SARA-G3 and SARA-U2 series - System Integration Manual
UBX-13000995 - R08
Objective Specification
Design-in
Page 114 of 188
2.6
Serial interfaces
2.6.1
Asynchronous serial interface (UART)
2.6.1.1
Guidelines for UART circuit design
Providing the full RS-232 functionality (using the complete V.24 link)
If RS-232 compatible signal levels are needed, two different external voltage translators (e.g. Maxim MAX3237E
and Texas Instruments SN74AVC8T245PW) can be used to provide full RS-232 (9 lines) functionality. The Texas
Instruments chip provides the translation from 1.8 V to 3.3 V, while the Maxim chip provides the translation
from 3.3 V to RS-232 compatible signal level.
If a 1.8 V application processor is used, for complete RS-232 functionality as per
ITU Recommendation
[10] in the
DTE/DCE serial communication, the complete UART interface of the module (DCE) must be connected to a 1.8 V
application processor (DTE) as described in Figure 58.
TxD
Application Processor
(1.8V DTE)
RxD
RTS
CTS
DTR
DSR
RI
DCD
GND
SARA-G3 / SARA-U2
(1.8V DCE)
12
TXD
9
DTR
13
RXD
10
RTS
11
CTS
6
DSR
7
RI
8
DCD
GND
0
Ω
TP
0
Ω
TP
0
Ω
TP
0
Ω
TP
Figure 58: UART interface application circuit with complete V.24 link in DTE/DCE serial communication (1.8 V DTE)
If a 3.0 V Application Processor is used, appropriate unidirectional voltage translators must be provided using the
module
V_INT
output as 1.8 V supply, as described in Figure 59.
4
V_INT
TxD
Application Processor
(3.0V DTE)
RxD
RTS
CTS
DTR
DSR
RI
DCD
GND
SARA-G3 / SARA-U2
(1.8V DCE)
12
TXD
9
DTR
13
RXD
10
RTS
11
CTS
6
DSR
7
RI
8
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 59: 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 - Texas Instruments
Table 31: Component for UART application circuit with complete V.24 link in DTE/DCE serial communication (3.0 V DTE)
