTS/RFSYNC pins of the modem in the interface voltage level of the modem as selected by jumper J4
  • LED Indications

There are 3 indicator LEDs that indicate power and network status of the modem

  • PWRLits up when the modem is powered
  • STS – Lits up when SIM800C is powered
  • NET – Indicates the network status of the modem as given below

J1External antenna interface. The modem comes with SMA and UFL connectors for external antenna interface.

J2 – This comes in handy when 5V is given as Vcc. If J2 is populated, VINT gets connected to Vcc and the modem interface automatically becomes 5V.

J3 – Selects the signal available on CTS/RI pin of K7

J4 – Selects the signal available on RTS/RFSYNC pin of K7

Dimension Diagram

Powering the modem

To power the modem, it is recommended to use 5-12V, 2A power source. When powered, you can see that the indicator LEDs – PWR, STS, NET lit up. Initially the NET LED blinks at a faster rate indicating that the SIM has not registered to network. The blink rate gets slower once the SIM gets registered.

However if you want to turn ON/OFF the SIM module manually, you can use the PKEY pin as mentioned above.

Interfacing with PC / Laptop

Having gone through the various hardware aspects of the modem, now we can see how to communicate with it from PC / laptop. As you know, the modem has TTL UART interface. So you cannot communicate directly with it from PC. You can use a USB-TTL converter based on FTDI chip as the mediator which allows you to communicate with PC via USB miniB Cable. You can get wide variety of such converters from rhydoLABZ. Before starting communication, don’t forget to install the driver in your PC.

The modem is powered from an external source and the converter is powered by USB.

In the connection diagram shown above, you can see that the VINT pin of modem is tied to VCCO of the converter [middle red line] that determines the interface level as 3.3 / 5V as selected by the PCB solder jumper on the other side of the converter.

Now you can communicate using any terminal software such as Realterm, Docklight etc..

Interfacing with 5V Microcontroller

To communicate with MCUs that use +5V level, you can use the following connections.

  • In the first case, VINT of modem is tied separately to +5V. The maximum voltage that the pin can tolerate is 5.5V. This connection can be followed if your power source is > 5V.

If you have a stable 5V power supply source, the VINT pin can be left open with J2 populated

Interfacing with 3.3V Microcontroller

CAUTION! When you are interfacing the modem with 3.3V MCU, ensure that J2 is open


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