Neo_M680 GPRS Module Hardware User Guide
Copyright © Neoway Technology Co., Ltd
18
Figure 3-14
Recommended circuit for the communication between 5V MCU and UART
In Figure 3-14, INPUT is connected to TXD of the MCU and VCC_IN is connected to the 5 V power supply
of the MCU. OUTPUT is connected to RXD of the module and VCC_OUT is connected to
VDD_EXT(2.8V) of the module. If the circuit is far away from the VDD_EXT pin, add a 0.1 μF decoupling
capacitor to VDD_EXT.
Level shifting between RXD of the MCU and TXD of the module can be implemented in the same way.
The pull-up resistor R3 ranges from 4.7 K to 10 K; R2 ranges from 2 K to 10 K. Resistors are selected based
on the voltage of the power supply and UART baudrate. You can select resistors with great resistance to
reduce the power consumption when the power supply has great voltage or the baudrate is low. But, the
resistance will affect the quality of the square wave. In addition, the circuit performance is affected by the
signal traces during PCB layout.
It is recommended that you choose a high-speed NPN transistor because the Q1 switch rate will affect the
wave quality after level shifting. MMBT3904 or MMBT2222 is recommended.
Avoid data produced at UART when the module is powered on. You are advised to send data to the UART
3 seconds after the module is powered on so that the module would not respond wrongly.
3.3
USB Interfaces
Table 3-3
USB interface
Signal
I/O
Function
Remarks
VBUS
AI
USB voltage check
4.3 V < VBUS < 7 V, typical value: 5 V
USB_DP
DIO
Positive signal of USB data
USB_DM
DIO
Negative signal of USB data
INPUT
OUTPUT
VCC_IN
VCC_OUT
4.7K
10K
Q1
R2
R3
