Ublox SARA-G300 - Page 135

SARA-G3 and SARA-U2 series - System Integration Manual
UBX-13000995 - R26 Design-in
Page 135 of 217
Providing the TXD, RXD, RTS, CTS and DTR lines only (not using the complete V.24 link)
If the functionality of the DSR, DCD and RI lines is not required, or the lines are not available:
 Leave the DSR, DCD and RI lines of the module unconnected and floating
If RS-232 compatible signal levels are needed, two different external voltage translators (e.g. Maxim MAX3237E
and Texas Instruments SN74AVC4T774) can be used. The Texas Instruments chips provide 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.
Figure 68 describes the circuit that should be implemented as if a 1.8 V Application Processor (DTE) is used,
given that the DTE will behave properly regardless of the DSR input setting.
TxD
Application Processor
(1.8V DTE)
RxD
RTS
CTS
DTR
DSR
RI
DCD
GND
SARA-G3 / SARA-U2
(1.8V DCE)
15
TXD
12
DTR
16
RXD
13
RTS
14
CTS
9
DSR
10
RI
11
DCD
GND
0 Ω
0 Ω
TP
TP
0 Ω
0 Ω
TP
TP
Figure 68: UART interface application circuit with partial V.24 link (6-wire) in the 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 69, given that the DTE will
behave properly regardless of the DSR input setting.
4
V_INT
TxD
Application Processor
(3.0V DTE)
RxD
RTS
CTS
DTR
DSR
RI
DCD
GND
SARA-G3 / SARA-U2
(1.8V DCE)
15
TXD
12
DTR
16
RXD
13
RTS
14
CTS
9
DSR
10
RI
11
DCD
GND
0 Ω
0 Ω
TP
TP
0 Ω
0 Ω
TP
TP
1V8
B1 A1
GND
U1
B3A3
VCCBVCCA
Unidirectional
Voltage Translator
C1
C2
3V0
DIR3
DIR2 OE
DIR1
VCC
B2 A2
B4A4
DIR4
1V8
B1 A1
GND
U2
VCCBVCCA
Unidirectional
Voltage Translator
C3
3V0
DIR1
OE
B2 A2
DIR2
C4
Figure 69: UART interface application circuit with partial V.24 link (6-wire) 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
Unidirectional Voltage Translator
SN74AVC4T774
43
- Texas Instruments
U2
Unidirectional Voltage Translator
SN74AVC2T245
43
- Texas Instruments
Table 45: Component for UART application circuit with partial V.24 link (6-wire) in DTE/DCE serial communication (3.0 V DTE)
43
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