I finished assembling the first revision of the tracking device. Unfortunately there was a slight problem with the footprint for the BMA180 so the accelerometer portion of the board is non functional. I've also come to the realization that I'm no longer going to use 402 SMD components for future designs. As much as I love the challenge of trying to place these tiny little parts on a board I think I'm going to move to 603 and up from now on.
So how did the rest of the tests go? Fairly well I'd have to say. Initially I was a little concerned about possible interference on the GPS because of the close proximity to the GSM module and high frequency buck converter for the power supply, but the impact seems to be minimal if any.
The charging circuit was designed around an MCP73833 single cell Lipo charger configured to 500mA charge rate supplied through a a mini USB port. There are also three on board LED's to show the charging state.
For the 3.3 volt power supply I decided to go with a Fairchild FAN5362 3MHz buck regulator. It's capable of sourcing 500 mA which is plenty of current for powering any other 3.3 volt external devices that would be connected to the board. Another benefit to using the buck regulator over a standard LDO is that the buck has under voltage lockout protection (UVLO). Basically the regulator will shutdown if the battery voltage falls below 2.5 volts. It's a nice feature to have when you're powering something with a lipo battery.
As I mentioned earlier I had a problem with the accelerometer footprint so I wasn't able to test much of the sleep mode options with the board. I wanted to see how low I could get the current draw in sleep mode as compared to the average 50 mA it draws while fully powered up. I guess I'll have to wait for the next revision. For the next revision I'm planning on switching from a BMA180 to a BMA250. It's a comparable accelerometer with a little lower resolution but features built in algorithms for orientation detection.
Another option I'm adding on the next revision is a connection for an external battery backup for the GPS. Currently the battery backup pin is wired directly to the Lipo battery which, as long as the battery is connected, does provide backup power. Unfortunately if I disconnect the battery the GPS resets and I have to wait for the ephemeris data to download again which takes about 12 minutes.
I'm sending out for the next rev board this week so stay tuned...