Friday, July 11, 2014

Getting 1PPS signal from a Bluetooth GPS

Recently some new experiments have been added to the (large) TO-DO list. For some of them, a time reference signal will be needed. So why not to use GPS timing? I have some old GPS in the junk box, so I could try to get the desired 1PPS signal from one of them.

One of the units is a Fortuna On Clip Bluetooth unit. This unit has been with me more than five years. The bluetooth side died, but the GPS side was still working, so I keep it to use in a future, maybe with a microcontroller. Also, this unit has connector for external antenna, so it is ideal for me. But, does it has the 1PPS signal?

The GPS is based on the SiRF Star IIe chipset, and uses the LP-7450 integrated circuit as signal processor. There is not too much info about this IC in the net, but some brochures show a signal called timemark. Is timemark the 1PPS signal I need?

I applied 5 volts to the DC input (the battery was dead) and connected to an external antenna I placed outside home. After some minutes the GPS got position, so I started to check every pin of this IC (it has 100) looking for the 1PPS signal.

There are some interesting signals in this IC, but one of them looked very promising:

100ms wide pulses and 900ms between pulses. Looks nice, but it can be anything, for example a signal to make a blinking led. The final test was made while listening a time and frequency standard station, RWM on 14.996 kHz.

The pulses on that pin were in perfect synchronise with the radio beeps. I turned off and on several times the GPS and every time the pulses were always synchronised with the radio beeps. No doubt, this is the 1 PPS signal I was looking for.

This 1PPS signal is available on pin 42 of LP-7450 integrated circuit.

Because the GPS bluetooth module was bad, I removed it. I reused the blue led and its driver to get an optical indication of the 1PPS signal. I also installed a small connector in the plastic case to get this 1PPS signal and the NMEA serial data, so it can still be used as a conventional serial GPS for a microcontroller, or with the aid of a TTL-RS232 converter, with the main PC.

Miguel A. Vallejo, EA4EOZ


  1. Estimado Miguel Angel,
    Aquí tienes el datasheet del integrado:
    Efectivamente en el pin 42 tienes la señal "timemark".