User's Manual

SIM900B Hardware Design
Figure 8: Timing of turn on system using PWRKEY
When power on procedure completes, the SIM900B will send out following result code to indicate the module is
ready to operate when set as fixed baud rate.
RDY
This result code does not appear when autobauding is active.
3.4.2 Turn Off SIM900B
Following ways can be used to turn off the SIM900B:
z Normal power down procedure: Turn off SIM900B using the PWRKEY pin
z Normal power down procedure: Turn off SIM900B using AT command
z Over-voltage or under-voltage automatic shutdown: Take effect if over-voltage or under-voltage is detected
z Over-temperature or under-temperature automatic shutdown: Take effect if over-temperature or
under-temperature is detected
3.4.2.1 Turn Off SIM900B Using the PWRKEY Pin (Power down)
The SIM900B can be turned off by driving the PWRKEY to a low level voltage for a short time and then release.
Please refer to the turn on circuit. The power down scenario illustrates as following figure.
This procedure lets the module log off from the network and allows the software to enter into a secure state and
save data before completely disconnecting the power supply.
Before the completion of the switching off procedure the module will send out result code:
NORMAL POWER DOWN
After this moment, the AT commands can’t be executed. The module enters the POWER DOWN mode, only the
RTC is still active. POWER DOWN can also be indicated by VDD_EXT pin, which is a low level voltage in this
mode.
VDD_EXT
（OUTPUT）
V
IL
<0.9V
V
IH
> 2.55V
Pulldown >1s
PWRKEY
(INPUT)
V
OL
< 0.1V
Delay > 1.7s
Logout net
Figure 9: Timing of turn off system using PWRKEY
SIM900B_HD_V1.03 24.06.2010
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