N21
Hardware User Guide
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17
In the above circuit, note VCC_EX:
VCC_EX=VCC_IO when V
IH
≤2.8V
VCC_EX=VDDIO_2P8 when V
IH
≥
2.8V
Components:
R2, R4: 2 kΩ to 10 kΩ
The greater the UART baud rate is, the lower the R2 and R4 values are.
R1, R3: 4.7 kΩ to 10 kΩ
The greater the UART baud rate is, the lower the R1 and R3 values are.
Q1, Q2: MMBT3904 or MMBT2222.
High-speed transistor is better.
If the low level at MCU_UART (V
IL
) is greater than 200mV and baudrate is lower than 1 MHz, adopt
recommended level shifting circuit 3. Otherwise, low level at UART might be higher than required,
resulting in failure to identify signals.