Integration Manual

SARA-R4/N4 series - System Integration Manual
UBX-16029218 - R13 Design-in Page 75 of 119
Providing the TXD and RXD lines only
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If the functionality of the CTS, RTS, DSR, DCD, RI and DTR lines is not required in the application, or the lines are not
available, then:
• Connect the module RTS input line to GND or to the CTS output line of the module, since the module requires RTS
active (low electrical level) if HW flow-control is enabled (AT&K3, which is the default setting)
• Connect the module DTR input line to GND using a 0 Ω series resistor, because it is useful to set DTR active if not
specifically handled, in particular to have URCs presented over the UART interface (see SARA-R4/N4 series AT
Commands Manual [1], &D, S0, +CNMI AT commands)
• Leave DSR, DCD and RI lines of the module floating, with a test-point on DCD
If RS-232 compatible signal levels are needed, the Maxim MAX13234E voltage level translator can be used. This chip
translates voltage levels from 1.8 V (module side) to the RS-232 standard.
If a 1.8 V Application Processor (DTE) is used, the circuit that should be implemented as in Figure 46.
TxD
Applicat ion Processor
(1.8V DTE)
RxD
RTS
CTS
DTR
DSR
RI
DCD
GND
SARA- R4/N4
(1.8V DCE)
12
TXD
9
DTR
13
RXD
10
RTS
11
CTS
6
DSR
7
RI
8
DCD
GND
0Ω
TP
0Ω
TP
0Ω
TP
TP
Figure 46: UART interface application circuit with a 3-wire link in the 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 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 in Figure 47.
4
V_INT
TxD
Applicat ion Processor
(3.0 V DTE)
RxD
DTR
DSR
RI
DCD
GND
SARA-R4/ N4
(1.8V DCE)
12
TXD
9
DTR
13
RXD
6
DSR
7
RI
8 DCD
GND
1V8
B1 A1
GND
U1
VCCBVCCA
Unidirect ional
Volt age T rans lat or
C1
C2
3V0
DIR1
DIR2 OE
VCC
B2 A2
RTS
CTS
10
RTS
11
CTS
TP
0 Ω
TP
0 Ω
TP
0 Ω
TP
TP
Figure 47: UART interface application circuit with a partial V.24 link (3-wire) in DTE/DCE serial communication (3.0 V DTE)
Reference Description Part Number - Manufacturer
C1, C2 100 nF Capacitor Ceramic X7R 0402 10% 16 V GRM155R61A104KA01 - Murata
U1 Unidirectional Voltage Translator SN74AVC2T245
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- Texas Instruments
Table 32: UART application circuit components with partial V.24 link (3-wire) in DTE/DCE serial communication (3.0 V DTE)
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Flow control is not supported by ‘00’, ‘01’ and SARA-R410M-02B-00 product versions, but the RTS input must be set low to use the UART on ‘00’ and ‘01’
versions. The DTR input must be set low to have URCs presented over UART on ‘00’, ‘01’ and ‘x2’ product versions.
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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