Thursday, November 04, 2010

Keeping Track of Time

The Navitrino controller has been running the heating system for over two weeks now and logging the internal and external temperatures. However, with it doing a full-time logging job, there has been little opportunity to take it off line and develop the firmware further.

I have a simple milliseconds timer to update the hours minutes and seconds, and unfortunately this has been running a bit fast. I really need to incorporate the real time clock code and make use of the DS1302 real time clock chip fitted to the NuElectonics shield.

Earlier this week I came across a simple task scheduler to run on the ATmega/Arduino - developed by Daniel Bradberry, and documented in his blog

Yesterday, using a spare DIY Arduino clone, I had a go implementing his simple task scheduler .

It was simply a matter of installing his "Timer" library and adapting his example sketch to suit.
In his header file Timer.h you need to change this line of code to get the required task-tick time


(this interval is then used in one place in the file Timer.cpp)

At the moment his second tick lasts something like 1.63 seconds, so I reduced the 200 above to 122 to make it much closer to 1 second. Unfortunately with the Timer 2 inside the ATmega, clocked at 16MHz, its not easy to get a very accurate second with the divide ratios available - closest you can get is 999.4mS which gains about 2 seconds per hour - but for most task timing it's fine.

His library allows you to set up a series of "once only" or repetetive tasks - with up to 10 independent timers scheduling them.

I set up tasks t_sec, t_15 and t_60 which kick off every second, 15 seconds and minute.

I have now re-jigged my central heating control/datalogger program so that it is controlled by the task scheduler.

The code looks a bit like this:

t_sec() {

// these are tasks you want to do every second eg update the time hh:mm:ss registers





t_60() {

// these are tasks you want to do every minute





If this partial re-write is successful, the next thing will be to add in the code for the SDcard datalogging, 1-wire and the RTC. However, these tasks will be much easier to implement now that I have the task-scheduler working.

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