Path: csiph.com!v102.xanadu-bbs.net!xanadu-bbs.net!news.glorb.com!usenet.killfile.org!.POSTED!not-for-mail
From: c28f62@TheWorld.com (Mark Kramer)
Newsgroups: comp.os.linux.misc,comp.sys.raspberry-pi
Subject: Re: Keeping 'order' without RTC.
Date: Tue, 12 May 2015 06:09:08 +0000 (UTC)
Organization: The World : www.TheWorld.com : Since 1989
Lines: 50
Message-ID: <mis5e4$56e$1@flea.killfile.org>
References: <mcsbdn$uh8$1@dont-email.me> <alpine.LNX.2.02.1504161412190.18103@darkstar.example.org> <mi6pvj$hsu$1@flea.killfile.org> <mi6tei$qmd$1@dont-email.me>
NNTP-Posting-Host: shell01.theworld.com
X-Trace: flea.killfile.org 1431410948 5326 192.74.137.71 (12 May 2015 06:09:08 GMT)
X-Complaints-To: news@usenet.killfile.org
X-Newsreader: trn 4.0-test72 (19 April 1999)
Xref: csiph.com comp.os.linux.misc:14762

In article <mi6tei$qmd$1@dont-email.me>,
William Unruh  <unruh@invalid.ca> wrote:
>On 2015-05-04, Mark Kramer <c28f62@TheWorld.com> wrote:
>> The software clock is not an RTC. It counts interrupts. Yes, you can
>> decode that count to a wallclock time, but that extra step is an extra
>> step for a true RTC.
>
>Agreed. It counts "seconds" (well after it has been calibrated) 

No, it counts interrupts. The frequency of interrupts depends on the
hardware. For old DOS on a PC, it was 18.2Hz. For some linuxes I've used,
it is 1kHz. Some were, as I recall, 100Hz. 

>but
>those are at best "seconds since the machine has been turned on". 

Since that counter can be set to any value, and is set by the time-setting
commands, no, it isn't "at best" a simple counter of power-on ticks, it is
"at best" a counter of some multiple of seconds since 1 Jan 1970 0000 GMT.

>To
>have it count useful seconds (eg seconds since Jan 1 1970) you need to
>feed it the Real Time at one of those seconds. Taht is the job of the
>Real Time Clock. 

Which the Pi does not have unless you've added that hardware. Otherwise, 
it is the job of the user, or of the NTP demon. 

>Note that the Real Time Clock also counts seconds. 

I've had an RTC that did quite well at counting 0.01 seconds. None of
them I know actually count seconds. They count to 60 and then go back
to 0 while incrementing a counter called "minutes". A 32.something kHz
crystal is very common. 

>Seconds since epoch IS a "real time" Seconds since the computer was
>switched on is not. 

Seconds since epoch is not "real time". It is a count of seconds since an
arbitrary point in time. 

>> On the other hand, the true RTC devices do maintain the "real time".
>> You can ask it directly "what hour of the day is it?" because its API
>> returns hour of the day. You don't have to get a 32 or 64 bit integer and 
>
>Actually intrnally it too just keeps count of seconds. 

No. It keeps count of seconds of the minute, minute of the hour, hour
of the day, etc...