SIM900 problem. Is it dead? - Page 1
Forum section: electronics / projects

author: ktulu
author_rank: Regular Contributor
timestamp: April 03, 2015, 07:44:33 am
content: Hello.My SIM900 module has some strange behavior.After power up, by pulling PWRKEY low for 1sec, NETLIGHT goes high for about 2secs, than off.After that for about 800ms ON, 200ms OFF continous alternating follows, (of the NETLIGHT pin) and the module does nothing. STATUS pin remains low during the whole time. I get some short crap out the TX pin during the ON times.In the datasheet there are only 4 valid 'states' for the netlight pin, but nothing similar I get.Any help is welcome.Thanks.

author: picandmix
author_rank: Frequent Contributor
timestamp: April 03, 2015, 08:39:19 am
content: This site has some good working examples and a video showing the correct flashing sequence of the leds at power up.http://tronixstuff.com/2014/01/08/tutorial-arduino-and-sim900-gsm-modules/As for the rubbish on the TX pin, is your software correct and set to the correct baud rate ?

author: Berni
author_rank: Super Contributor
timestamp: April 03, 2015, 09:01:14 am
content: It does spit out junk on the TX line during bootup but to talk to it later you need 115200 by default. As for the led behavior its probably failing to connect to the network. Make sure it has a good antenna if you have bad reception in your area and make sure it can talk to the SIM card.

author: bookaboo
author_rank: Frequent Contributor
timestamp: April 03, 2015, 09:20:17 am
content: ftp://imall.iteadstudio.com/IM120417009_IComSat/DOC_SIM900_Hardware%20Design_V2.00.pdfOn page 45 you can see the meanings of the netlight timings.Sounds like you are out of network coverage or not registered to the network. Try the CSQ and COPS AT commands to find out what's up.

author: ktulu
author_rank: Regular Contributor
timestamp: April 03, 2015, 09:25:01 am
content: The module is actually built into a GPRS module, and controlled by a PIC micro. I have disabled the microcontroller, and toggled the PWRKEY manually with the same results. (with or without SIM card) I was monitoring the TX line with a terminal software, trying out every baud rate from 1200 to 115200.I am not sure if it needs some initialisation over the serial port for it to operate correctly, but I'm guessing not.The NETLIGHT should always be on for 64ms and not more in any situation, while mine is around 800ms.First i tought it is some error signalling, but the datasheet sais nothing about.As far as I can tell, it doesn't even try to connect to the network. It draws around 40mA.

author: bookaboo
author_rank: Frequent Contributor
timestamp: April 03, 2015, 10:28:02 am
content: Is the netlight PIN high for 800mS or is the LED output inverted by design?There is a hardware reset pin somewhere on that module I think.Another trap is that with it's talking with the network it takes some short 2A pulses, so make sure your power supply can handle it. Scope the power supply in at the SIM900 Vcc pins and watch for drops.

author: ktulu
author_rank: Regular Contributor
timestamp: April 03, 2015, 06:46:27 pm
content: The LED is driven trough a NPN transistor, and it is not inverted, so the netlight pin is high for 800mS. Aswell the LED stays ON for that long.Unfortunately I don't have a DSO, only a regular 20MHz scope, but absolutely no spikes can be seen.When I pull the reset pin low, the netlight pin stops blinking. After release, it continues but without the ~2sec first ON than 800/200 periods as when powerup.More weird, that I'm unable to power it down with the PWRKEY. Only by reapplying the power.I've found some info on updating the firmware:http://www.geekonfire.com/wiki/index.php?title=GPRS_Shield%EF%BC%88SIM900%EF%BC%89_Firmware_Updatehttp://dostmuhammad.com/blog/sim900-firmware-update-tutorials-appnotes/Maybe I should give a try? Just unsure which one to flash in. (enhance, dtmf, mms...) if it even works.

author: ktulu
author_rank: Regular Contributor
timestamp: April 04, 2015, 10:51:20 am
content: OK, I've reflashed the firmware, and seems to work fine now.Thank you all for your help.