SimCom SIM900B EVB Supplement

SIM900B_Hardware Design_V2.01

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SIM900B_Hardware Design_V2.01 2 2012-05-07
Document Title SIM900B Hardware Design
Version 2.01
Date 2012-05-07
Status Release
Document Control ID SIM900B_Hardware Design_V2.01
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. 2011

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SIM900B_Hardware Design_V2.01 3 2012-05-07
Contents
Contents.................................................................................................................................................................... 3
Version History........................................................................................................................................................ 7
1Introduction....................................................................................................................................................... 8
2SIM900B Overview........................................................................................................................................... 8
2.1 SIM900B Key Features ............................................................................................................................ 8
2.2 Operating Modes .................................................................................................................................... 10
2.3 SIM900B Functional Diagram ................................................................................................................11
3Application Interface...................................................................................................................................... 12
3.1 Pin Description ....................................................................................................................................... 12
3.2 Power Supply.......................................................................................................................................... 14
3.2.1 Minimizing Voltage Drop of VBAT................................................................................................... 15
3.2.2 Monitoring Power Supply ................................................................................................................... 15
3.3 Power on/down Scenarios ...................................................................................................................... 15
3.3.1 Power on SIM900B............................................................................................................................. 15
3.3.1.1 Turn on SIM900B Using the PWRKEY Pin (Power on) ..................................................................... 15
3.3.2 Power down SIM900B........................................................................................................................ 16
3.3.2.1 Power down SIM900B by the PWRKEY Pin ...................................................................................... 17
3.3.2.2 Power down of SIM900B by AT Command........................................................................................ 17
3.3.2.3 Over-voltage or Under-voltage Power down ....................................................................................... 17
3.3.2.4 Over-temperature or Under-temperature Power down ......................................................................... 18
3.3.3 Restart SIM900B by PWRKEY Pin.................................................................................................... 18
3.4 Power Saving Mode................................................................................................................................ 19
3.4.1 Minimum Functionality Mode ............................................................................................................ 19
3.4.2 Sleep Mode 1 (AT+CSCLK=1) .......................................................................................................... 19
3.4.3 Wake Up SIM900B from Sleep Mode 1 (AT+CSCLK=1)................................................................. 19
3.4.4 Sleep Mode 2 (AT+CSCLK=2) .......................................................................................................... 20
3.4.5 Wake Up SIM900B from Sleep Mode 2 (AT+CSCLK=2)................................................................. 20
3.5 RTC Backup ........................................................................................................................................... 20
3.6 Serial Interfaces ...................................................................................................................................... 21
3.6.1 Function of Serial Port and Debug Port .............................................................................................. 22
3.6.2 Software Upgrade and Debug ............................................................................................................. 23
3.7 Audio Interfaces ..................................................................................................................................... 23
3.7.1 Speaker Interface Configuration ......................................................................................................... 24
3.7.2 Microphone Interfaces Configuration ................................................................................................. 25
3.7.3 Earphone Interface Configuration....................................................................................................... 25
3.7.4 Audio Electronic Characteristics......................................................................................................... 25
3.8 SIM Card Interface ................................................................................................................................. 26
3.8.1 SIM Card Application......................................................................................................................... 26
3.8.2 Design Considerations for SIM Card Holder...................................................................................... 27
3.9 LCD Display/SPI Interface..................................................................................................................... 29
3.10 ADC........................................................................................................................................................ 30
3.11 RI Behaviors........................................................................................................................................... 30

