SimTech SIMCom SIM900-DS Supplement

SIM900-DS_Hardware Design_V1.00

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SIM900-DS_Hardware Design_V1.00 2 2013-01-20
Document Title SIM900-DS Hardware Design
Version 1.00
Date 2013-01-20
Status Release
Document Control ID SIM900-DS_Hardware Design_V1.00
General Notes
SIMCom offers this information as a service to its customers, to support application and engineering efforts that
use the products designed by SIMCom. The information provided is based upon requirements specifically
provided to SIMCom by the customers. SIMCom has not undertaken any independent search for additional
relevant information, including any information that may be in the customer’s possession. Furthermore, system
validation of this product designed by SIMCom within a larger electronic system remains the responsibility of the
customer or the customer’s system integrator. All specifications supplied herein are subject to change.
Copyright
This document contains proprietary technical information which is the property of SIMCom Limited, copying of
this document and giving it to others and the using or communication of the contents thereof, are forbidden
without express authority. Offenders are liable to the payment of damages. All rights reserved in the event of grant
of a patent or the registration of a utility model or design. All specification supplied herein are subject to change
without notice at any time.
Copyright © Shanghai SIMCom Wireless Solutions Ltd. 2013

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Contents
Contents........................................................................................................................3
1Introduction............................................................................................................8
2SIM900-DS Overview ............................................................................................8
2.1 SIM900-DS Key Features...................................................................................................8
2.2 Operating Modes...............................................................................................................10
2.3 SIM900-DS Functional Diagram.....................................................................................11
3Package Information............................................................................................12
3.1 Pin out Diagram................................................................................................................12
3.2 Pin Description..................................................................................................................13
3.3 Package Dimensions..........................................................................................................15
4Application Interface ...........................................................................................17
4.1 Power Supply.....................................................................................................................17
4.1.1 Minimizing Voltage Drop of VBAT................................................................................................... 18
4.1.2 Monitoring Power Supply ................................................................................................................... 18
4.2 Power on/down Scenarios.................................................................................................18
4.2.1 Power on SIM900-DS ......................................................................................................................... 18
4.2.2 Power down SIM900-DS .................................................................................................................... 19
4.2.3 Restart SIM900-DS by PWRKEY Pin................................................................................................ 21
4.3 Power Saving Mode ..........................................................................................................21
4.3.1 Minimum Functionality Mode ............................................................................................................ 21
4.3.2 Sleep Mode 1 (AT+CSCLK=1) .......................................................................................................... 22
4.3.3 Wake Up SIM900-DS from Sleep Mode 1 (AT+CSCLK=1)............................................................. 22
4.3.4 Sleep Mode 2 (AT+CSCLK=2) .......................................................................................................... 22
4.3.5 Wake Up SIM900-DS from Sleep Mode 2 (AT+CSCLK=2)............................................................. 23
4.4 RTC Backup......................................................................................................................23
4.5 Serial Interfaces.................................................................................................................24
4.5.1 Function of Serial Port and Debug Port .............................................................................................. 25
4.5.2 Software Upgrade and Debug ............................................................................................................. 26
4.6 Audio Interfaces................................................................................................................26
4.6.1 Speaker Interface Configuration ......................................................................................................... 26
4.6.2 Microphone Interfaces Configuration ................................................................................................. 27
4.6.3 Earphone Interface Configuration....................................................................................................... 28
4.6.4 Audio Electronic Characteristics......................................................................................................... 28
4.7 SIM Card Interface...........................................................................................................29
4.7.1 SIM Card Application......................................................................................................................... 29
4.7.2 Design Considerations for SIM Card Holder...................................................................................... 30

