N27 Hardware User Guide
Chapter 4 Application Interfaces
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recommended level shifting circuit.
Figure 4-8 Recommended level shifting network
VL
IO_VL1
GND
IO_VL2
VCC
IO_VCC1
IO_VCC2
EN
NLSX4373
VDD_1P8
UART_TXD
UART_RXD
VDD_1P8
VCC_IO
MCU_RXD
MCU_TXD
0.1
μ
F
0.1
μ
F
Schematic guidelines:
NLSX4373 is a dual-supply level shifter, the rate of which can be up to 20 Mb/s.
VL is the reference voltage of IO_VL1 and IO_VL2, ranging from 1.5V to 5.5V.
VCC is the reference voltage of IO_VCC1 and IO_VCC2, ranging from 1.5V to 5.5V.
EN is the enable pin, which works at a voltage of greater than VL-0.2V. In the above circuit, the
EN pin is connected to VDD_1P8 and the level shifter is always working.
For more design guidelines, see the NLSX4373 chipset datasheet.
PCB layout guidelines:
Do not route UART traces across other traces if possible. If crossing is inevitable, route the I2C
traces perpendicular to other traces.
Keep traces far away from areas that might introduce ESD.
Surround the traces with the ground plane if possible.
4.3.3 SIM
Signal
Pin
I/O
Function
Remarks
USIM_VCC
6
PO
USIM power output
1.8V only
USIM_DATA
7
B
USIM data IO
USIM_CLK
8
DO
USIM clock
USIM_RESET
9
DO
USIM reset
USIM_DET
5
DI
USIM detection
A pull-up resistor is recommended.
