Ublox SARA-G300, System Integration Manual

Page: 134 / 217

SARA-G3 and SARA-U2 series - System Integration Manual
UBX-13000995 - R26 Design-in
Page 134 of 217
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 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 an RS-232 compatible signal level.
If a 1.8 V Application Processor (DTE) is used and complete RS-232 functionality as per
ITU Recommendation [10]
is required in the DTE/DCE serial communication, then the complete 1.8 V UART interface of the module (DCE)
must be connected to a 1.8 V DTE as described in Figure 66.
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 66: 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 (DTE) is used, then it is recommended to connect the 1.8 V UART interface 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 Figure 67.
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 67: 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 42 - Texas Instruments
Table 44: Components for UART application circuit with complete V.24 link in DTE/DCE serial communication (3.0 V DTE)
42
Voltage translator providing partial power down feature so that the DTE 3.0 V supply can be also ramped up before
V_INT
1.8 V supply