an ISSUE with the SIM900...POWER Key Problem - Page 1
Forum section: electronics / projects

author: amirengineer
author_rank: Newbie
timestamp: June 19, 2014, 05:07:49 pm
content: I had SIM900 Shield and i use a 12V-2A power adapter , the Power Led is light on when i connect the power,but when i press the power key it was lighting the STA's LED but now nothing is happening to that led....ichecked all the voltages on the I/O to the LDO,Power Pins of the SIM900 & checked the LEDs if they arefault...every thing is OK....but the Module is still not responding to the Power Key Press...This is How it Should Look Like (STA's LED is ON...Blue Led)-----> 1st Attachment ImageThis is What I get even if i press the Power button-----> 2nd Attachment Imageso what could be the problem !!?while i was searching for a solution i found some one who made a video about the same Issue , if you didn'tunderstand my explaination of the problem here's his Video

author: gardner
author_rank: Regular Contributor
timestamp: June 19, 2014, 11:45:13 pm
content: The guy in the video you referenced said his unit was responsive to serial data. Is yours?Have a look at the AT+CFUN command. It is possible to place the SIM900 into a low-power mode that has to be reverted via this command. Perhaps it needs an AT+CFUN=1,1 to bring it back to life.

author: Harvs
author_rank: Super Contributor
timestamp: June 20, 2014, 12:39:18 am
content: When I first got a "shield" (as it was the dev board I could get quickest), I never found that led behaved the way it was suppose to. Instead I tapped a wire off the STATUS pin to a GPIO so I could actually follow the power up sequence, and reset sequence properly as per the SIM900 hardware manual. If you've got a uC that's performing the power up and power down, you really need to be monitoring the status line.

author: Harvs
author_rank: Super Contributor
timestamp: June 20, 2014, 12:41:27 am
content: BTW if you haven't yet, download the SIM900_HD_Vxxx Hardware Design manual. Under the power section it shows how each of the power and status pins behave.

author: amirengineer
author_rank: Newbie
timestamp: June 20, 2014, 08:46:30 am
content: Quote from: gardner on June 19, 2014, 11:45:13 pmI'd tried but the sim900 doesn't response to my AT Commands (i do have echo back of what i send) but no responseQuote from: Harvs on June 20, 2014, 12:39:18 amI power it up manually as a beginning then if it works fine i was going to write the code for controlling it....but it seems not goodQuote from: Harvs on June 20, 2014, 12:41:27 amDo you mean to connect the (pwrkey,rstkey,s_rstkey) by external switches ?

author: What_NZ
author_rank: Regular Contributor
timestamp: June 20, 2014, 11:17:19 am
content: These SIM900 units are power hungry at power on.Are you sure you are not getting a sudden drop in the VBAT voltage at power on? i.e. after you press the power switch for more than 3 seconds.According to the SIM900 specifications, a 100uF or greater should be placed as close to the VBAT pin as possible. The EFCOM design only uses x2 10uF.Also the drop could be from a dodgy 2A Power adapter but I would say less likely but worth checking.

author: amirengineer
author_rank: Newbie
timestamp: June 20, 2014, 03:52:24 pm
content: Quote from: What_NZ on June 20, 2014, 11:17:19 amIf that was the Problem , i think that the led (Blue for Status) would flash for a few seconds then turn Off but that doesn't happens.......Quote from: What_NZ on June 20, 2014, 11:17:19 amYes I notice that after i read your comment & i changed those 10uf Cap with a Tantalum 220uF but nothing happensQuote from: What_NZ on June 20, 2014, 11:17:19 amthat was very interesting...as i connect my multimeter with the power sockets (in series) i discovered that the module doesn'tconsume any Amps.!!!...i don't know why....i begin to suspect my meters,So tomorrow I'll test it on another meter on my Workbecause i don't have any High Quality one right now, but if it's really don't consume any current , What that means ??

author: What_NZ
author_rank: Regular Contributor
timestamp: June 20, 2014, 11:34:23 pm
content: Quote from: amirengineer on June 20, 2014, 03:52:24 pmAccording to Page 18 of the Hardware Design Manual Figure 11 there is up to a 2.2 second delay before activation of the STATUS output (LED) if VBAT drops in that time it may reset the module before the LED could flash. This is mentioned in 4.2.2 on the same page under Abnormal Power Down........Quote from: amirengineer on June 20, 2014, 03:52:24 pmIt was only a guess but worth a try. I'm sure they would have done a lot of testing before releasing the product to market but strange why they didn't follow the manufactures recommendations, probably cost, size, availability, something else(?)Quote from: amirengineer on June 20, 2014, 03:52:24 pmI reread your post and you say Quote "I'd tried but the sim900 doesn't response to my AT Commands (i do have echo back of what i send) but no response"Just to clarify - do you mean echo back from the sim900 module or you have 'Local Echo' turned on in your terminal program.Oh I would also suggest to check that the S_RST Switch isn't stuck down (I have had problems with those types of switches in the past) even if the S_RST switch seems mechanically ok, check the NRST (Pin 16) of the SIM900 module, it should be HIGH.