GSM UART Port Application Note
GSM_UART_Application_Note_V1.0
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3. Level Shifter
Normally, the voltage of VDD_EXT is 2.8V and the DC characteristics of UART port are
listed in Table 3.
Table 1: Logic levels of the UART port
Symbol
MIN
MAX
Unit
V
IL
0
0.67
V
V
IH
1.7
0.3
V
V
OL
0
0.34
V
V
OH
2.0
VDD_EXT
V
NOTE: If the voltage level of UART pins in DTE and the module doesn’t match with each
other, level shifter circuit should be inserted.
3.1. Communicate with DTE
It is recommended to design the level shift circuit by choosing open drain output buffer
(e.g. NC7WZ07) or discrete transistor.
Note:VDD_EXT is supplied by the module(2.8V). VCC_MCU is the voltage for host
MCU UART interface.
Figure
3
:
The reference level shifter by using open drain output buffer
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