Behavior of ON/OFF pin



I have a Wism0228 modem on a custom board, controlled by a PIC, and I am observing a very strange behavior with the ON/OFF pin. My sequence for switching on the modem is as follows:

  1. Force ON/OFF pin low
  2. Wait at 700 ms (datasheet says to wait at least 685 ms)
  3. Wait until the READY pin goes high (datasheet says at most it takes 7 seconds)
  4. Release the ON/OFF pin (it floats and is not controlled by the PIC)

With a logic analyzer, I can see the waveform for the READY pin and it is as the datasheet says: low -> high -> low -> high.
However, about 80 ms after the READY pin is high for the last time, I can see the ON/OFF pin going low for a couple of ms, then back high again. It is the modem that is toggling the pin because the processor is not controlling the pin.

The problem is that if I try to setup a GPRS connection before the modem toggles the ON/OFF pin I get the error code 10 (SIM not found). But 5 seconds latter, I retry again to setup the connection and now it works without a problem.

Is there a waiting time before I can use the modem ? The datasheet clearly states that the modem is available as soon as the READY pin completes the sequence: low->high->low->high sequence

The other problem I am observing is related to the reset pin. After 2 failed attempts to setup a GPRS connection, I toggle the reset pin low for 50 ms and then pull it back up again. A couple of seconds latter I repeat the procedure for turning on the modem. However, sometimes the ON/OFF pin is kept low indefinitely by the modem and not even a reset pulls it out of this state. I must manually switch off the power supply to reset the modem. This is a very strange behavior.

Should I keep the reset pin low for longer than 50 ms ? Must I force the ON/OFF pin high before resetting the modem ?

Is there a better/alternative way to initialize the modem ?

This is very annoying because sometimes the modem simply locks and I cannot use it anymore without manually switching off the power.




Hi Nelson,

From what I can extract from your description: ON/OFF toggling occurs only in the case when the SIM is not active i.e. not registered to network. Please confirm this.

Regarding the problem related to reset, as you said, set the ON/OFF pin high before resetting the modem.




I am sorry for replying over a month latter, but I was shifted to another project. At the moment, our Wismo module has the ON/OFF pin always at a low state. It is the modem, not our processor, that is forcing that pin low.

At some point, we just gave up on using the ON/OFF, RESET and READY pins and we switch on/off the module energy, wait a few seconds and try again to configure and use the modem. The strangest thing (for us) is that the modem is working fine most of the time, with the ON/OFF pin always low.


I am not sure how to do this. As far as I know, before we can use the modem (that is, before we can sent AT commands) we need to toggle the ON/OFF pin. So, how can I know if the modem is registered or not without sending AT commands ? Otherwise I do not know if I am toggling the ON/OFF when the modem is not registered.