SIM900 Hardware Design Guide Jeffrey Song, 2010
SIM900 Hardware Overview (Top View) Application: M2M Smart phone Tracker The GPRS/GSM engine for the global market. Quad-band : GSM850, EGSM900, DCS1800, PCS1900 GPRS multi-slot : Class 10 / Class 8 GPRS class : Class B GPRS coding schemes : CS-1, CS-2, CS-3, CS-4 Voice coders : HR, FR, EFR, AMR Noise Suppression Echo Cancellation Encryption : A5/1, A5/2, A5/3 RF : SAIC (Single antenna interference cancellation) Input voltage: 3.2~4.8V Sleep mode current consumption: 1.5mA Operation temperature: -40 ~+85 2
SIM900 Hardware Feature Tiny Configuration Size: 24 X 24X 3 mm (SMT), Weight: 3.4g, Packing : Auto-tray, Tape & Reel Low Power Consumption Sleep Mode : 1.5 ma High-Speed Processing ARM926EJ-S (156MHz) 3
Chipset Solution PNX4851 : RF7161 : Platform Solution A single-chip integrated base band processor and transceiver IC RF Power Amplifier supporting quad-band Memory Chipset (ST/SA) : NOR Flash + RAM Memory Software Solution RTOS : RTK-E Protocol Stack: 3GPP GSM/GPRS R99 4
Pins Diagram 5
VBAT --- Input Pin Power Supply Design Rules : Vmax = 4.8V, Vmin = 3.2V, Vnorm = 4.0V, Current Peaks : 2 A Current Peaks Frequency: 217Hz Any voltage drops that may occur in a transmit burst should not exceed 400mV. 6
Power Supply Schematic design guide lines 7
Power Supply How to select Power chip When input source(dc_in) is 5~9V,you d better select LDO such as MIC29302. When input source(dc_in) is above 9V,you d better select switch regulator such as LM2596. Of cause the switch regulator will generate EMC interferences, so it is due to your application. Regarding switching frequency, normally we suggest 500Khz or a little more. 8
How to select TVS Power Supply TVS has high ESD/spike protection performance. When you select TVS, you must notice the Working Peak Reverse Voltage (V RWM ). In normal, V RWM should be above working voltage 1~2V. If V RWM is too high, it can not protect the circuit. For example, for a 5V input source as above picture, we recommend you select TVS for1smb6cat3. 9
Power Supply How to select capacitor We recommend two kinds of capacitors in parallel as above circuit. C1:0.1u; C2: 100u/2200u C2 value depends on power chip. For example, for MIC29302(output current 3A), C2 should be 100u(Tantalum capacitor,low ESR), but for LM317(output current 1.5A), C2 should be 1000u. 10
Power Supply For example: 11
Power Supply Power supply PCB layout guidelines: The protection diode(tvs) must be placed close to the power source connector input. The bypass low ESR capacitor must be placed close to the module s power input pads. The PCB traces must be as wide enough to ensure no voltage drops occur during the 2A current peaks. Recommend 80mil. The PCB traces should be as short as possible. The use of a good common ground plane is suggested. 12
PWRKEY --- Input Pin Turning on/off be used to turn on or turn off SIM900 Timing of turning on 13
Turning on/off Timing of turning off 14
Turning on/off PWRKEY reference deisign circuit 15
Note: Turning on/off 1. PWRKEY pin has been pulled up internally(+3v), so any pulled up resistor on PWRKEY line is forbidden. We suggest to use open drain transistor to isolate the internal power system. 2. To check if the module has powered on/off, STATUS pin should be monitored. 3. It is mandatory to avoid sending data to module during the first 200ms of the module startup. 4. A HEX string such as 00 49 49 49 49 FF FF FF FF will be sent out through serial port at the baud rate of 115200 immediately after SIM900 is powered on. The string shall be ignored since it is used for synchronization with PC tool. 16
Hardware Reset To unconditionally restart the SIM900, the RESET pin must be tied low at least for 200ms and then released. A simple circuit as below: 17
