Recently, I have been working on SIMCom's module peripheral circuit design, and now I will sort out the peripheral circuit design ideas.
1. Power scheme
Generally, the instantaneous current of this type of communication module can reach about 2A, so it is necessary to pay great attention to the power supply scheme, otherwise it is easy to cause the module to crash or restart. The power supply scheme can use DC-DC, LDO or DC-DC+LDO scheme. In addition to the basic power chip, because the instantaneous current of the module is relatively high, the capacitor configuration of the power supply of the module also needs to be considered.
(1) DC-DC solution
advantage:
① Wide input range of external power supply voltage;
②High power efficiency;
③The current is relatively large, generally 2-3A;
④The calorific value is low.
shortcoming:
① The power supply ripple is relatively large, and a magnetic bead needs to be added to the output to improve the power quality;
②Generally, the DC-DC solution occupies a relatively large PCB area;
③Because the DC-DC uses PWM to adjust the voltage, it will cause great interference;
④ The quiescent current is high, which is not suitable for low-power systems that work continuously.
Commonly used DC-DC chips are: MP1482, MP2303, LM2596ADJ, etc.
(2) LDO solution
advantage:
①Low ripple and stable output voltage;
② Linear devices, rarely have interference problems.
shortcoming:
① low efficiency;
②The calorific value is large;
③ The input voltage cannot be very wide (the input and output voltage difference is too large, and the power dissipation is large).
Many manufacturers of this kind of chip are doing it, such as TI, Xinlong, etc. Generally, the chip recommended by SIMCom is MIC29302.
(3) DC-DC+LDO solution
This solution is to reduce the external voltage to about 5V through DC-DC, and use LDO to reduce the voltage to 4.2V.
advantage:
①High efficiency;
②Low ripple and stable output voltage.
shortcoming:
①BOM cost is high;
②Similar to the DC-DC scheme, the interference is relatively large.
(4) Module power supply interface capacitance and protection configuration
Recommended external capacitors: 220uF tantalum capacitor*2+1uF ceramic capacitor+33pF ceramic capacitor+10pF ceramic capacitor+5.1V/500mW Zener diode, these devices should be placed close to the module. Zener diodes cannot be omitted. In many cases, the module is damaged due to the ESD interference of the power line. If the power supply circuit cannot provide enough 2A current, it is recommended to increase the capacitor capacity appropriately, and the capacitor capacity should not be less than 1000uF. When it can provide 2A current, the capacitor capacity should not be less than 330uF. Ensure that the module launch voltage drop is less than 300mV.
(5) Others
① The width of the power line should not be less than 80mil, and it is best to route it on the same side of the PCB;
②It is better to add a power-off function to the module to prevent the module from crashing. You can use a MOS tube or directly control the enable pin of the power chip.
2. Problems with MCU connection
(1) Serial port wiring problem
The module generally provides a full-featured serial port. If the MCU serial port speed is fast enough, you can only connect RX and TX. If the MCU serial port speed is relatively slow, it is recommended that the hardware flow control related pins should also be connected.
The two connection methods are shown in the following figure:
(2) Interface level problem
The common I/O level standards for SIMCom modules are 2.8V and 1.8V.
When the I/O level is 2.8V and the MCU level is 3.3V, the resistor divider method can be used to convert the I/O level, as shown in the following figure:
When the I/O level is 1.8V and the MCU level is 3.3V, it is best to use a special level conversion chip, such as the TXB0108 recommended by SIMCom. The circuit diagram is as follows:
(3) Others
The remaining I/Os are connected as needed, such as module status pins, etc. It is recommended to connect a 51 ohm resistor in series on the pins connecting the module and MCU to enhance protection and solve some EMI/EMC problems.
3. Switch pin processing
The switch circuit can refer to the following figure:
The triode can be selected from the commonly used low-power NPN triode, and a 100nF capacitor and a TVS tube can be added to the switch pin to enhance the protection of the module.
4. SIM card interface
The communication rate of the SIM card is relatively high, and there is no protection circuit inside the SIM card, so special attention should be paid to the design, otherwise the SIM card cannot be recognized, the card is dropped, or the SIM card is damaged. The SIM card circuit is shown in the following figure:
have to be aware of is:
①SIM card lead is best controlled within 10cm, preferably within 5cm;
②SIM_RST, SIM_CLK and SIM_DATA need to be connected in series with a 51 ohm resistor for impedance matching, and a 22pF capacitor to ground to prevent radio frequency interference;
③The junction capacitance of the ESD device cannot be greater than 50pF;
④The SIM card wiring should be kept away from the power supply, antenna and high-speed signal line;
⑤ The ground of the SIM card must maintain good connectivity with the ground of the module to ensure equipotentiality;
⑥ In order to prevent SIM_CLK from interfering with other signals, it is recommended to process SIM_CLK as a packet.
5. Antenna interface processing
The antenna reference circuit is shown below:
In the design, the impedance of 50 ohms should be guaranteed to facilitate the debugging of the RF engineer. The impedance design can be designed using the Si9000 software. The matching circuit should be reserved, and the RF test head should be reserved according to the actual situation. Note that the antenna circuit should be kept away from the power supply.
So far the SIMCom module design is completed.
Part of this blog post is excerpted from SIMCom's official website documentation.
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