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4.8 LCD Display/SPI Interface ..............................................................................................31
4.9 PCM Interface...................................................................................................................32
4.10 ADC....................................................................................................................................32
4.11 RI Behaviors......................................................................................................................32
4.12 Network Status Indication ...............................................................................................33
4.13 General Purpose Input/Output (GPIO)..........................................................................34
4.14 External Reset ...................................................................................................................34
4.15 I2C Bus ...............................................................................................................................35
4.16 Antenna Interface .............................................................................................................35
5Electrical, Reliability and Radio Characteristics..............................................37
5.1 Absolute Maximum Ratings.............................................................................................37
5.2 Recommended Operating Conditions.............................................................................37
5.3 Digital Interface Characteristics......................................................................................37
5.4 SIM Card Interface Characteristics................................................................................38
5.5 VDD_EXT Characteristics...............................................................................................38
5.6 SIM1_VDD and SIM2_VDD Characteristics.................................................................38
5.7 VRTC Characteristics ......................................................................................................38
5.8 Current Consumption (VBAT = 3.8V)............................................................................39
5.9 Electro-Static Discharge...................................................................................................40
5.10 Radio Characteristics .......................................................................................................40
5.10.1 Module RF Output Power................................................................................................................ 40
5.10.2 Module RF Receive Sensitivity ....................................................................................................... 42
5.10.3 Module Operating Frequencies........................................................................................................ 42
6Manufacturing......................................................................................................43
6.1 Top and Bottom View of SIM900-DS..............................................................................43
6.2 Typical Solder Reflow Profile..........................................................................................44
Appendix ....................................................................................................................45
A. Related Documents...................................................................................................................45
B. Terms and Abbreviations........................................................................................................46
C. Safety Caution..........................................................................................................................47

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Table Index
Table 1: SIM900-DS key features ...................................................................................................................... 8
Table 2: Coding schemes and maximum net data rates over air interface........................................................ 10
Table 3: Overview of operating modes............................................................................................................. 10
Table 4: Pin description .................................................................................................................................... 13
Table 5: The Current consumption of Minimum Functionality Mode ............................................................. 22
Table 6: Microphone Input Characteristics....................................................................................................... 28
Table 7: Audio Output Characteristics ............................................................................................................. 28
Table 8: Pin description (Amphenol SIM card holder)..................................................................................... 30
Table 9: Pin description (Molex SIM card holder)........................................................................................... 31
Table 10: ADC specification ............................................................................................................................ 32
Table 11: RI Behaviors ..................................................................................................................................... 32
Table 12: Status of the NETLIGHT pin ........................................................................................................... 34
Table 13: Pin definition of the GPIO interface................................................................................................. 34
Table 14: Absolute maximum ratings............................................................................................................... 37
Table 15: Recommended operating conditions................................................................................................. 37
Table 16: Digital interface characteristics ........................................................................................................ 37
Table 17: SIM card interface characteristics..................................................................................................... 38
Table 18: VDD_EXT characteristics ................................................................................................................ 38
Table 19: SIM1_VDD and SIM2_VDD characteristics ................................................................................... 38
Table 20: VRTC characteristics........................................................................................................................ 38
Table 21: Current consumption* ...................................................................................................................... 39
Table 22: The ESD characteristics (Temperature: 25℃, Humidity: 45 %)...................................................... 40
Table 23: SIM900-DS GSM850 and EGSM900 conducted RF output power ................................................. 40
Table 24: SIM900-DS DCS1800 and PCS1900 conducted RF output power.................................................. 41
Table 25: SIM900-DS conducted RF receive sensitivity.................................................................................. 42
Table 26: SIM900-DS operating frequencies ................................................................................................... 42
Table 27: Illustration of module information.................................................................................................... 44
Table 28: Related documents............................................................................................................................ 45
Table 29: Terms and Abbreviations.................................................................................................................. 46
Table 30: Safety caution ................................................................................................................................... 47

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Figure Index
Figure 1: SIM900-DS functional diagram ........................................................................................................ 11
Figure 2: SIM900-DS pin out diagram (Top view) .......................................................................................... 12
Figure 3: Dimensions of SIM900-DS（Unit: mm）........................................................................................ 15
Figure 4: Recommended PCB footprint outline（Unit: mm）........................................................................ 16
Figure 5: Reference circuit of the LDO power supply...................................................................................... 17
Figure 6: Reference circuit of the DC-DC power supply ................................................................................. 17
Figure 7: VBAT voltage drop during transmit burst......................................................................................... 18
Figure 8: The minimal VBAT voltage requirement at VBAT drop.................................................................. 18
Figure 9: Powered on/down module using transistor........................................................................................ 18
Figure 10: Powered on/down module using button .......................................................................................... 19
Figure 11: Timing of power on module............................................................................................................ 19
Figure 12: Timing of power down SIM900-DS by PWRKEY......................................................................... 20
Figure 13: Timing of restart SIM900-DS ......................................................................................................... 21
Figure 14: RTC supply from capacitor ............................................................................................................. 23
Figure 15: RTC supply from non-chargeable battery ....................................................................................... 23
Figure 16: RTC supply from rechargeable battery ........................................................................................... 23
Figure 17: Seiko XH414H-IV01E Charge-Discharge Characteristic ............................................................... 24
Figure 18: Connection of the serial interfaces .................................................................................................. 24
Figure 19: Connection of RXD and TXD only................................................................................................. 25
Figure 20: Connection for software upgrading and debugging ........................................................................ 26
Figure 21: Speaker reference circuit ................................................................................................................. 26
Figure 22: Speaker with amplifier reference circuit ......................................................................................... 27
Figure 23 : Microphone reference circuit ......................................................................................................... 27
Figure 24: Earphone reference circuit............................................................................................................... 28
Figure 25: Reference circuit of the 8-pin SIM card holder............................................................................... 29
Figure 26: Reference circuit of the 6-pin SIM card holder............................................................................... 29
Figure 27: Amphenol C707 10M006 5122 SIM card holder............................................................................30
Figure 28: Molex 91228 SIM card holder ........................................................................................................ 31