Note: Hardware Reset 1. Don t use any pulled up resistor on the RESET line. The RESET pin must be connected with open collection configuration. 2. Hardware RESET is unconditionally shut down, without doing any detach operation from the GSM network. So you must use it as an emergency exit procedure. 3. The RESET signal is internally pulled up so the pin can be left open if not used. 18
Serial Port Level matching and isolating Auto bauding notes 19
Serial Port Level matching and isolating The serial port on SIM900 is a 2.8V UART, if customers CPU voltage is different with this, it must use level-shift circuit. We suggest two methods to implement level shift: Level-shift chip match Transistor match 20
Serial Port 21
Serial Port 22
Serial Port Notes: Using this method, you must care the switch frequency of transistor. In general, when baud rate is below 19200bps,we suggest you select this method. Furthermore, In Non-modem mode, we suggest to pull down RTS Pin to GND directly. 23
Auto bauding notes Serial Port In fixed baud rate mode, after module powers on, Unsolicited Result Codes RDY will be received from serial port, but in auto-bauding mode, no this info coming. When module powers on, we suggest to wait 1~2s then send first AT to module. In general, MCU needs to send 2 AT at least for synchronization baud rate. All of AT commands must be capital letter in autobauding mode. If CPU baud rate changed, you have to control DTR signal(high-low-high) 24
SIM Interface SIM_VDD, SIM_DATA, SIM_CLK, SIM_RST, SIM_PRESENCE 25
Hardware notes: SIM Interface SIM card lines should be layout in the bottom layer, and the length should as short as possible. It can not exceed 5cm. SIM card interface should far away from high speed signal such as crystal. The capacitor of SIM_VDD should be 100nF-1u, too small or too big will lead unpredictable problem. 26
SIM Interface As following figure, if the 10uF capacitor is used in the SIM_VDD line, the SIM card may not be detected. The 220nF capacitor is recommended. 27
Software notes: SIM Interface You must wait URC CALL READY (with fixed baud rate) then you can operate SIM card when you power on the module, or you can use AT+CCALR to inquire the states of CALL READY When you meet error 512 or 515, First you should check if you operate SIM card before CALL READY, and second you should check if memory of SIM card is full. 28
RF interface RF_ANT 29
Antenna Requirement VSWR: Voltage Standing Wave Ratio TRP: Total Radiated Power TIS: Total Isotropic Sensitivity For details, please refer to AN_SMT Module RF Reference Design Guide. 30
MIC_P, MIC_N, SPK_P, SPK_N Audio Interface 31
Audio PCB layout guide The audio lines (MIC&SPK) must be protected (must lay copper around the MIC&SPK lines) by ground layer. Ground layer (top and bottom etc.) must be connected by placing many vias, especially around the audio lines and under the module's shield. The width of MIC wires should be 8-10mil. The width of SPK wires should be 12-14mil. The audio lines had better use difference connection and keep parallel. The ground layout is very important. You should keep ground area as big as possible. For two layers PCB, most of lines should be layout on the bottom layer, especially the audio lines. For multilayers PCB, audio lines should be layout in the middle layer, and keep ground layer under audio layer. 32
DBG_TXD, DBG_RXD Debug Port Update Firmware & Capture Debugging Trace. It s recommended to keep a connector on customer s main board. 33
ADC, VRTC, VDD_EXT If not used, Keep these pins open. Others NETLIGHT State SIM900 function Off SIM900 is not running 64ms On/ 800ms Off SIM900 does not find the network 64ms On/ 3000ms Off SIM900 find the network 64ms On/ 300ms Off GPRS communication PWRKEYOUT, DISP_DATA,DISP_CLK,DISP_C/D,DISP_CS,LINEIN_R,LINEIN_L, PWM1, PWM2, KBR0~KBR4, KBC0~KBC4, GPIO11, GPIO12 ---these pins are available in the customized firmware! 34
Evaluation Board Kit 35
Thank You! 36