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How to enable sleep mode in sim900?

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Newbie level 5
May 10, 2011
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"Sleep mode (Slow Clocking mode)
Enter sleep mode
- at+csclk=1 and keep DTR high
Wakeup from sleep mode
- Pull DTR low
- Receive an SMS or incoming call

How I could know that the SIM900 module is entered to the SLEEP mode?

As I know the power consumption in SLEEP mode is 2.5mA, but when I enter at+csclk=1 and keep DTR high the power consumption is not decreasing.
May be I am not taking into consideration something?

Could anybody help me to enter SIM900 in SLEEP mode?

Hi v57ali

I have this same issue in a design. Did you succeed on entering sleep mode so that current lowers to the 2-3mA range??

Thanks in advance, best regards


Hi Guys,

Sleep Mode as follows:

AT+CSCLK=1 to set mode.

To be able to issue any commands to the SIM900 after this requires that DTR is low.

Note: DTR should be driven with open collector/drain output or use a level translator.

The sleep mode is autonomous operation in the SIM900, after DTR is set high sleep mode is entered after a couple of seconds and from time to time the SIM900 will "wake".

To confirm sleep mode is active with DTR high the SIM900 will not process commands on the serial port.

Incoming SMS or Call is indicated by the RI line (see SIM900 hardware reference doc for timing and phase) which can be used to wake up controlling MCU if also in low power mode.

The sleep mode current I have measured is <2mA.

Hi Batang

Thanks for your answer. To be honest, I had tried the CSCLK command before. But the thing is that my module is not exactly a SIM900 but a SIM908 (GSM/GPRS + GPS engine, but the GSM/GPRS part is a SIM900) and this may be the reason by which my module takes about... 30 SECONDS!!! to enter sleep mode. After that time, power consumption reduces, as you say, to some 1,5mA. I found it out by chance, as i sent the CSCLK command and went to the wc... :) May be it´s only that the GPS engine needs that time to completely switch off... I have no clue. Anyway, it works!!

By the way... there is an strange behaviour in my module that you may help to explain. I set pin55 to interrupt. The main thing is to detect a movement with an accel, amplify it with an opamp and set the interrupt with an TS372 comparator. Pullup R of 10k.

when I test this functionality alone, everything goes fine. every time i shake the PCB an interrupt happens. But if I am connected to the GSM network, every time I send/receive anything, this pin goes high on its own!!! Sets to vcc for a couple of seconds and returns to low level. Is this a matter of the core FW??? I have tried pulldown resistors, capacitors... May be I'm not setting the pin correctly but I get FL_OK messages when i configure the pin. And, as i mentioned before, when not connected to GSM network, it works fine!!!

Thanks in advance best regards


UPDATE: By the way... I can´t understand why CPOWD=1 command shuts off the module and the power consumption is then in the order of tens of mA-s. If it´s shut off... I understand that its consumption should be the minimum, even less that in sleep mode... Am I wrong? My intention was first to switch the module ON and OFF with the accel, opamp, a flipflop and comparator. But this consumption issue forced me to change the strategy and use sleep mode and DTR pin instead. And I have a 1k pulldown resistor from PWR_KEY to gnd to ensure module switches on.
Last edited:

Hi Iban,

If you are using a SIM908 the GPS must turned off before asserting DTR high.

As for the high current draw after module shutdown a guess would be leakage on the serial port pins etc but without seeing your circuit hard to say.

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Hi Batang

Thanks for your answer.

Yes, as you say, I have to power down the GPS engine before entering sleep mode. This works fine and I have cut my module's consumption down to 2,5mA. But about the 30mA current when shut down (CPOWD =1)... The circuitry I have attached to my module is the same in power down and in sleep mode, so current should be in the same order of magnitude, I guess, not 10 times bigger... May be I don´t switch GPS off before shutting down.. I'll check this. And I may disconnect everything from my module so that SIM908 remains connected only to vcc and GND. I'll try this two.

Thanks, and best regards.


Hi all, seems you do make sim900 enter sleep state, but I still cannot , even if use the same procedure as above, that is 1)semd "AT+CSCLK=1" to module, 2) pull up DTR, then wait, but nothing happens after even minutes, dont know why, I 'm using EAT, maybe this is the fault

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