Assembly Question

This forum is used for discussing hardware, software, and technical details of the AVRcam embedded system.

Assembly Question

Postby Guest » Sat Feb 26, 2005 12:07 pm

Hello.

I just finished soldering my board and plugged the battery in to test for the green led. For some reason both leds are comming on. The green LED comes on right away. After a little bit the yellow LED starts getting brighter and brighter. Has any one seen something like this? I have gone over my solder and I dont seen any bridges. This is before adding the ICs and I am getting 5V between Vcc and ground.

Any ideas on where I can look?

Thanks
Guest
 

Postby Guest » Sat Feb 26, 2005 2:14 pm

Hello,
The yellow LED is directly connected to one of the i/o lines of the mega8 (I know you haven't installed any of the ICs yet, but what I mean is that this is normally driven by one of the i/o lines on the mega8; specifically, it is hooked up to pin 12 of the mega8 , which is Port D bit 5).

Obviously, with no mega8 in circuit, the yellow LED should not be coming on at all. Can you do a few tests with a ohmmeter to see if this line is bridged to anything other than pin 12 of the mega8? An LED that starts off and gradually turns on sounds like a capactor that is charging up slowly...but this doesn't really make any sense since the resistors in circuit aren't all that large, and the caps are fairly small as well. Can you give me a feel for the following:

-after applying power, how long before the yellow LED comes on at all?
-how long until it reaches it maximum brightness?
-can you monitor the voltage at pin 12 of the mega8 while this phenomenon is happening?
-can you monitor the overall current draw of the system during this time frame (by putting an ammeter in series with the power supply)?

This is a strange one. For reference, what version of the PCB do you have, 1.1 or 1.2?

If you have an ohmmeter, I would seriously start poking around the circuit to see if anything else is connected that shouldn't be. My initial guess is that there is some short someplace which needs to be fixed before proceeding.

Post back and let me know what you find.
Guest
 

Postby Chimp87 » Sat Feb 26, 2005 5:19 pm

Sorry, I am the original poster, I thought I was logged in.

I have v1.2 of the PCB.

After tracing arround the PCB using the schematic in the manual the only bad point that is showing up is pins 5 and 6 on the mega8 which go to p14 on the camera header also seams to be connected to ground. None of those pins have a solder bridge. It looks to be a short run between the points and I do not see anything connecting to the trace.

It took a good 2 or 3 seconds to get to full brightness but now it is turning on right when power is applied. Wierd. The voltage at pin 12 is 4.5 and it is 5V at Vcc. The circuit is drawing about .8 mA. It is reading .5 kOhm between pin 12 and Vcc. I have checked all the points I can with pin 12 but am not finding any shorts. My only guess is that there is something happening under the socket header. I will try and desolder that and see if it helps.
Chimp87
 
Posts: 1
Joined: Sat Feb 26, 2005 12:41 am

Postby Guest » Sat Feb 26, 2005 7:01 pm

Ok...I just ran a few tests with a board that is essentially at the same assembly point that yours is at (i.e., I removed all the ICs, and removed the camera).

The circuit seems to draw around 4.5 mA with just the green LED on (this current is measured at the Vin polarized terminal). 0.8 mA doesn't seem right, as I would think the LEDs would be drawing more than this.

Pins 5 and 6 on the mega8 should be connected, and connect to pin 14 on the camera, but this should definitely not be connected to ground. I just checked a dozen blank v1.2 PCBs to see if any of them had a short between pin 5 on the mega8 and ground, and none of them did. This is interesting though...a short between pin 5 and ground would explain a problem another user was having, where any command requiring the detection of a transition on pin5/6 never seemed to happen (i.e., if the lines were shorted to ground, no transition would ever occur). Anyway...

A voltage of 4.5 volts at pin 12 is, again, strange. Lets see...something is dropping 0.5 volts someplace...I'm not sure what that could be though.

Another item to note is that the output of pin 12 also goes to the three-pin header along the left side of the board. SIG1 labelled on the board is directly connected to pin 12 of the mega8. The thinking here is that the user could drive some external device with this line if they wanted to. Check around this header to see if anything looks fishy.

Also, the 500 ohms between pin 12 and Vcc...two 1K resistors in parallel would give this resistance...there are several 1K resistors in the circuit, one of which should go from pin 12 to the yellow diode...but this doesn't really explain how you could get the 500 ohms...

Can you measure the resistance between pin 12 of the mega8 and the anode of the yellow LED? This should read 1K...

I'll keep thinking about what could be going on here...post back if you find anything else out.
Guest
 


Return to AVRcam Embedded System

Who is online

Users browsing this forum: No registered users and 21 guests

cron