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3.12 Network Status Indication ...................................................................................................................... 31
3.13 General Purpose Input/Output (GPIO) ................................................................................................... 32
3.14 Keypad Interface .................................................................................................................................... 32
3.15 Buzzer..................................................................................................................................................... 33
4Electrical, Reliability and Radio Characteristics......................................................................................... 34
4.1 Absolute Maximum Ratings................................................................................................................... 34
4.2 Recommended Operating Conditions..................................................................................................... 34
4.3 Digital Interface Characteristics ............................................................................................................. 34
4.4 SIM Card Interface Characteristics ........................................................................................................ 34
4.5 SIM_VDD Characteristics...................................................................................................................... 35
4.6 VRTC Characteristics ............................................................................................................................. 35
4.7 Current Consumption (VBAT = 3.8V) ................................................................................................... 35
4.8 Electro-Static Discharge ......................................................................................................................... 36
4.9 Radio Characteristics.............................................................................................................................. 37
4.9.1 Module RF Output Power ................................................................................................................... 37
4.9.2 Module RF Receive Sensitivity........................................................................................................... 38
4.9.3 Module Operating Frequencies ........................................................................................................... 39
5Manufacturing ................................................................................................................................................ 40
5.1 Mechanical Dimensions of SIM900B .................................................................................................... 40
5.2 Mounting SIM900B onto the application platform ................................................................................ 41
5.3 Board-to-board connector....................................................................................................................... 41
5.4 Mechanical dimensions of the RUNFENG BB530-06001-20R............................................................. 42
5.5 RF connector .......................................................................................................................................... 43
5.6 Top and Bottom View of the SIM900B .................................................................................................. 44
5.7 PIN Assignment of SIM900B................................................................................................................. 45
Appendix ................................................................................................................................................................ 46
A. Related Documents.......................................................................................................................................... 46
B. Terms and Abbreviations ................................................................................................................................. 47
C. Safety Caution ................................................................................................................................................. 48

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Table Index
TABLE 1: SIM900B 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 ....................................................................................................................................... 12
TABLE 5:THE CURRENT CONSUMPTION OF MINIMUM FUNCTIONALITY MODE ......................................... 19
TABLE 6: MICROPHONE INPUT CHARACTERISTICS ............................................................................................ 26
TABLE 7: AUDIO OUTPUT CHARACTERISTICS ...................................................................................................... 26
TABLE 8: PIN DESCRIPTION (AMPHENOL SIM CARD HOLDER)......................................................................... 28
TABLE 9: PIN DESCRIPTION (MOLEX SIM CARD HOLDER) ................................................................................ 29
TABLE 10: ADC SPECIFICATION ................................................................................................................................30
TABLE 11: RI BEHAVIORS ........................................................................................................................................... 30
TABLE 12: STATUS OF THE NETLIGHT PIN.............................................................................................................. 31
TABLE 13: PIN DEFINITION OF THE KEYPAD INTERFACE................................................................................... 32
TABLE 14: ABSOLUTE MAXIMUM RATINGS........................................................................................................... 34
TABLE 15: RECOMMENDED OPERATING CONDITIONS ....................................................................................... 34
TABLE 16: DIGITAL INTERFACE CHARACTERISTICS ........................................................................................... 34
TABLE 17: SIM CARD INTERFACE CHARACTERISTICS........................................................................................ 35
TABLE 18: SIM_VDD CHARACTERISTICS................................................................................................................ 35
TABLE 19: VRTC CHARACTERISTICS ....................................................................................................................... 35
TABLE 20: CURRENT CONSUMPTION ...................................................................................................................... 35
TABLE 21: THE ESD CHARACTERISTICS (TEMPERATURE: 25℃, HUMIDITY: 45 %) ....................................... 36
TABLE 22: SIM900B GSM 900 AND GSM 850 CONDUCTED RF OUTPUT POWER.............................................. 37
TABLE 23: SIM900B DCS 1800 AND PCS 1900 CONDUCTED RF OUTPUT POWER ............................................ 38
TABLE 24: SIM900B CONDUCTED RF RECEIVE SENSITIVITY............................................................................. 39
TABLE 25: SIM900B OPERATING FREQUENCIES.................................................................................................... 39
TABLE 26: PIN ASSIGNMENT...................................................................................................................................... 45
TABLE 27: RELATED DOCUMENTS ........................................................................................................................... 46
TABLE 28: TERMS AND ABBREVIATIONS................................................................................................................ 47
TABLE 29: SAFETY CAUTION..................................................................................................................................... 48

