N720 OpenLinux
Hardware User Guide
Copyright © Neoway Technology Co., Ltd
27
Schematic Design Recommendations
This reference design is applicable to USIM1 and USIM2 as well as USIM card and chipset.
Reserve a position for pull-up resistor externally in design since the USIM_DATA pin is not pulled
up internally.
USIM_VCC is the pin to supply power for SIM card and its maximum load is 30 mA. Do not use it
for any other purpose.
USIM_CLK is the clock signal pin, supporting a clock frequency of 3.25 GHz.
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 20Ω resistor respectively to USIM_DATA, USIM_RESET, USIM_CLK, and USIM_DET
in series to enhance the ESD performance.
N720 OpenLinux supports USIM card detection. USIM_DET is a 1.8V interrupt pin.
The UIM detection circuit works by checking the levels across the USIM_DET pin before and
after a UIM card is inserted. In the reference circuit, USIM-DET is floating before a USIM card is
inserted and is grounded after a USIM card is inserted. Low level means USIM card detected
while high level mean no USIM card detected.
PCB Design Guidelines
Place USIM far away from RF circuits. USIM signals are likely to be jammed by RF radiation,
resulting in failure to detect the SIM card.
Place USIM card close to the module and USIM traces should be as short as possible.
Place ESD protection resistors and components close to USIM card.
Surround USIM traces with ground to enhance EMC capability.
3.3.4 UART
Signal
Pin
I/O
Function
Remarks
UART5_TXD
46
DO
UART5 data transmitting
Used for data transmission
UART5_RXD
47
DI
UART5 data receiving
Used for data transmission
N720 OpenLinux provides one UART interface. To support hardware flow control, multiplex pin 51 and
pin 52 as UART5_CTS and UART5_RTS. UART interfaces support 4 Mbps at most. The level at the
interfaces 1.8V. To use multiple UART interfaces, see Section 3.7 MUX Interfaces. Figure 3-16 And
Figure 3-17 show the reference designs of the UART interfaces.