This article will introduce several other level conversion circuits to the example of the remote electronic MiniARM industrial control core board. Through the bipolar transistor, the collector is connected to the power supply by a pull-up resistor, and 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. Figure 1 transistor level shifting circuit When the GPRS module TXD is at a high level, since the V1 of the Q1 is Vb, the triode is turned off, and the pull-up resistor R1 pulls the RXD of the MiniARM to a high level. When the GPRS module TXD is low, due to the V1 of Q1 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 to a high level. When MiniARM's TXD is low, due to Q2's Ve When selecting the resistance of the collector pull-up resistor, it is necessary to consider the input communication rate and the current consumption on the pull-up resistor. Reducing the value of the pull-up resistor can increase the communication speed and obtain a shorter switching time, but increase the current consumption on the resistor at a low level. Increase the resistance value, the switching time is extended, and the communication speed is reduced. Level shifting is achieved using MOSFET devices, which is similar to Method 3. Figure 2 MOSFET level shifting circuit 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 to a high level. When the GPRS module TXD is low, since Ugs>0, Uds>0, the NMOS is turned on, and the MiniARM RXD will get a low level with a voltage value of 0.1V+Uds. In addition, the use of this circuit requires attention: 1, VDD_EXT ≤ VCC_MCU 2, MiniARM's low-level threshold should be greater than NMOS tube voltage drop +0.1V 3, Vgs ≤ VDD_EXT 4, Vds ≤ VCC_MCU 5, 74xHCT series chips (3.3V to 5V) CMOS devices compatible with 5V TTL levels can be used as 3.3V to 5V level shifting chips. This is because the level of 3.3V CMOS is just compatible with the 5V TTL level (as shown in Figure 3). This method can be used to select a cheap 74xHCT series chip to achieve TTL compatibility. Figure 3 5V and 3.3V threshold voltage 6, dedicated level conversion chip A dedicated level shifting chip (such as 74LVC16245, SN74LVC1T45, SN74LVC2T45) is used. With the level shifting chip, flexible bidirectional level shifting between different voltage nodes that the chip can withstand can be achieved. This method has higher flexibility but higher cost.
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