Time to think about time....

With the RFID system on a postal adventure for the next few days I decided to give some thought to the T of ODOTS. (It is indeed, T time...)

As a timing system the ODOTS will need to be able to accurately (and to an extent, precisely) track the time at each of the recording stations. The microcontroller unit has the ability to track time to an extent by looking at its own clock (digital signal clock, a simple oscillator) but these tend to be rather inaccurate (because they don't need to be) and also loose track of time when you pull the plug. 

This is where an external clock chip comes in handy both having dedicated hardware ensuring that it is keeping time well, and the ability to run on a very small power supply, allowing it to be powered with a small coin cell while the rest of the system is having a plug-pull induced nap. As mentioned in the last post the proof of concept system is staying in DIP land to reduce required soldering complexity/need for PCBs, on top of this requirement I also wanted the chip to require a minimum of external components. Luckily the handiest chip to use is Maxim's DS1337(+) a brilliant little chip that not only satisfies my generational requirement to meme, but also already has drivers written for it (though I only learned this after getting halfway through writing my own).

Choosing the RTC with the power of 1337.

RTC (Real Time Clock) time will be synchronised with a computer, probably during initial setup and then periodically by some sort of Tech Wiz (capitals necessary), while the microcontroller will periodically re-align its own time estimate with the RTC's during operation. Ideally the update timing slack will be less than 10ms (about the maximum precision I could really expect from the RFID system). The RTC can generate interrupts on second boundaries (effectively showing the 0ms point for that second) which I will be exploiting to allow the Microcontroller to gain some sort of sub-second accuracy. 

The frequency of RTC referrals is interesting, ideally I want to keep the interval small (to prevent inaccuracy buildup), but not so small that referring to the RTC is the only thing the MCU has any time time to do. Initially I had thought 10 minutes would be good, but this would require the Microcontroller to set the next interrupt time after each referral, increasing the time each refferal takes. However, the RTC has inbuilt repeating alarm abilities, once every unit of time (once a second, once a minute e.c.t.) for the time being I will attempt to use the once a minute interrupt to see if this noticeably impacts performance for users. That throws as many 'this is going to cause more work in the future' flags as I have seen but oh well....