General GPS project notes

I currently have three OEM GPS modules to play with. The Trimbles are 8 channel. (There is a pin compatible Lassen iQ that has 12 channels, might try one of those later).

The Garmin is 12 channel. The Trimbles are interesting because they can generate TSIP output which I can post-process in Pathfinder Office. The Garmin is interesting because I can try collecting RINEX data and postprocess that.

I have external antennas for all three. All three are similar in that they produce a datastream on a TTY signal level serial port, so I need to interface to that. My intention is to do things in 2 stages; first build a generic board to take serial data out to an RS232 port connected to a computer then to build a PIC based datalogger/interface to manage the GPS and talk to a handheld device.

At the PIC stage I want to use a Bluetooth module on the serial port so that my Dell Axim can talk to it without having any clumsy cable connection.

Trimble

Lassen

Lassen SQ

Serial interfaces

Serial TTY to USB

The Lassen SQ eval board has a Silicon Labs CP2102 USB-to-UART chip on it. Silicon Labs has a bunch of interesting evaluation kits for sale at $11 - $30. For example, their ethernet kit has light and temperature sensors on it, so you can connect it to your network and program it to send emails with the readings in the message. They have a digital compass+accelerometer for $72. But I digress...

The CP2102 has a 100 ma 3.3 voltage regulator on board; the Lassen SQ board uses it to power the SQ from USB power. Since the SQ uses about 100 ma there is nothing left to power the Bluetooth stamp so I need to go to a separate regulator when running in Bluetooth mode.

Serial TTY to Bluetooth

I am going to try using the Bluetooth stamp from SparkFun. The stamp requires a 3.3v supply, and responds to special AT commands (similar to a modem). It comes up 9600 8n1, so I had to connect a 3.3v TTL to RS232 converter to it and reprogram it to do 9600 8-odd-1 to work with my SQ.

The 3 pin regulators I got only do 100 ma as well. Should have ordered so beefier ones. I can always just use TWO. :-) I have one on the Bluestamp and one on the Lassen SQ. It's working though it's a bit out of spec for the Bluestamp.

I should look at the STM LD1117 which does 800ma and costs about 75 cents.

Bluetooth with ArcPad

To get it working you have to power on the bluetooth GPS receiver, then go into the Bluetooth Manager in the PocketPC and set up the connection. Then you have to configure ArcPad to use the correct bluetooth COM port.

Each time you restart the GPS receiver you pretty much have to reboot the PDA and reconnect in the Bluetooth Manager and then start ArcPad. It's pathetically unstable and locks up if you do things in the wrong order.

Power supply

7805 and friends turn too much energy into heat... sure are easy to use though. The 78SR05 is a switcher disguised as a 3-pin replacement

The National "Simple switcher" is efficient but not that simple. I bought a couple anyway to experiment. National LM2597 3.3 and 5.0 "Simple Switcher" are available in 8-DIP for $3.78 Digikey Requires 1N5817 Schottky diode and 100 uH inductor (M9799-ND) and the usual handful of caps.

Backup / standby power: Button cell batteries - CR2032

Instead of using several cells and a step-down regulator, another option is to use a step-up regulator. The Maxim MAX1674/MAX1675/MAX1676 is one. They are about $5 from Digikey. Using one of these allows generating 3.3v or 5v from a single cell (or two or three cells for more run time.)

I need to get some 22uH inductors and 47uF caps to use the 1674

For a few cents more there is also the MAX1774 ($5.43) which is a cool chip that has high efficiency, step-down for connection to a car battery, and single cell battery backup, It even has two regulators so you can have two output levels. It is really the bee's knees but once again, too complicated for my simple requirements. I just want a unit that runs on batteries for a long, long time.

See app note From Blackouts: Dirt Cheap to Fully Integrated

Parts orders