3 ms·
Well-written article. But professionally, although most Real Time Clock module does generate interrupts, it's misleading to call this simple circuit a "Real Ti
by segfaultbuserr 7y ago
Well-written article.
But professionally, although most Real Time Clock module does generate interrupts, it's misleading to call this simple circuit a "Real Time Clock", there is no "Real Time". It's better to use the term "counter", or "timer".
- lisper 7y ago"The 25c clock" seemed like a snappier title than "A simple hack that lets an Apple II keep track of real time (while it's powered on) and do some basic multitasking" even though that would have been more accurate.
- NikkiA 7y agoGiven that it's a second off after every 16 seconds (interrupt tate: 59.939Hz, assumed rate: 60Hz), it's not even really keeping track of time while powered on.
- lisper 7y agoYou need to check your math. It loses about 1.5 minutes per day on a naive implementation. Very few people left their computer running all the time in those days. (Also, it's pretty to fix the problem in software.)
- NikkiA 7y agoYou're right that my math is off, it loses 0.06s every minute, so a second every 16 minutes, still not 'real time'.
- mmjaa 7y agoIn those days, having a clocking mechanism which produced consistent time - i.e. a monotonic clock - was of quite high value. On boot up, it was typical to ask the user what time it was, and continue with the program. But then how do you measure time since boot? With simple hacks like this of course. Also, this hack allowed a form of multi-tasking, an 8-bit feature quite well targeted back then. Consistency was key in such clocks, not necessarily accuracy. The fact of the drift is not important, since it was consistent and could be accomodated for in software - as many peripherals of the day, indeed, required.