The 60hz mains are accurate only over long durations, like a day or
so. From what I've read, they adjust the frequency occasionally to
make the total number of cycles accurate over time, but on a moment to
moment basis they may be off.
Crystals are usually pretty precise (compared to
Randall -
On Fri, May 02, 2008 at 12:33:03PM -0400, Randall Nortman wrote:
Just a quick non-gEDA design question -- I have the choice between
using the zero crossings of the 60Hz mains voltage or my MCU clock
(generated from an 18.432MHz quartz crystal producing a 48MHz CPU
clock via PLL
At 12:33 PM 5/2/2008, you wrote:
Just a quick non-gEDA design question -- I have the choice between
using the zero crossings of the 60Hz mains voltage or my MCU clock
(generated from an 18.432MHz quartz crystal producing a 48MHz CPU
clock via PLL built into the MCU) for low-resolution timing. The
-
From: [EMAIL PROTECTED]
[mailto:[EMAIL PROTECTED] On Behalf Of Randall Nortman
Sent: Friday, May 02, 2008 12:33 PM
To: gEDA user mailing list
Subject: gEDA-user: Using 60Hz mains frequency for timing?
Just a quick non-gEDA design question -- I have the choice
between using the zero
Just a quick non-gEDA design question -- I have the choice between
using the zero crossings of the 60Hz mains voltage or my MCU clock
(generated from an 18.432MHz quartz crystal producing a 48MHz CPU
clock via PLL built into the MCU) for low-resolution timing. The
crystal is not designed as a
Crystal usually cut to + or - 100 ppm for a general use like a CPU and it
will not change too much with temperature and age. Ethernet crystals were
at one time cut to a better spec 50 ppm. Special communications crystal can
be a lot better. The mains are very good in most places for the morning
On Fri, May 02, 2008 at 01:06:47PM -0400, Ian Chapman wrote:
Crystal usually cut to + or - 100 ppm for a general use like a CPU and it
will not change too much with temperature and age. Ethernet crystals were
at one time cut to a better spec 50 ppm. Special communications crystal can
be a
Just a quick non-gEDA design question -- I have the choice between
using the zero crossings of the 60Hz mains voltage or my MCU clock
(generated from an 18.432MHz quartz crystal producing a 48MHz CPU
clock via PLL built into the MCU) for low-resolution timing.
It depends. :-)
Either one is
Thanks very much to all who responded... and so quickly. I should
have included more information on the application -- This is for
building a power meter (watts and watt-hr) for measuring power
consumption of particular circuits as well as the whole house (but not
for billing purposes -- just
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