We just launched a KickStarter project for ClockTHREEjr! We can significantly reduce the price if we achieve a volume buy. Help us spread the word!
Justin
UPDATE: We have exceeded our minimum threshold by a factor of 2 and still three weeks to go! Help us keep up the momentum! Spread the word [clock]!!
UPDATE: We have added 2- and 3-board designs. Check out the super cool “artist rendition” of “to-the-minute”.
Here’s a quick pic of my custom faceplate Clock3. My clock has a custom black faceplate with a row of ‘dots’ across the second to last row on the bottom. Each dot represents 15 seconds and move one dot from left to right in groups of four dots, or one minute. When the dots reach the right side of the clock five minutes is up and the words change.
Prior to the actual doomsday, we plan to use DOOMSDAY as a race timer for mountain bike races. To that end we need precision time keeping. We have a 1 ms accuracy goal relative to an absolute GPS time reference. The GPS module provides time data in NEMA format as well as a one pulse per second (1pps) reference signal. When the GPS signal goes out of view, a real time clock chip keeps track of time and provides its own 1 Hertz signal: a 1 Hertz square wave.
In order to sync the 1pps signals to the real time clock square wave, I made some modifications to two libraries: Time, and TinyGPS (by Mikal Hart). Precision timing is made possible by adding a 1Hz reference to the existing library. Millisecond accuracy is obtained by keeping track of the last 1 Hz transistion and the 1 Hz duration. So in addition to the standard year(), month(), day(), hour(), minute(), and second() functions, I added millisecond() which returns an integer between 0 and 999.
Some other functions were added to support the 1Hz reference:
/* TJS: one Hertz interrupt to be called on rising edge of one Hz square wave. * Used to sync with GPS clock or other 1Hz source to get millisecond time accuracy * trigger is one of LOW, CHANGE, RISING, or FALLING */ void set_1Hz_ref(time_t current_time, int interrupt_pin, void(*cb_ptr)(), int trigger); /* * TJS: stop counting ticks. Used to sync with absolute time. */ void pause_1Hz(); /* * TJS: start counting ticks. Used to sync with absolute time. */ void unpause_1Hz();
The TinyGPS library also needed to be modified to support precision time synchronization. The key enabler is a fix notification scheme. At the heart is a user provided function that gets called when a new fix message arrives. This function was added to support this:
/* * TJS: Add a callback routine to be called when a new fix arrives. */ void TinyGPS::add_callback(fix_cb_t fct_ptr); // Example fix callback signature void grab_datetime(unsigned long _date, unsigned long _time, long lat, long lon, long alt, unsigned long speed, unsigned long course);
Arduino 1.0 has hit the street. And with that comes some backward compatibility issues. These are the ones I needed to address to get ClockTHREE to run on Arduino 1.0.
ISSUE #1: “BYTE” keyword is no longer supported, but it is not required with the new Serial.write command.
FIX: Replace Serial.print(val, BYTE); with Serial.write(val);
ISSUE #2: “wprogram.h” not found. It has been renamed “Arduino.h”
FIX: The release notes includes a handy pre-compiler directive to check of the arduino flavor you are using.
#if defined(ARDUINO) && ARDUINO >= 100 #include "Arduino.h" #else #include "WProgram.h" #endif
ISSUE #3: Wire.send, Wire.recieve renamed to Wire.write and Wire.read resp.
FIX: At first I tried writing a couple of macros like FIX 1, but after it took me a long time to discover my mistake, I decided not to be clever here. I now I just add a pre-compiler conditional for Arduino 1.0. So you end up wrapping the old code with the check wherever “Wire.send” and “Wire.receive()” where. Like this (and so on for read/receive).
// Arduino 1.0 compatibility
#if defined(ARDUINO) && ARDUINO >= 100 Wire.write("DATA", 4) #else Wire.send("DATA", 4) #endif
ISSUE #4: NewSoftSerial renamed to SoftwareSerial
FIX: This fix stinks, I don’t know a good way to solve it (suggestions welcome!) Arduino will scan the PDE file for ANY #includes EVEN when precluded by a pre-processor directive. The fix is to comment/uncomment the code for the correct version of Arduino.
For Arduino 1.0:
#include <SoftwareSerial.h>
SoftwareSerial sws(4, 5);
//#include <SoftwareSerial.h>
//SoftwareSerial sws(4, 5);
Most people have little need to synchronize clocks to the minute, much less to the millisecond….. but if you do need it, here is how to get millisecond (ms) accuracy in absolute time with Arduino using GPS.
We start with the 1pps signal from the GPS receiver. The 1pps signal is available from most GPS modules, but double check if you are ordering one for this purpose. 1pps stands for 1 pulse per second. The pulse is a very short (20 uS = 20/1,000,000 seconds) pulse whose rising edge coincides with the beginning of a second in GPS time.
For some reason, the GPS shield I got from spark fun, did not connect to the 1pps signal even though it had a spot for it. Checking further showed that only one of the three GPS headers was connected to the 1pps signal. I red-wired the missing link.
I hooked this 1pps signal into digital pin 2 (INT0) so that I could use the hardware interrupt to capture the rising edge of that pulse. Using this as the absolute 1 second reference, we can measure the drift of the DS3231 real time clock chip. This is the same chip that is used in the ChronoDot.
I did this by hooking up the 1Hz square wave to digital pin 3 (INT1) and used interrupts to measure time drift DS3231. My initial test showed it to be loosing about second per year to the GPS time. Not bad considering they only claim to be accurate to about a minute per year. Another way to look at the drift is that it will take about 6 hours to loose a milli second (1/1000 of a second). As long as we can sync with GPS once every 6 hours, we should be able to maintain timing precision and accuracy to a millisecond.
