Monday, July 24, 2017

A GPS adjusted clock

We recently got a new cable box.  The new box has a clock that is a little smaller than the old one, and my wife has a hard time seeing it. We had an LCD clock with huge letters but there just isn't enough light in the place we want it.

So, NE06 GPS modules are getting pretty cheap. Why not get some huge seven segment LEDs and build up a clock based on a GPS?  Turns out large LED displays are fairly hard to come by, and pretty expensive. Plus, all that wiring to all those pins is a pain.

Then it hit me: I have some TFT displays, not huge, but plenty adequate for inch or inch and a half high digits.  If I used the TFT, I wouldn't even have to build the board; I could use my serial graphics terminal board.

First step was to see if I could make huge letters without too much hassle (the largest font I have is only about 3/8" high).  The first whack isn't too bad. It will work, although I might yet do some prettying up of the font.

So, on to the GPS.

I started with my serial terminal code, which may have added more complexity than I needed, but it works.  This code fills a circular buffer with serial interrupts from the GPS and the mainline has to process the contents of the buffer. This makes missing characters less likely but it does mean the mainline is futzing with trying to understand whether there are characters in the buffer and identify the start and end of the GPS data.

Parsing the GPS data is tedious, but not especially difficult.  With the PIC32MX150F128B there is plenty of memory, so I can be pretty aggressive with my code.  Most of the code is the TFT library so the stuff I'm working on is pretty small anyway.

At this point I can get the time and coordinates (which I don't need).  I am displaying all sorts of values which eventually won't be used.

Next up is to get the date, and go through the tedious business of determining whether it is daylight savings or standard time. The correction is trivial but knowing when to apply it is a pain.

Following that I'll get after the PIC's real time clock calendar.  Supposedly the PIC can maintain the time to within a third of a second per month.  I'm not convinced can actually accomplish that, but adjusting it from time to time through the GPS should keep the time always correct.

Since even in the basement with overcast skies I can still see 8 to 11 satellites that might be overkill.  If I have too much trouble with the RTCC I might just skip that, but the next priority is the daylight savings time thing.

The code is currently partial, and pretty rough, but as always I keep everything in git and the current code is in gitlab:

The board hardware is at

And the TFT library code at

They will be updated as time goes on.

OK, so some code cleanup, and slightly larger digits, but still significant work to do:

The digits are actually a tan color, but it doesn't show in the picture.

Sunday, July 16, 2017


It has been a long time since I wrote about the dsPIC-EL-GM. This thing has turned out to be a very successful tool for me.

Over the past couple of years, this has been the platform for dozens of projects.  In some cases the entire project was done on the board plus a shield. In others, it became the prototype for a purpose built board.

The choice of the dsPIC33EV was a good one.  Besides being 5 volts, meaning inexpensive displays work with it, it is fast, available in a variety of memory sizes (32-cheap, up to 256-huge), has a huge range of peripherals, and PPS.

The decision to put connectors for shields on the board turns out to be critical. Most projects need an LCD.  Hand wiring all those pins from the PIC to the display, plus power and programming (which I seemed to always get wrong) tended to present a barrier to starting new projects. With the shields I can just grab a shield and get started, and all the tedious stuff is behind me.

Plus, by having a standard part, standard display/LED/button pinouts, I have built up a number of libraries that make getting a project started easier and faster.

There are a couple of features that turn out not to have been so useful.  The jumper allowing me to use PIC24EV parts hasn't gotten used very much. I was attracted by the low price of some of those parts, but the 33EV32 isn't all that much more expensive, and it has more memory and way more speed.

Also, I had added a connector offset slightly from the shield connectors to allow me to plug in standard perfboards. But the extremely low cost of custom prototyping boards made that less useful. The demise of Radio Shack also shoved that feature into obscurity.  It was a significant advantage to be able to run down to the corner to get a perfboard. Now that it has to be mail order, I just keep an adequate supply of prototyping shields on hand.

I have been tempted to build another PIC-EL for one of the PIC32s. The PIC32MZ series offers blinding speed, crazy big memory, and even floating point. By having a flexible platform, the need for a DIP package is mitigated, and the price of the MZ, while not cheap, isn't really crazy. I had a lot of fun doing a graphics card with the PIC32MX250F128B, so I feel like the PIC32 isn't that alien of an animal.  But that is a project for another day.

I am not trying to sell dsPIC-ELs. However, needed information for building your own is available on gitlab:
  • Gerbers for the dsPIC-EL-GM are available here.
  • And for the proto shields here.
We did kit a few for the high school electronics club, which necessitated creating detailed construction instructions:
  • Build instructions here.
Should you build your own, let me know how you did in the comments below.