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Figure Index
FIGURE 1: SIM900B FUNCTIONAL DIAGRAM......................................................................................................... 11
FIGURE 2: REFERENCE CIRCUIT OF THE LDO POWER SUPPLY ......................................................................... 14
FIGURE 3: REFERENCE CIRCUIT OF THE DC-DC POWER SUPPLY..................................................................... 14
FIGURE 4: VBAT VOLTAGE DROP DURING TRANSMIT BURST........................................................................... 15
FIGURE 5: THE MINIMAL VBAT VOLTAGE REQUIREMENT AT VBAT DROP .................................................... 15
FIGURE 6: POWERED ON/DOWN MODULE USING TRANSISTOR....................................................................... 16
FIGURE 7: POWERED ON/DOWN MODULE USING BUTTON ............................................................................... 16
FIGURE 8: TIMING OF POWER ON MODULE........................................................................................................... 16
FIGURE 9: TIMING OF POWER DOWN SIM900B BY PWRKEY ............................................................................. 17
FIGURE 10: TIMING OF RESTART SIM900B.............................................................................................................. 18
FIGURE 11: RTC SUPPLY FROM CAPACITOR........................................................................................................... 20
FIGURE 12: RTC SUPPLY FROM NON-CHARGEABLE BATTERY ......................................................................... 21
FIGURE 13: RTC SUPPLY FROM RECHARGEABLE BATTERY .............................................................................. 21
FIGURE 14: SEIKO XH414H-IV01E CHARGE-DISCHARGE CHARACTERISTIC ................................................. 21
FIGURE 15: CONNECTION OF THE SERIAL INTERFACES..................................................................................... 22
FIGURE 16: CONNECTION OF RXD AND TXD ONLY ............................................................................................. 22
FIGURE 17: CONNECTION FOR SOFTWARE UPGRADING AND DEBUGGING.................................................. 23
FIGURE 18: SPEAKER REFERENCE CIRCUIT .......................................................................................................... 24
FIGURE 19: SPEAKER WITH AMPLIFIER REFERENCE CIRCUIT ......................................................................... 24
FIGURE 20: MICROPHONE REFERENCE CIRCUIT.................................................................................................. 25
FIGURE 21: EARPHONE REFERENCE CIRCUIT....................................................................................................... 25
FIGURE 22: REFERENCE CIRCUIT OF THE 8-PIN SIM CARD HOLDER............................................................... 27
FIGURE 23: REFERENCE CIRCUIT OF THE 6-PIN SIM CARD HOLDER............................................................... 27
FIGURE 24: AMPHENOL C707 10M006 5122 SIM CARD HOLDER......................................................................... 28
FIGURE 25: MOLEX 91228 SIM CARD HOLDER ...................................................................................................... 29
FIGURE 26: RI BEHAVIOUR OF VOICE CALLING AS A RECEIVER...................................................................... 30
FIGURE 27: RI BEHAVIOUR OF DATA CALLING AS A RECEIVER........................................................................ 31
FIGURE 28: RI BEHAVIOUR OF URC OR RECEIVE SMS ........................................................................................ 31
FIGURE 29: RI BEHAVIOUR AS A CALLER ............................................................................................................... 31
FIGURE 30: REFERENCE CIRCUIT OF NETLIGHT .................................................................................................. 32
FIGURE 31: REFERENCE CIRCUIT OF THE KEYPAD INTERFACE ....................................................................... 33
FIGURE 32: TOP AN SIDE MECHANICAL DIMENSIONS OF MODULE （UNIT: MM）.................................... 40
FIGURE 33: RECOMMENDED PCB FOOTPRINT OUTLINE（UNIT: MM）.......................................................... 41
FIGURE 34: BB530-06001-20R BOARD-TO-BOARD CONNECTOR ........................................................................ 42
FIGURE 35 : BOARD-TO-BOARD CONNECTOR PHYSICAL PHOTO .................................................................... 42
FIGURE 36: U.FL-R-SMT............................................................................................................................................... 43
FIGURE 37: U.FL SERIES RF ADAPTER CABLE ....................................................................................................... 44
FIGURE 38: TOP VIEW OF THE SIM900B................................................................................................................... 44
FIGURE 39: BOTTOM VIEW OF THE SIM900B ......................................................................................................... 44

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SIM900_Hardware Design_V2.01 7 2012-05-07
Version History
Date Version Description of change Author
2010-04-08 1.01 Origin Huangqiuju
2010-05-31 1.02 Modify voltage domain ，current consumption and
figure37 Huangqiuju
2010-06-23 1.03 §2.1, §3.3. §3.4 Modify the power supply range from
3.2V~4.8V to 3.1V~4.8V
§3.7, Modify the VRTC pin connection when RTC
backup is not needed.
Huangqiuju
2010-08-19 1.04 Modify the power supply range to 3.2v~4.8v.
§3.3.2 Add Figure 6:The minimal VBAT voltage at
VBAT drop.
§3.4 Modify figure 7.
§3.4 Add table 7.
§3.5 Add 3.5.4 and 3.5.5 description.
Delete chapter 3.6.
Add figure 29,figure 30,figure 31,figure 32
Modified figure 39 and B2B connector’s manufacture
Huangqiuju
2011-02-09 2.00 Arrange the structure of document. Huangqiuju
2012-05-07 2.01 Add illustration of SIM900 module information and
some notes in chapter 5. Juntao.zhao
2012-05-07 2.01 Add some notes in chapter 4. Juntao.zhao

