Smart Module Series
SC20_Series_Hardware_Design 44 / 133
In order to ensure USB performance, comply with the following principles while designing USB interface.
Route the USB signal traces as a differential pair with total grounding. The impedance of USB
differential trace is 90 Ω.
Keep the ESD protection devices as close as possible to the USB connector. Pay attention to the
influence of junction capacitance of ESD protection devices on USB data lines. Typically, the
capacitance value should be less than 2 pF.
Do not route signal traces under crystals, oscillators, magnetic devices or RF signal traces. It is
important to route the USB differential traces in inner-layer with ground shielding on not only upper
and lower layer but also right and left sides.
Make sure the trace length difference between USB_DM and USB_DP is not exceeding 6.6 mm.
Table 12: USB Trace Length Inside the Module
3.10. UART Interfaces
The module provides two UART interfaces:
UART1:
4-wire UART interface with RTS/CTS hardware flow control; baud rate up to 4 Mbps
UART2:
2-wire UART interface used for debugging
Table 13: Pin Definition of UART Interfaces
Pin No.
Signal
Length (mm)
Length Difference (DP-DM)
13
USB_DM
29.43
-0.07
14
USB_DP
29.36
Pin Name
Pin No.
I/O
Description
Comment
UART1_TX
34
DO
UART1 transmit
1.8 V power domain.
If it is unused, keep it open.
UART1_RX
35
DI
UART1 receive
UART1_CTS
36
DI
DTE clear to send signal
from DCE
Connect to DTE’S CTS;
1.8 V power domain.
If unused, keep these pins
open.
UART1_RTS
37
DO
DTE request to send signal
to DCE
Connect to DTE’S RTS;
1.8 V power domain.