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SIM900-DS_Hardware Design_V1.00 7 2013-01-20
Version History
Date Version Description of change Author
2013-01-20 1.00 Origin Zhaojuntao

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SIM900-DS_Hardware Design_V1.00 8 2013-01-20
1Introduction
This document describes SIM900-DS hardware interface in great detail.
This document can help user to quickly understand SIM900-DS interface specifications, electrical and mechanical
details. With the help of this document and other SIM900-DS application notes, users can use SIM900-DS to
design various applications quickly.
2SIM900-DS Overview
Designed for global market, SIM900-DS is a quad-band GSM/GPRS module that works on frequencies GSM
850MHz, EGSM 900MHz, DCS 1800MHz and PCS 1900MHz. SIM900-DS features GPRS multi-slot class 10/
class 8 (optional) and supports the GPRS coding schemes CS-1, CS-2, CS-3 and CS-4.
SIM900-DS with special features, Dual-SIM Dual-Standby, which allows customers to use two SIM cards in one
device simultaneously.
With a tiny configuration of 24*24*3mm, SIM900-DS can meet almost all the space requirements in user
applications, such as M2M, smart phone, PDA and other mobile devices.
SIM900-DS has 68 SMT pads, and provides all hardware interfaces between the module and customers’ boards.
zSerial port and debug port
zTwo analog audio channels
zPCM interface
zProgrammable general purpose input and output.
zThe SPI display interface
SIM900-DS is designed with power saving technique so that the current consumption is as low as 1.5mA in sleep
mode.
SIM900-DS integrates TCP/IP protocol and extended TCP/IP AT commands which are very useful for data
transfer applications. For details about TCP/IP applications, please refer to document [2].
2.1 SIM900-DS Key Features
Table 1: SIM900-DS key features
Feature Implementation
Power supply 3.2V ~ 4.8V
Power saving Typical power consumption in sleep mode is 1.5mA ( BS-PA-MFRMS=9 )
Frequency bands
zSIM900-DS Quad-band: GSM 850, EGSM 900, DCS 1800, PCS 1900.
SIM900-DS can search the 4 frequency bands automatically. The frequency
bands also can be set by AT command “AT+CBAND”. For details, please
refer to document [1].
zCompliant to GSM Phase 2/2+