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SIM900B_Hardware Design_V2.01 8 2012-05-07
1Introduction
This document describes SIM900B hardware interface in great detail.
This document can help user to quickly understand SIM900B interface specifications, electrical and mechanical
details. With the help of this document and other SIM900B application notes, user guide, users can use SIM900B
to design various applications quickly.
2SIM900B Overview
Designed for global market, SIM900B is a quad-band GSM/GPRS module that works on frequencies GSM
850MHz, EGSM 900MHz, DCS 1800MHz and PCS 1900MHz. SIM900B features GPRS multi-slot class 10/
class 8 (optional) and supports the GPRS coding schemes CS-1, CS-2, CS-3 and CS-4.
With a tiny configuration of 40mm*33mm *3mm, SIM900B can meet almost all the space requirements in user
applications, such as M2M, smart phone, PDA, FWP, and other mobile devices.
The physical interface to the mobile application is a 60-pin board-to-board connector, which provides all hardware
interfaces between the module and customers’ boards except the RF antenna interface.
zSerial port and Debug port can help user easily develop the applications.
zTwo audio channels include two microphone inputs and two speaker outputs.
zProgrammable general purpose input and output.
zThe keypad and SPI display interface will give user the flexibility to develop customized applications.
SIM900B 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 SIM900B Key Features
Table 1: SIM900B key features
Feature Implementation
Power supply 3.2V ~ 4.8V
Power saving Typical power consumption in sleep mode is 1.0mA ( BS-PA-MFRMS=9 )
Frequency bands
zSIM900B Quad-band: GSM 850, EGSM 900, DCS 1800, PCS 1900.
SIM900B 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+
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

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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.
z1200bps to 115200bps.
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 1200 bps to 57600bps.
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: 40mm * 33mm*3mm
Weight: 7g
Firmware upgrade Firmware upgradeable by debug port.
*SIM900B does work at this temperature, but some radio frequency characteristics may deviate from the GSM specification.

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SIM900B_Hardware Design_V2.01 10 2012-05-07
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 SIM900B.
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 SIM900B Functional Diagram
The following figure shows a functional diagram of SIM900B:
zThe GSM baseband engine
zFlash and SRAM
zThe GSM radio frequency part
zThe antenna interface
zThe board-to-board interface
zThe Other interfaces
SIM900B
SIM900B_Hardware Design_V2.01 11 2012-05-07
Flash+
SRAM Baseband
Engine
Antenna
connector
Radio
Frequency
LCD
KEYPADS
AUDIO
SIM
ADC
UART
GPIO
POWER
Board-to-board Connector
Figure 1: SIM900B functional diagram

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3Application Interface
3.1 Pin Description
Table 4: Pin description
Pin name Pin number I/O Description Comment
Power supply
VBAT 1,2,3,4,5,6,7,8 I Power supply
VRTC 15 I/O Power supply for RTC
It is recommended to
connect with a battery or
a capacitor (e.g. 4.7uF).
VDD_EXT 17 O 2.8V output power supply If it is unused, keep open.
AGND 50，51 Analog ground
Separate ground
connection for external
audio circuits. If unused
connect to GND
directory.
GND 9,10,11,12,13,
14 Ground
Power on/down
PWRKEY 34 I
PWRKEY should be pulled low at least
1 second and then released to power
on/down the module.
VILmax=0.9V
VIHmin=2.6V
VImax=3.3V
VILmin= 0V
It has been pulled up
internally (3V).
Audio interfaces
MIC1P 53
MIC1N 55
I Differential audio input
SPK1P 54
SPK1N 56
O Differential audio output
MIC2P 57
MIC2N 59
I Differential audio input
SPK2P 58
SPK2N 60
O Differential audio output
If these pins are unused,
keep open.
Status
NETLIGHT 30 O Network status
LCD interface
DISP _CLK 20 O
DISP_DATA 18 I/O
DISP _D/C 24 O
Display interface If these pins are unused,
keep open.

