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Forums > C64 Coding > Excact Vertical Frequency / Refresh Rate
2017-07-18 23:22
TWW

Registered: Jul 2009
Posts: 396
Excact Vertical Frequency / Refresh Rate

I am making a clock by using a CIA timer. Basically I calculate how many cycles are executed each second by calculating lines x cycles/line x vertical refresh rate.

I then want to make the clock run correct on all systems (NTSC/PAL/DREAN/NTSC_Old) and in this context, I was wondering what is the EXACT vertical refresh rates for these systems (50 and 60 plus 3 decimals or more)?

(I've found conflicting information around the web so that's why I'm asking)
 
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2017-07-21 09:19
Style

Registered: Jun 2004
Posts: 479
Quote: ehrm, no they are not. please read up on the topic. they are putting a lot of effort into keeping all grids in sync and keeping the frequency stable.

edit: to make it clear.... shorttime this is correct, the frequency may drift up and down quite a bit, sth like +/- 1% or so at max (it usually does so during the day - you can use the mentioned test program to examine this). however, the number of periods for long time periods (a weeek, a month... i dont remember what is guaranteed) is very accurate, and certainly ok to use for a wall clock.


Dude. I work in power generation :)
2017-07-21 14:20
Groepaz

Registered: Dec 2001
Posts: 8120
burning speccies i hope :)

edit: care to give the exact numbers then? IIRC its worse in north america than in central europe (and its really terrible in other, less developed areas, i guess)
2017-07-21 22:52
JackAsser

Registered: Jun 2002
Posts: 1226
Quote: burning speccies i hope :)

edit: care to give the exact numbers then? IIRC its worse in north america than in central europe (and its really terrible in other, less developed areas, i guess)


There are real time data for it: UK f.e. http://www2.nationalgrid.com/uk/Industry-information/electricit..
2017-07-21 23:25
Groepaz

Registered: Dec 2001
Posts: 8120
yeah i know these sites (there are quite a bunch of them) ... i was talking about the guaranteed maximum deviation etc
2017-07-21 23:54
JackAsser

Registered: Jun 2002
Posts: 1226
Quote: yeah i know these sites (there are quite a bunch of them) ... i was talking about the guaranteed maximum deviation etc

Some info regarding central europe with allowed deviations: http://www.mainsfrequency.com/frequ_info_en.htm
2017-07-22 00:09
lft

Registered: Jul 2007
Posts: 266
Quoting Groepaz
trying to confuse the centiseconds counter like you said SHOULD work imho ... kinda like opening the borders =) it should count to $f then and wrap around. make a test program and have a look (it will break on every emulator right now, i guess ....)


I've been thinking of opening the TOD border too, but I assumed it was a 3-bit counter. It only needs to count to 5 or 6 before incrementing the decisecond register.

But who knows; perhaps it is an LFSR?

If this trick works, one could use it repeatedly to make the clock go faster or slower. Totally pointless, but maybe a fun exercise.
2017-07-22 01:11
Groepaz

Registered: Dec 2001
Posts: 8120
jusdging from my observations (and existing test programs) all the counters are simple binary counters with a binary compare for the proper wrap around (if you initialize them with "invalid" values the will happily count further until they overflow)
2017-07-22 18:30
Style

Registered: Jun 2004
Posts: 479
Quote: burning speccies i hope :)

edit: care to give the exact numbers then? IIRC its worse in north america than in central europe (and its really terrible in other, less developed areas, i guess)


50Hz will quite often drop to 49.5 or so with a sizeable forced outage here. And we have one of the most secure networks in the world, since my state decided to operate a capacity market (as opposed to energy only).

Due to frequency response it's ok, but I certainly wouldnt do any form of timing with it.
2017-07-22 22:45
Groepaz

Registered: Dec 2001
Posts: 8120
Quote:
50Hz will quite often drop to 49.5

wow, thats quite a bit worse than it is in europe then :)
2017-07-23 01:21
MagerValp

Registered: Dec 2001
Posts: 879
Occasionally being off by 1%, but good most of the time - I bet that's still more stable than most 30 year old crystals if you account for different units and temperature ranges.
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