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Transmitting power
zClass 4 (2W) at GSM 850 and EGSM 900
zClass 1 (1W) at DCS 1800 and PCS 1900
GPRS connectivity
zGPRS multi-slot class 10（default）
zGPRS multi-slot class 8 (option)
Temperature range
zNormal operation: -30°C ~ +80°C
zRestricted operation: -40°C ~ -30°C and +80 °C ~ +85°C*
zStorage temperature -45°C ~ +90°C
Data GPRS
zGPRS data downlink transfer: max. 85.6 kbps
zGPRS data uplink transfer: max. 42.8 kbps
zCoding scheme: CS-1, CS-2, CS-3 and CS-4
zIntegrate the TCP/IP protocol.
zSupport Packet Broadcast Control Channel (PBCCH)
CSD zSupport CSD transmission
USSD zUnstructured Supplementary Services Data (USSD) support
SMS
zMT, MO, CB, Text and PDU mode
zSMS storage: SIM card
FAX Group 3 Class 1
SIM interface Support SIM card: 1.8V,3V
External antenna Antenna pad
Audio features
Speech codec modes:
zHalf Rate (ETS 06.20)
zFull Rate (ETS 06.10)
zEnhanced Full Rate (ETS 06.50 / 06.60 / 06.80)
zAdaptive multi rate (AMR)
zEcho Cancellation
zNoise Suppression
Serial port and
debug port
Serial port:
zFull modem interface with status and control lines, unbalanced,
asynchronous.
z600bps to 460800bps.
zCan be used for AT commands or data stream.
zSupport RTS/CTS hardware handshake and software ON/OFF flow control.
zMultiplex ability according to GSM 07.10 Multiplexer Protocol.
zAutobauding supports baud rate from 600bps to 115200bps.
Debug port:
zNull modem interface DBG_TXD and DBG_RXD.
zCan be used for debugging and upgrading firmware.
Phonebook management Support phonebook types: SM, FD, LD, RC, ON, MC.
SIM application toolkit GSM 11.14 Release 99
Real time clock Support RTC
Physical characteristics Size: 24*24*3mm
Weight: 3.4g
Firmware upgrade Firmware upgradeable by debug port.
*SIM900-DS does work at this temperature, but some radio frequency characteristics may deviate from the GSM specification.

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Table 2: Coding schemes and maximum net data rates over air interface
Coding scheme 1 timeslot 2 timeslot 4 timeslot
CS-1 9.05kbps 18.1kbps 36.2kbps
CS-2 13.4kbps 26.8kbps 53.6kbps
CS-3 15.6kbps 31.2kbps 62.4kbps
CS-4 21.4kbps 42.8kbps 85.6kbps
2.2 Operating Modes
The table below summarizes the various operating modes of SIM900-DS.
Table 3: Overview of operating modes
Mode Function
GSM/GPRS
SLEEP
Module will automatically go into sleep mode if the conditions of sleep
mode are enabling and there is no on air and no hardware interrupt (such as
GPIO interrupt or data on serial port).
In this case, the current consumption of module will reduce to the minimal
level.
In sleep mode, the module can still receive paging message and SMS.
GSM
IDLE
Software is active. Module registered to the GSM network, and the module
is ready to communicate.
GSM
TALK
Connection between two subscribers is in progress. In this case, the power
consumption depends on network settings such as DTX off/on,
FR/EFR/HR, hopping sequences, antenna.
GPRS
STANDBY
Module is ready for GPRS data transfer, but no data is currently sent or
received. In this case, power consumption depends on network settings and
GPRS configuration.
Normal
operation
GPRS
DATA
There is GPRS data transfer (PPP or TCP or UDP) in progress. In this case,
power consumption is related with network settings (e.g. power control
level); uplink/downlink data rates and GPRS configuration (e.g. used
multi-slot settings).
Power down
Normal power down by sending the AT command “AT+CPOWD=1” or using the PWRKEY.
The power management unit shuts down the power supply for the baseband part of the
module, and only the power supply for the RTC is remained. Software is not active. The
serial port is not accessible. Power supply (connected to VBAT) remains applied.
Minimum
functionality
mode
AT command “AT+CFUN” can be used to set the module to a minimum functionality mode
without removing the power supply. In this mode, the RF part of the module will not work or
the SIM card will not be accessible, or both RF part and SIM card will be closed, and the
serial port is still accessible. The power consumption in this mode is lower than normal
mode.

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2.3 SIM900-DS Functional Diagram
The following figure shows a functional diagram of SIM900-DS:
zThe GSM baseband engine
zFlash
zThe GSM radio frequency part
zThe antenna interface
zThe other interfaces
SIM900-DS_Hardware Design_V1.00 11 2013-01-20
Analog base
band Digital base band
Power management unit Radio
Frequency
Power
supply
Analog Interface
Audio
ADC
Digital Interface
LCD/SPI
I2C
FLASH
SIM1
SIM2
GPIOs
PCM
RTC
·UART
Figure 1: SIM900-DS functional diagram

