ARM core board level conversion circuit (below)
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3. Transistor + pull-up resistor through the bipolar transistor, the collector is connected to the power supply by the pull-up resistor, the input high-level voltage value is the power supply voltage value. Take the MiniARM core board and GPRS module as an example, as shown in Figure 1. 
When the GPRS module TXD is at a high level, the transistor is turned off because of Ve=Vb of Q1, and the pull-up resistor R1 pulls the RXD of the MiniARM high.
When GPRS module TXD is low, due to Q1's Ve
When the TXD of the MiniARM is high, the transistor is turned off due to Ve > Vb of Q2, and the pull-up resistor R5 pulls the RXD of the GPRS module high.
When the MiniARM's TXD is low, due to Q2's Ve
When selecting the resistance of the collector pull-up resistor, consider the input communication rate and the current consumption on the pull-up resistor. Decreasing the resistance of the pull-up resistor can increase the communication speed and obtain a shorter switching time, but it increases the current consumption of the resistor at a low level. Increasing the resistance of the resistors increases the switching time and reduces the communication speed.
4. The MOS transistor + pull-up resistor uses MOSFET devices for level conversion. This design method is similar to method 3. 
When the GPRS module TXD is at a high level, since Ugs=0, the NMOS is turned off and the pull-up resistor pulls the RXD of the MiniARM high.
When the GPRS module TXD is at a low level, since Ugs>0, Uds>0, NMOS is turned on, the RXD of the MiniARM will get a low level voltage value of 0.1V+Uds.
In addition, the use of this circuit requires attention:
1. VDD_EXT ≤ VCC_MCU
The 2.MiniARM low level threshold should be greater than the NMOS tube voltage drop +0.1V.
3.Vgs ≤ VDD_EXT
4. Vds ≤ VCC_MCU
5.74xHCT Series Chip (3.3V to 5V)
A CMOS device compatible with 5VTTL levels can be used as a 3.3V to 5V level conversion chip. This is because the level of 3.3VCMOS is just compatible with the 5VTTL level (as shown in Figure 3). This method can be used to select the cheap 74xHCT series chip to achieve TTL compatibility. 
6. Dedicated level-shifting chips use dedicated level-shifting chips (eg 74LVC16245, SN74LVC1T45, SN74LVC2T45). Through the level conversion chip, it is possible to perform flexible bidirectional level conversion between different voltage nodes that the chip can withstand. This method has higher flexibility but higher cost.
Zhiyuan Electronics' MiniARM industrial control core board has powerful functions and reliable stability. Through the selection of this series of core boards for product development, users can make the product development process shorter and the products developed more reliable. Its brief description is shown in the table below. 
