N27 Hardware User Guide
Chapter 4 Application Interfaces
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N27 provides one SIM interface, which supports 1.8V only. Figure 4-9 shows the reference design of
the SIM card interface.
Figure 4-9 Reference design of the SIM card interface
1 uF
CLK
RST
VCC
VPP
GND
SIM card
DATA
GND
SIM-DET
20
Ω
47 k
Ω
VDD_1P8
USIM_DATA
USIM_CLK
USIM_RESET
USIM_VCC
USIM_PRESENT
N27 module
20
Ω
20
Ω
20
Ω
10 k
Ω
Schematic guidelines
USIM_VCC is the pin to supply power for SIM card and it can load 30 mA at most. Do not use it
for any other purpose.
Connect USIM_DATA to USIM_VCC through a pull-up resistor, the recommended resistance of
which is 10 kΩ.
USIM_CLK is the clock signal pin, which operates at a frequency of 3.25 MHz commonly.
Add ESD protectors, such as ESD diodes or TVS diodes (with a junction capacitance less than
33pF) on the USIM signal lines in applications with a high requirement of ESD protection.
Connect a 20 Ω resistor respectively to USIM_DATA, USIM_RESET, USIM_CLK, and
USIM_PRESENT in series to enhance the ESD performance.
N27 supports SIM card detect, but this function is determined by the structure of the SIM card
connector.
The module determines the SIM card status based on the level of the USIM_PRESENT pin.
USIM_PRESENT is at a high level when the pin is floating. It is at a low level after the SIM card
is inserted and the pin contacts the ground of the connector.
PCB layout guidelines:
USIM signals are like to be jammed by RF radiation, resulting in failure to detect the SIM card.
Place USIM far away from RF circuits.
Place the USIM card close to the module and USIM traces should be as short as possible.
Place ESD protection resistors and components close to the USIM card connector.
Surround USIM traces with ground to enhance EMC capability.
