hardware design v1 00 2

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Figure 36: Reset reference design circuit
NOTE: It is recommended to cut off the VBAT power supply directly instead of using external reset pin when SIM900R can not
respond to the AT command “AT+CPOWD=1” and PWRKEY pin.
4.15 PWM
SIM900R provides two PWMs which can be used to drive a vibrator, and a backlight LED for display or
keyboard.
Each PWM output frequency varies from 25.6KHz to 3.25MHz.Two 7-bit unsigned binary parameters are used
for the output period and for the duty cycle. The AT command “AT + SPWM” is used to set the output period and
duty cycle of the PWM. For details,please refer to document [1].
4.16 I
2
C Bus
The SIM900R provides an I
2
C interface which is only used in the embedded AT application.
Note: This function is not supported in the standard firmware. If user wants this function, the firmware must be customized.
Please contact SIMCom for more details.
4.17 Antenna Interface
SIM900R provides a RF antenna interface. The customer’s antenna should be located in the customer’s main
board and connect to module’s antenna pad through microstrip line or other type RF trace which impendence must
be controlled in 50Ω. To facilitate the antenna tuning and certification test, a RF connector and an antenna
matching circuit should be added. The following figure is the recommended circuit.
59
MODULE
SIM900R_Hardware Design_V1.00 35 2012-07-20
R102
C101
60
RF_ANT
GND
C102
61
GND
R101
Antenna feed pad
Matching circuit
RF test connector
J101
J102
Figure 37: Antenna matching circuit