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DISP _CS 22 O
DISP_RST 26 O
Keypad interface / GPIOs
GPIO1/KBC4 35 I/O
GPIO2/KBC3 33 I/O
GPIO3/KBC2 31 I/O
GPIO4/KBC1 29 I/O
GPIO6/KBR3 45 I/O
GPIO7/KBR4 43 I/O
GPIO8/KBR2 41 I/O
GPIO9/KBR1 39 I/O
Defaults are as GPIO, they can be
multiplexed as keypad
If these pins are
unused ,keep open
GPIO5/KBC0 27
GPIO10/KBR0 37
I/O
GPIO
Just can be used as
GPIO, if these pins are
unused, keep open
Serial port
RXD 40 I Receive data This pin should be
pulled up to 3V
externally.
TXD 42 O Transmit data
RTS 44 I Request to send
CTS 46 O Clear to send
RI 48 O Ring indicator
DCD 28 O Data carry detect
DTR 38 I Data terminal Ready
If these pins are unused,
keep open.
Debug interface
DBG_TXD 49 O
DBG_RXD 47 I
Serial interface for debugging and
firmware upgrade
If these pins are unused,
keep open.
SIM interface
SIM_VDD 19 O Voltage supply for SIM card. Support
1.8V or 3V SIM card
SIM_DATA 21 I/O SIM data input/output
SIM_CLK 23 O SIM clock
SIM_RST 25 O SIM reset
SIM_PRESEN
CE
16 I SIM card detection
All signals of SIM
interface should be
protected against ESD
with a TVS diode array.
If SIM_PRESENCE is
unused, just keep open
ADC
ADC0 52 I General purpose analog to digital
converter. Input voltage range: 0V ~
2.8V
If it is unused ,keep open
Pulse Width Modulation
BUZZER 36 O PWM Output If it is unused, keep open

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3.2 Power Supply
The power supply range of SIM900B 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 SIM900B
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 2: 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 3: Reference circuit of the DC-DC power supply
The single 3.6V Li-ion cell battery can be connected to SIM900B 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.
SIM900_Hardware Design_V2.01 7 2012-05-07

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Figure 4: VBAT voltage drop during transmit burst
3.2.1 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 5: The minimal VBAT voltage requirement at VBAT drop
3.2.2 Monitoring Power Supply
The AT command “AT+CBC” can be used to monitor the VBAT voltage. For detail, please refer to document [1].
3.3 Power on/down Scenarios
3.3.1 Power on SIM900B
3.3.1.1 Turn on SIM900B Using the PWRKEY Pin (Power on)
User can power on SIM900B 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.
SIM900B_Hardware Design_V2.01 15 2012-05-07

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Figure 6: Powered on/down module using transistor
Figure 7: Powered on/down module using button
The power on scenarios is illustrated as following figure.
SIM900B_Hardware Design_V2.01 16 2012-05-07
VIL<0.9V VIH > 2.55V
Pulldown >1s
VBAT
PWRKEY
(INPUT)
Delay > 2.2s
13m
s
VDD_EXT
Serial Port Undefined Active
Figure 8: Timing of power on module
When power on procedure is completed, SIM900B 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
SIM900B is powered on. For details, please refer to the chapter “AT+IPR” in document [1].
3.3.2 Power down SIM900B
SIM900B will be powered down in the following situations：
zNormal power down procedure: power down SIM900B by the PWRKEY pin.
zNormal power down procedure: power down SIM900B 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.

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3.3.2.1 Power down SIM900B by the PWRKEY Pin
User can power down SIM900B 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 the following figure.
SIM900B_Hardware Design_V2.01 17 2012-05-07
VDD_EXT
（OUTPUT）
VIL<0.9V
VIH > 2.55V
5s >Pulldown >1s
PWRKEY
(INPUT)
VOL < 0.1V
Delay > 1.7s
Logout net
Figure 9: Timing of power down SIM900B 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.
3.3.2.2 Power down of SIM900B by AT Command
SIM900B 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]
3.3.2.3 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.

