LEXI-R422 - System integration manual
UBX-23007449 - R02
Design-in
Page 70 of 108
C1-Public
2.6
Data communication interfaces
2.6.1
UART interfaces
2.6.1.1
Guidelines for UART circuit design
Providing 1 UART with full RS-232 functionality (using the complete V.24 link)
If RS-232 compatible signal levels are needed, two different external voltage translators can be used
to provide full RS-232 (9 lines) functionality: e.g. using the Texas Instruments SN74AVC8T245PW for
the translation from 1.8 V to 3.3 V, and the Maxim MAX3237E for the translation from 3.3 V to RS-
232 compatible signal level.
If a 1.8 V Application Processor (DTE) is used and complete RS-232 functionality is required, then the
complete 1.8 V UART of the module (DCE) should be connected to a 1.8 V DTE, as in
TxD
Application Processor
(1.8V DTE)
RxD
RTS
CTS
DTR
DSR
RI
DCD
GND
LEXI-R422
(1.8V DCE)
F1
TXD
K1
DTR
E1
RXD
H1
RTS
G1
CTS
M1
DSR
L1
RI
J1
DCD
GND
0
Ω
TP
0
Ω
TP
0
Ω
TP
0
Ω
TP
Figure 46: UART interface application circuit with complete V.24 link in DTE/DCE serial communication (1.8V DTE)
If a 3.0 V Application Processor (DTE) is used, then it is recommended to connect the 1.8 V UART of
the module (DCE) by means of appropriate unidirectional voltage translators using the module
V_INT
output as 1.8 V supply for the voltage translators on the module side, as described in
A11
V_INT
TxD
Application Processor
(3.0V DTE)
RxD
RTS
CTS
DTR
DSR
RI
DCD
GND
LEXI-R422
(1.8V DCE)
F1
TXD
K1
DTR
E1
RXD
H1
RTS
G1
CTS
M1
DSR
L1
RI
J1
DCD
GND
1V8
B1
A1
GND
U1
B3
A3
VCCB
VCCA
Unidirectional
voltage translator
C1
C2
3V0
DIR3
DIR2
OE
DIR1
VCC
B2
A2
B4
A4
DIR4
1V8
B1
A1
GND
U2
B3
A3
VCCB
VCCA
Unidirectional
voltage translator
C3
C4
3V0
DIR1
DIR3
OE
B2
A2
B4
A4
DIR4
DIR2
TP
0
Ω
TP
0
Ω
TP
0
Ω
TP
0
Ω
TP
Figure 47: UART interface application circuit with complete V.24 link in DTE/DCE serial communication (3.0 V DTE)
Reference
Description
Part number - Manufacturer
C1, C2, C3, C4
100 nF capacitor ceramic X7R 0402 10% 16 V
GRM155R61A104KA01 - Murata
U1, U2
Unidirectional voltage translator
SN74AVC4T774
3
- Texas Instruments
Table 30: Component for UART application circuit with complete V.24 link in DTE/DCE serial communication (3.0 V DTE)
3
Voltage translator providing partial power down feature so that the DTE 3 V supply can be also ramped up before
V_INT
1.8 V supply