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3Package Information
3.1 Pin out Diagram
Figure 2: SIM900-DS pin out diagram (Top view)
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3.2 Pin Description
Table 4: Pin description
Pin name Pin number I/O Description Comment
Power supply
VBAT 55,56,57 I Power supply
VRTC 26 I/O Power supply for RTC
It is recommended to
connect with a battery or
a capacitor (e.g. 4.7uF).
VDD_EXT 15 O 2.8V output power supply If it is unused, keep open.
GND
2,17,18,29,39,
45,46,53,54,
58,59,61,62,
63,64,65
Ground
Power on/down
PWRKEY 1 I
PWRKEY should be pulled low at
least 1 second and then released to
power on/down the module.
Pulled up internally.
Audio interface
MIC1_P 19
MIC1_N 20
I Differential audio input
SPK1_P 21
SPK1_N 22
O Differential audio output
SPK2_P 23 Single audio output
MIC2_P 24 Single audio input
If these pins are unused,
keep open.
Status
STATUS 66 O Power on status
NETLIGHT 52 O Network status
If these pins are unused,
keep open.
SPI interface
SSI _CLK 11 O
SSI_DATA 12 I/O
SSI _OUT 13 O
SSI _SEL1 14 O
Display interface If these pins are unused,
keep open.
I2C interface
SDA 37 O I2C serial bus data
SCL 38 I/O I2C serial bus clock
If these pins are unused,
keep open.
GPIOs
GPO1 68 GPO1
GPIO4 49 GPIO4
GPIO3 50 GPIO3
GPIO2 51
I/O
GPIO2
If these pins are unused,
keep open.

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GPIO1 67 GPIO1
Serial port
RXD 10 I Receive data
TXD 9 O Transmit data
RTS 8 I Request to send
CTS 7 O Clear to send
DCD 5 O Data carrier detect
RI 4 O Ring indicator
DTR 3 I Data terminal ready
If these pins are unused,
keep open.
Debug interface
DBG_TXD 27 O
DBG_RXD 28 I
For debugging and upgrading
firmware
If these pins are unused,
keep open.
SIM1 interface
SIM1_VDD 30 O Voltage supply for SIM card.
Support 1.8V or 3V SIM card
SIM1_DATA 31 I/O SIM1 data input/output
SIM1_CLK 32 O SIM1 clock
SIM1_RST 33 O SIM1 reset
All signals of SIM
interface should be
protected against ESD
with a TVS diode array.
SIM1_PRESENCE 34 I SIM1 card detection If it is unused, keep open.
SIM2 interface
SIM2_VDD 43 O Voltage supply for SIM card.
Support 1.8V or 3V SIM card
SIM2_DATA 35 I/O SIM2 data input/output
SIM2_CLK 36 O SIM2 clock
SIM2_RST 40 O SIM2 reset
All signals of SIM
interface should be
protected against ESD
with a TVS diode array.
SIM2_PRESENCE 44 I SIM2 card detection If it is unused, keep open.
ADC
ADC 25 I Input voltage range: 0V ~ 2.8V If it is unused, keep open.
External reset
NRESET 16 I Reset input(Active low) Recommend connecting
a 100nF capacitor.
PCM interface
PCM_SYNC 48 O PCM sync
PCM_OUT 47 O PCM data output
PCM_IN 41 I PCM data input
PCM_CLK 42 O PCM CLK
If these pins are unused,
keep open.
RF interface
RF_ANT 60 I/O Radio antenna connection Impendence must be
controlled to 50Ω.
Not connect
NC 6 - This pin should be kept
open.