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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.
3.3.2.4 Over-temperature or Under-temperature Power down
The module will constantly monitor the temperature of the module,
If the temperature > +80℃, the following URC will be reported:
+CMTE: 1
If the temperature < -30℃, the following URC will be reported:
+CMTE:-1
If the temperature > +85℃, the following URC will be reported, and the module will be automatically powered
down.
+CMTE: 2
If the temperature < -40℃, the following URC will be reported, and the module will be automatically powered
down.
+CMTE:-2
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.
The AT command “AT+CMTE” could be used to read the temperature when the module is running.
For details please refer to document [1].
3.3.3 Restart SIM900B by PWRKEYPin
When the module works normally, if the user wants to restart the module, follow the procedure below:
1) Power down the module.
2) Wait for at least 800mS after STATUS pin changed to low level.
3) Power on the module.
SIM900B_Hardware Design_V2.01 18 2012-05-07
VDD_EXT
（OUTPUT）
H
Delay > 800ms
PWRKEY
（INPUT）
Pull down the PWRKEY
to turn on the module
Restart
Turn off
Figure 10: Timing of restart SIM900B

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SIM900B_Hardware Design_V2.01 19 2012-05-07
3.4 Power Saving Mode
SIM900B has two sleep modes: sleep mode 1 is enabled by hardware pin DTR; sleep mode 2 is only enabled by
serial port regardless of the DTR. In sleep mode, the current consumption of the module is very low. The AT
command “AT+CFUN=<fun>” can be used to set SIM900B into minimum functionality. When SIM900B is in
sleep mode and minimum functionality, the current of module is the lowest.
3.4.1 Minimum Functionality Mode
There are three functionality modes, which could be set by the AT command “AT+CFUN=<fun>”. The command
provides the choice of the functionality levels <fun>=0,1,4.
z0: minimum functionality.
z1: full functionality (default).
z4: flight mode (disable RF function).
Minimum functionality mode minimizes the current consumption to the lowest level. If SIM900B is set to
minimum functionality by “AT+CFUN=0”, the RF function and SIM card function will be disabled. In this case,
the serial port is still accessible, but all AT commands correlative with RF function and SIM card function will not
be accessible.
For detailed information about the AT Command “AT+CFUN=<fun>”, please refer to document [1].
Table 5:The Current consumption of Minimum Functionality Mode
<fun> Current consumption(uA) (sleep mode)
0 651
1 1000
4 715
3.4.2 Sleep Mode 1 (AT+CSCLK=1)
User can control SIM900B module to enter or exit the sleep mode 1 (AT+CSCLK=1) by DTR signal. When DTR
is in high level and without interrupt (on air and hardware such as GPIO interrupt or data in serial port), SIM900B
will enter sleep mode 1 automatically. In this mode, SIM900B can still receive paging or SMS from network but
the serial port is not accessible.
Note: For SIM900B, it is requested to set AT command “AT+CSCLK=1” and to ensure DTR at high level to enable the sleep
mode 1; the default value is 0, which can not make the module to go into sleep mode. For more details please refer to document
[1].
3.4.3 Wake Up SIM900B from Sleep Mode 1 (AT+CSCLK=1)
When SIM900B is in sleep mode 1 (AT+CSCLK=1), the following methods can wake up the module:
zPull down DTR pin.
The serial port will be active after DTR pin is pulled to low level for about 50ms.

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zReceive a voice or data call from network.
zReceive a SMS from network.
3.4.4 Sleep Mode 2 (AT+CSCLK=2)
In this mode, SIM900B will continuously monitor the serial port data signal. When there is no data transfer over 5
seconds on the RXD signal and there is no on air and hardware interrupts (such as GPIO interrupt), SIM900B will
enter sleep mode 2 automatically. In this mode, SIM900B can still receive paging or SMS from network but the
serial port is not accessible.
Note: For SIM900B, It is requested to set AT command “AT+CSCLK=2” to enable the sleep mode 2; the default value is 0, which
can not make the module to enter sleep mode. For more details please refer to document [1].
3.4.5 Wake Up SIM900B from Sleep Mode 2 (AT+CSCLK=2)
When SIM900B is in sleep mode 2 (AT+CSCLK=2), the following methods can wake up the module:
zSend data to SIM900B via main serial port. *
zReceive a voice or data call from network.
zReceive a SMS from network.
Note: The first byte of the user’s data will not be recognized.
3.5 RTC Backup
Current input for RTC when the VBAT is not supplied for the system. Current output for backup battery when
the VBAT power supply is in present and the backup battery is in low voltage state. The RTC power supply of
the module can be provided by an external capacitor or a battery (non-chargeable or rechargeable) through the
VRTC. The following figures show various reference circuits for RTC back up.
zExternal capacitor backup
Figure 11: RTC supply from capacitor
zNon-chargeable battery backup
SIM900B_Hardware Design_V2.01 20 2012-05-07