LTE Module Series
EC21 Hardware Design
EC21_Hardware_Design Confidential / Released 41 / 94
In order to enhance the reliability and availability of the USIM card in your application, please follow the
criteria below in USIM circuit design:
Keep layout of USIM card as close to the module as possible. Keep the trace length as less than
200mm as possible.
Keep USIM card signals away from RF and VBAT traces.
Assure the ground between the module and the USIM card connector short and wide. Keep the trace
width of ground and USIM_VDD no less than 0.5mm to maintain the same electric potential.
To avoid cross-talk between USIM_DATA and USIM_CLK, keep them away from each other and
shield them with surrounded ground.
In order to offer good ESD protection, it is recommended to add a TVS diode array whose parasitic
capacitance should not be more than 50pF. The 22 ohm resistors should be added in series between
the module and the USIM card so as to suppress EMI spurious transmission and enhance ESD
protection. The 33pF capacitors are used for filtering interference of GSM900. Please note that the
USIM peripheral circuit should be close to the USIM card connector.
The pull-up resistor on USIM_DATA line can improve anti-jamming capability when long layout trace
and sensitive occasion are applied, and should be placed close to the USIM card connector.
3.10. USB Interface
EC21 contains one integrated Universal Serial Bus (USB) transceiver which complies with the USB 2.0
specification and supports high-speed (480Mbps) and full-speed (12Mbps) modes. The USB interface is
used for AT command communication, data transmission, GNSS NMEA sentences output, software
debugging, firmware upgrade and voice over USB*. The following table shows the pin definition of USB
interface.
Table 10: Pin Description of USB Interface
Pin Name
Pin No.
I/O
Description
Comment
USB Signal Part
USB_DP
69
IO
USB differential data bus (positive)
Require differential
impedance of 90Ω
USB_DM
70
IO
USB differential data bus (minus)
Require differential
impedance of 90Ω
USB_VBUS
71
PI
Used for detecting the USB connection Typical 5.0V
GND
72
Ground
For more details about the USB 2.0 specifications, please visit http://www.usb.org/home.
Quectel
Confidential
