Hi guys,
I'm on my 3rd board now, we received new 4 layer boards thanks to nice people on this list! Thanks again! After populating board 1 and after a careful visual inspection, the power supply on the board refused to work. Everything except the GPS and patch antenna was populated on all the boards. We looked around parts of the power supply, and could find no seemingly obvious problems, no shorts, and all component values were good. Since debugging the board is quite difficult, I decided to populate a second board, but work in steps this time. SO, I first populated components for the 5V supply looking at the app notes for the 8pin LDO IC, and when we got 5V, I continued onto the 3.3V line. The 3.3V line
worked too, so the supply problem was resolved. Since the supply was working OK, I decided to populate everything else and test. When we hooked it on to the current limited supply ( 100ma, 7V ) the current varied a lot ( I noticed this when only the power supply parts were populated too ).
The current varied from 25ma, to about 60-65ma, and one of the LED's ( The one towards the 3.3V reg ) stayed lit. We probed the supply voltages, and all was well. BUT, then we tried reset, and current did not go down. So, we tried probing to see if the oscillator was oscillating, and nothing, nothing at all. We switched it off, and tried looking for shorts around the crystal. At that point, it appeared as if one of the 2 caps ( 0.1Mf ) around the crystal had a short, and was grounding the LPC pin. So, I carefully got rid of the short and we tried powering up, but no oscillations at all. When I tried today morning, it gave up, just like the previous board. The
current goes up steadily, The LED lights up sowly ( fairly random, sometime it goes on bright, sometimes flickers, sometimes its dim ), the 2nd LED starts flickering, it hits current limit, and stops working. Turn off and on, it goes into current limit immediately. Before measuring, and without the LPC soldered on, resistance on the 3.3V line was around 577ohms. After the Tiny reaches current limit, and I shut the power supply down, the 3.3V line shows a 19ohm short.
I desoldered the LPC, and voila, the 570ish ohms was back, and stayed like that even after I resoldered the LPC. I powered it up, and it reached current limit again, slowly, like how I described above, and the 3.3V line now shows the 19ohm short again.
Anyone know what's happening here?
-Thanks in advance
Uttam
P.S. Thanks for the 900mhz antenna help guys!