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3.3 Package Dimensions
Figure 3: Dimensions of SIM900-DS（Unit: mm）
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Figure 4: Recommended PCB footprint outline（Unit: mm）
Note: Keep copper out of area A. Do not place via in area B to avoid short circuit between the via on customer board and the test
points on the bottom side of the module.
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4Application Interface
4.1 Power Supply
The power supply range of SIM900-DS is from 3.2V to 4.8V. The transmitting burst will cause voltage drop and
the power supply must be able to provide sufficient current up to 2A. For the VBAT input, a bypass capacitor
(low ESR) such as a 100 µF is strongly recommended; this capacitor should be placed as close as possible to
SIM900-DS VBAT pins. The following figure is the reference design of +5V input power supply. The designed
output for the power supply is 4.1V, thus a linear regulator can be used.
GND
Figure 5: Reference circuit of the LDO power supply
If there is a high drop-out between the input and the desired output (VBAT), a DC-DC power supply will be
preferable because of its better efficiency especially with the 2A peak current in burst mode of the module. The
following figure is the reference circuit.
Figure 6: Reference circuit of the DC-DC power supply
The single 3.6V Li-ion cell battery can be connected to SIM900-DS VBAT pins directly. But the Ni-Cd or Ni-MH
battery must be used carefully, since their maximum voltage can rise over the absolute maximum voltage of the
module and damage it.
When battery is used, the total impedance between battery and VBAT pins should be less than 150mΩ.
The following figure shows the VBAT voltage drop at the maximum power transmit phase, and the test condition
is as following:
VBAT=4.0V,
A VBAT bypass capacitor CA=100µF tantalum capacitor (ESR=0.7Ω),
Another VBAT bypass capacitor CB=1µF.
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Figure 7: VBAT voltage drop during transmit burst
4.1.1 Minimizing Voltage Drop of VBAT
When designing the power supply in user’s application, pay special attention to power losses. Ensure that the
input voltage never drops below 3.1V even when current consumption rises to 2A in the transmit burst. If the
power voltage drops below 3.1V, the module may be shut down automatically. The PCB traces from the VBAT
pins to the power supply must be wide enough (at least 60mil) to decrease voltage drops in the transmit burst. The
power IC and the bypass capacitor should be placed to the module as close as possible.
Figure 8: The minimal VBAT voltage requirement at VBAT drop
4.1.2 Monitoring Power Supply
The AT command “AT+CBC” can be used to monitor the VBAT voltage. For detail, please refer to document [1].
4.2 Power on/down Scenarios
4.2.1 Power on SIM900-DS
User can power on SIM900-DS by pulling down the PWRKEY pin for at least 1 second and release. This pin is
already pulled up to 3V in the module internal, so external pull up is not necessary. Reference circuit is shown as
below.
Figure 9: Powered on/down module using transistor
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Figure 10: Powered on/down module using button
The power on scenarios is illustrated as following figure.
Figure 11: Timing of power on module
When power on procedure is completed, SIM900-DS will send following URC to indicate that the module is
ready to operate at fixed baud rate.
RDY
This URC does not appear when autobauding function is active.
Note: User can use AT command “AT+IPR=x” to set a fixed baud rate and save the configuration to non-volatile flash memory.
After the configuration is saved as fixed baud rate, the Code “RDY” should be received from the serial port every time when
SIM900-DS is powered on. For details, please refer to the chapter “AT+IPR” in document [1].
4.2.2 Power down SIM900-DS
SIM900-DS will be powered down in the following situations：
zNormal power down procedure: power down SIM900-DS by the PWRKEY pin.
zNormal power down procedure: power down SIM900-DS by AT command “AT+CPOWD=1”.
zAbnormal power down: over-voltage or under-voltage automatic power down.
zAbnormal power down: over-temperature or under-temperature automatic power down.
4.2.2.1 Power down SIM900-DS by the PWRKEY Pin
User can power down SIM900-DS by pulling down the PWRKEY pin for at least 1 second and release. Please
refer to the power on circuit. The power down scenario is illustrated in following figure.
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Figure 12: Timing of power down SIM900-DS by PWRKEY
This procedure makes the module log off from the network and allows the software to enter into a secure state to
save data before completely shut down.
Before the completion of the power down procedure, the module will send URC:
NORMAL POWER DOWN
At this moment, AT commands can not be executed any more, and only the RTC is still active. Power down mode
can also be indicated by STATUS pin, which is at low level at this time.
4.2.2.2 Power down SIM900-DS by AT Command
SIM900-DS can be powered down by AT command “AT+CPOWD=1”. This procedure makes the module log off
from the network and allows the software to enter into a secure state to save data before completely shut down.
Before the completion of the power down procedure, the module will send URC:
NORMAL POWER DOWN
At this moment, AT commands can not be executed any more, and only the RTC is still active. Power down mode
can also be indicated by STATUS pin, which is at low level at this time.
For detail about the AT command “AT+CPOWD”, please refer to document [1]
4.2.2.3 Over-voltage or Under-voltage Power down
The module software monitors the VBAT voltage constantly.
If the voltage ≤3.3V, the following URC will be reported:
UNDER-VOLTAGE WARNNING
If the voltage ≥4.7V, the following URC will be reported:
OVER-VOLTAGE WARNNING
If the voltage < 3.2V, the following URC will be reported, and the module will be automatically powered down.
UNDER-VOLTAGE POWER DOWN
If the voltage > 4.8V, the following URC will be reported, and the module will be automatically powered down.
OVER-VOLTAGE POWER DOWN
At this moment, AT commands can not be executed any more, and only the RTC is still active. Power down mode
can also be indicated by STATUS pin, which is at low level at this time.
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