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A low cost GPS Monitor

For the Thunderbolt Monitor kit, click here

The purpose of this project is to build a low cost device that can monitor the serial packets generated by a Trimble Thunderbolt GPS Disciplined Oscillator (GPSDO).

The intention is to be able to decode either standard NMEA or Trimble TSIP packets and present sufficient information on a 2 line LCD display to ensure that the receiver is working and under what conditions.

It is very much work-in-progress, hard hat required before you read the rest of the page :-)

Here is the hardware for my prototype GPS Monitor, based on the following:

Here is the first prototype. I installed an LED on P0.7 for troubleshooting. As of v0.0.4, the LED only receives very narrow pulses and generates no visible light, but I use it as a test point for the scope probe.

The schematic (I have no shame). The part on the left is the component side of the Toolstick. Please note the very elaborate RS-232/TTL converter...

Component side view of the Toolstick and LCD. I used a 3 terminal voltage regulator for the 5V, and 3 diodes to drop the 5V down to ~3V for the processor. I used one of the LCD interface pad as a holder for the 3V supply to the Toolstick (the LCD is used in 4 bit mode, so data on the other 4 bits is irrelevant.)

The green LED on the Toolstick is connected to P1.3 and is not currently used by the firmware as of v0.0.4. The state of P1.3 is affected by the current LCD driver, so until this driver is rewritten, that pin cannot be used.

Here is a picture of the second prototype, intended for a Vacuum Fluorescent display (Noritake, Ref 6).

And here is a picture with the display, running:

Here is the schematic of Prototype #2:

The processor used, a C8051F330P, is in a 24 pin package, with 4 pins that are not used. That particular device is now obsolete and has been replaced with the C8051F330D (or C8051F330GP), which is the same processor packaged in a 20 pin package. The pinout of the P device is available in Ref 8 and the GP device pinout is available in Ref 9.

Here is the schematic with the D/GP processor:

Notes: Therefore, for firmware 0.3.3 and above to display GPS time like the original design, pin 18 shall be grounded.

Finally, here is a picture of both units side by side, in dim light. Which one do you prefer?
(to be honest, the LCD is equipped with LED backlighting, which was not connected)

The current state of the project and firmware download info is available in my Wiki (Ref 5).

And now, just for fun, here is a picture taken by Dan Karg of his GPSMonitor showing the 2008 leap second (the Thunderbolt was configured to output UTC):

Other source

Someone pointed to me a source of GPS Monitor hardware on eBay. The seller is fluke.l (note this is the letter "l", not the number "1").

I contacted him and he confirmed that he was using my software, in conjunction with overstock display modules. He was gracious enough to send me one of his monitors for the cost of shipping. The schematic is below:

I believe this is the second version, the first version did not have a voltage regulator.

In the unit I received from fluke.l, and as other people have reported to me, the three diodes D1-D3 have been replaced with 0 ohm resistors, which means the processor is powered directly from +5V. This is not good. The specification for the part has a max "normal" supply voltage of 3.6V, with an absolute max of 4.2V.

If that is the case in yours, you will need to take your fluke.l monitor apart to replace the jumpers with diodes. That requires desoldering the LCD connector. It is easier to unsolder the pins on the LCD side than on the processor board, as the holes on the processor board are a little too tight.

Alternately, if you do not have the desoldering tool necessary for that job, I recommend to just cut the pins, desolder them one at a time (which can be done with a conventional soldering iron) and replace the pins with bus wire. That will be much easier and will minimize the risk of damage to the PWB.


  1. Silabs ToolStick page: www.silabs.com/products/mcu/Pages/ToolStick.aspx, please note that you will need the 330 Toolstick and a Base Adapter. As of this writing, the total cost is about $28.00 + S/H ($10 for the Toolstick and $18 for the Base Adapter). The Base Adapter can be reused across various projects of course.
  2. The C8051F330-GP is no longer available. You may find some at brokers, but I recommend using the Toolstick instead. If you find the DIP device, you will need to buy a standalone programmer, or buy the small adapter that plugs in the Base Toolstick adapter to provide a standard JTAG interface.
  3. SDCC for Win32, at sourceforge.net/project/showfiles.php?group_id=599 (scroll down to sdcc-win32 and select 2.6.0, then click on "sdcc-2.6.0-setup.exe")
  4. There are many sources for the ubiquitous 2x16 character LCD displays. All Electronics has several models starting at $6.00 www.allelectronics.com
  5. GPS Monitor Wiki: /dokuwiki/doku.php?id=precision_timing:gps_monitor
  6. Noritake Vacuum Fluorescent Displays: www.noritake-elec.com/uversion.htm. Alternately, Mouser and Digikey sell VFDs made by Newhaven Display that are cheaper (about $40) and do work: Search for M0216SD-162SDAR2-1. Please note that unlike LCDs, VFDs have limited life. Brightness starts dropping after about 2 years of continuous operation, and particularly noticeable is that brightness becomes uneven across the display.
  7. Notes on SDCC and other 8051 development tools are in the Wiki: /dokuwiki/doku.php?id=tools_for_the_silabs_and_other_8051_micro_controllers
  8. Pinout of the C8051F330P: C8051F330P_Pinout.pdf
  9. Pinout of the C8051F330-GP: C8051F330-GP_Pinout.png

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November 19 2013 11:56:52