Figure 21: Resistor Matching Circuit - SimCom SIM800L Hardware Design

If the voltage of UART is 3V or3.3V, the following reference circuits are recommended:
Note: please make sure the minimum of client high limit should be less than 2.8V minus the diode drop.
If the voltage of UART is 5V, the following reference circuits are recommended:
SIM800L(MT6261)_Hardware Design_V1.01
TXD
RXD
RTS
CTS
DTR
DCD
RI
GND 5.6K
Module

Figure 21: Resistor matching circuit

10K
VDD_EXT
RXD
TXD
GND
Module
Figure 22: Diode isolation circuit
VDD_EXT
Module
VDD_EXT
47K
TXD
Figure 23: TX level matching circuit
1K
RXD
1K
TXD
1K
RTS
1K
CTS
1K
GPIO
1K
GPIO
1K
EINT
GND
5.6K 5.6K
TXD
RXD
10K
GND
DTE
VDD
4.7K
4.7K
RXD
30
Smart Machine Smart Decision
DTE
(3.3V)
DTE
2016-07-07