In the circuit design process, the electronic engineer often encounters the problem that the I/O level of the processor MCU is different from the I/O level of the module. In order to ensure normal communication between the two, level conversion is required. Below, we will make a detailed analysis of the level shifting circuit.
For most MCUs, the pins are basically CMOS, so the input voltage range is: high level is not lower than 0.7VCC, low level is not higher than 0.3VCC.
But before we introduce the level shifting circuit, we need to understand the following points:
1, to solve the level conversion problem, the most fundamental is to solve the level compatibility problem, and the level compatibility principle has two: 1VOH> VIH 2VOL
Figure 1 threshold voltage
2. For multiple power systems, some devices do not allow the input level to exceed the supply voltage. For devices with similar requirements, the circuit should be properly protected.
3, the level conversion circuit will affect the communication speed, so you should pay attention to the communication rate requirements when using.
4. The driving ability of different conversion modes is different, and should be properly considered in the selection.
5. When there are many ways to convert, improper selection of the conversion method will result in more components or inconvenient wiring.
The following examples of the remote electronic MiniARM core board are used to share common level conversion circuit methods.
Resistance division methodThe resistor divider method is the easiest method. The circuit works by dividing the logic level high. Take the MiniARM core board and the GPRS module as an example. The IO level of the MiniARM core board is 3.3V, and the IO level of the GPRS module is 2.8V. Level 2 can be used to achieve level matching when communicating between the two.
Figure 2 resistor divider method
The conversion circuit of the resistor divider is tested, and the waveform after conversion is as shown in FIG.
Use diodes for level matching, using the MiniARM core board and GPRS module as an example.
Figure 4 diode clamp method
When the GPRS module TXD is at a high level, due to the clamping effect of the diode D2, the MiniARM RXD will obtain a 2.8V+VF high level voltage. When the TXD of the MiniARM is high, the RXD of the GPRS module will get a high level voltage of 2.8V+VF due to the clamping effect of the diode D1.
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