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.
Garmin
Trimble
Lassen
Serial interfaces
Serial TTY to RS232
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 AT commands. It comes up 9600 8n1. I will need to reset it to 9600 8-odd-1 to work with my SQ.
Drat. 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 should look at the STM LD1117 which does 800ma and costs about 75 cents.
PIC datalogger
Power supply
High efficiency switching voltage regulators National LM2597 3.3 and 5.0 "Simple Switcher" 8-DIP $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
