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Category: FAQ:

  • 1.1 Is it possible to use diode to convert 5V to 4.4V for power supply?

    It is not recommended to use this method, because the forward voltage of diode will change as the current changes, which will cause the instability of the module’s power supply and make the module work abnormally.It is not recommended to use this method, because the forward voltage of diode will change as the current changes, which will cause the instability of the module’s power supply and make the module work abnormally.

  • 1.2 Is it possible to use standard USB to supply power for the module?

    The output current of standard USB interface is max.500mA, which does not meet the peak current demand of GPRS module. If USB interface is required to supply power for the module, you can operate like below:

    1) Use AT+QGPCLASS to set the GPRS level to 8. This command will take effect after next restart of the module. There is only one uplink slot when the GPRS level is 8.

    2) Connect two 2200uF capacitors with low ESR value to VBAT pin and keep them close to the module.

    3) It is recommended that the VBAT voltage should be adjusted between 4.2V and 4.5V which can prevent the module from working abnormally due to the voltage drop of VBAT.

    4) Test the module under the weak signal environment (such as CSQ value is less than 14). If the module works properly, it indicates that USB interface can supply power for the module.

  • 1.3 How to use higher DC voltage (e.g. automotive 12V/24V/48V) to power the module?

    It is suggested to implement it by two stage conversion. First stage is to step down to 5V via DC-DC converter and the second stage is to convert to 4V from 5V via LDO as shown in the figure below.

  • 1.4 How to test the power consumption in the sleep mode?

    The best way to test the power consumption of module is to supply power for module separately in case that peripheral circuit might cause extra power consumption.ONLY VBAT/GND/SIM/RF/PWRKEY should be connected, in this case, the power consumption on VBAT will be the exact consumption of module.

    To test the power consumption on the EVB board, please operate like below:

    (1) Remove the resistor R110 and R102.

    (2) Solder R224 with a 0Ω resistor.

    (3) Plug the power adapter and turn on the 5V power switch. See figure below.



    1. Removing R110 is to make sure that the power supply of module is directly from the external power via J103. 

    2. Power adapter should be plugged to offer the power for the RS232 converter and LEDs, which will avoid the increase of power consumption caused by sink current.

    3. VDD_EXT of module is used to supply the power for RS232 converter. In order to avoid the RS232 converter being supplied by VDD_EXT, R102 should be removed. Meanwhile, R224 should be soldered a 0R resister to supply the power for RS232 converter with 5V power adapter.

  • 2.1 How to deal with the UART communication failure?

    First of all, check the level of STATUS pin to judge whether the module is powered on or not. High level means the module is on and low level means the module is off. If module has already been powered on, you need to analyze it according to the following two situations.

    2.1.1 Abnormal Communication between PC and EVB

    1) Check whether the serial cable is OK or not. It is recommended to use the Quectel serial cable, which can avoid the abnormal communication between PC and EVB when using three-wire serial cable under the hardware flow control.

    2) Check the usage of the serial tool is right or wrong. If you use QCOM tool provided by Quectel, please tick RTS as the following figure.

    2.1.2 Abnormal Communication between MCU and module

    1) Please check whether the baud rate of MCU is corresponding with the module’s or not. Quectel module is set as autobauding by default. Send 2~4 AT Commands to synchronize the baud rate of the module. Some modules might need more AT commands to get synchronized. Once the module gets synchronized, it is suggested to use AT+IPR command to fix the baud rate and use AT&W to save it.

    2) If using three wires to connect MCU and the module, it is needed to check if the hardware control is closed.

  • 2.2 How to match the UART level between MCU and module?

    1) If the UART level of MCU is 3.3V, the level shift circuit is shown as below.

    2) If the UART level of MCU is 3V, just change the resistor value of 5.6K in the above Figure to 15K.

    3) If the UART level of MCU is 5V, the level shift circuit is shown as below.

  • 3.1 How to enable hot-swap function of SIM card?

    1. Ensure that the SIM_PRESENCE pin of module has been  connected.

    2. Enable the SIM card detection function by AT command.


    AT+QSIMSTAT=1               // Enable SIM inserted status report

    AT+QSIMDET=1,1 or AT+QSIMDET=1,0    // Enable SIM card presence detection

    When SIM card is pulled out or plugged in, module will immediately respond:


    +QSIMSTAT:1,0  or  +QSIMSTAT:1,1

    Call Ready

  • 3.3 SIM card cannot be recognized after the module works for a while

    Description: module responds +CPIN: READY after powering on, but after a while, it responds +CPIN: NOT READY. You can analyze the issue from the following aspects.

    1) RF interference

         a. You can use AT+CFUN=4,1 to disable the RF function and check if the issue still exists.

         b. Put the antenna close to the SIM card and SIM card signal line to check if the issue appears more frequently.

         c. Please check if there is electrical/magnetic field around the testing field, for example, high voltage transmission lines, high-power wireless devices, etc. Try to use shielding case to cover the SIM card and all the SIM signal lines to confirm if the issue will be resolved.

         d. Try to connect the pins of SIM card signal (SIM_DATA/SIM_CLK/SIM_RST/SIM_VDD) to ground via 15~33PF capacitors to eliminate the RF interference.

    2) Hardware design issue

    If the VBAT trace is too close to the SIM card signal line, module can still recognize the SIM card because of the low ripple of VBAT when powering on. But when module starts to register to the network, the ripple will become higher and interfere the SIM card signal line, which might cause the module not to recognize the SIM card. You can cut off the power supplied to the module and use external power source to directly feed to VBAT pin of module to see if the issue is resolved.

    3) Poor quality of SIM card

         a. Change another SIM card to see if there is the same problem on the new SIM card.

         b. Put the problematic card on the Quectel EVB or cell phone to see if there is the same problem.

  • 3.4 What reasons can cause the module not to register to the network after powering on?

    1. Use AT+CPIN to check whether module can recognize the SIM card or not.

    2. Check whether the SIM card is out of service or not.

    3.  Use AT+QBAND and AT+CFUN to check the working band and working mode of the module respectively.

    4. Use AT+CSQ to check whether the RF signal is normal or not.

    5. Use AT+GSN to check whether the IMEI number of the module is legal or not. Some customers may change the IMEI number, which causes network in some areas to regard this changed IMEI number as illegal and forbid the module to register to the network.

  • 4.1 What are the design considerations when using embedded antenna?

    1. Choose DC-DC converter with low switching frequency.

    2. The input and output line of DC-DC converter should be far away from antenna. Note: All Power Supplies should be far away from antenna.

    3. Antenna should be far away from LCD, especially the FPC of LCD.

    4. Antenna and RF trace should be far away from the main CPU, including the SDRAM and FLASH. It is recommended to add EMI filters at the high-speed line of CPU, SDRAM and FLASH.

    5. Use the shielding case to cover the CPU, SDRAM, FLASH and DC-DC converter.

    6. Antenna should be far away from the SIM card.

    7. Please ensure that the enclosure surrounding the antenna and structural components are not metal materials.

    8. As the enclosure and components surrounding the embedded antenna will have influence on the RF performance, it is strongly suggested to provide the device to the antenna vendor to optimize the antenna performance. Also Quectel provides the document <Antenna_Design_Notes>  for your reference.