Having a sidereal clock is handy thing for astronomical observation / tracking, as the the 24 hr sidereal clock follows the stars positions.
I came across this great resource for interesting time things, one being an atomic clock wrist watch
Other than that here is a some useful programs for pic12F675's to get some more use out of your 10MHz oscillators:
http://leapsecond.com/pic/
pd29.asm got my attention with: "PIC 10 MHz to best sidereal 1PPS, with sync and milli-step"
from the pd29.asm:
very clever.
here are the other dividers: other src and hex
I came across this great resource for interesting time things, one being an atomic clock wrist watch
Other than that here is a some useful programs for pic12F675's to get some more use out of your 10MHz oscillators:
http://leapsecond.com/pic/
pd29.asm got my attention with: "PIC 10 MHz to best sidereal 1PPS, with sync and milli-step"
from the pd29.asm:
Code:
; ------------------------------------------------------------------------
;
; Title:
;
; PD29 -- PIC 10 MHz to sidereal 1PPS divider, with sync and milli-step
;
; Function:
;
; This PIC program implements a digital frequency divider: the external
; 10 MHz input clock is divided by nearly ten million to produce a 1PPS
; output that very closely approximates sidereal time.
;
; - The 1PPS pulse width is 10 ms.
;
; - Two Step inputs are checked once a second:
; to advance 1PPS by 1 ms, hold StepA low
; to retard 1PPS by 10 ms, hold StepB low
; to advance 1PPS by 100 ms, hold both StepA and StepB low
;
; - Two inputs support optional manual 1PPS synchronization. Pull Arm
; pin low for a second to stop divider. The output will synchronize
; to next rising edge of Sync pin (within one instruction cycle).
;
; Diagram:
; ---__---
; 5V (Vdd) +++++|1 8|===== Ground (Vss)
; 10 MHz clock in ---->|2 pD 7|<+--- StepA
; 1PPS out <----|3 29 6|<+--- StepB
; Arm o--->|4 5|<+--- Sync
; --------
; Notes:
;
; o External pull-up required on Arm input (pin4/GP3).
; + Step and Sync inputs have internal WPU.
; Output drive current is 25 mA maximum per pin.
; Coded for Microchip 12F675 but any '609 '615 '629 '635 '675 '683 works.
;
; Theory:
;
; A sidereal day is approximately 23 hours, 56 minutes, 4.0916 seconds
; long (86164.0916 seconds). Thus sidereal clocks run fast compared to
; conventional clocks (based on solar time). Sidereal clocks gain a day
; per year (by definition), which is about one second every six minutes.
;
; The rate difference is 86164.0916 / 86400, or 0.9972695787 (2730 ppm).
; One second for a 10 MHz PIC is 2500000 instructions.
; One sidereal second for a 10 MHz PIC is 2493173.946759 instructions.
;
; Rounding up to a whole number of 2493174 instructions per loop means
; the error is 2493174 - 2493173.946759 * 400 ns = 2.13e-8.
; This is 21 ns/second, or 1.8 ms/day, or 0.7 seconds per year, which
; is almost as good as the definition of the sidereal day is known.
;
; To further improve precision, this version implements a dual-modulus
; fractional-N synthesizer technique in order to create the irrational
; 2493173.946759 cycle count. The best (8-bit) rational approximation
; of 0.946759 is 160/169 (= 0.946746).
;
; The loop is made to be 2493173 + 160/169 instruction cycles long.
; Over 169 seconds, 9 loops are 2493173 instructions long and 160
; loops are 2493174 instructions long. The error drops to 5.20e-12.
; This is 5.2 ps/second, or 450 ns/day, or 0.00016 seconds per year,
; which is below the uncertainty in sidereal day length itself.
;
; Version:
;
; 09-Feb-2013 Tom Van Baak (tvb) www.LeapSecond.com/pic
;
; ------------------------------------------------------------------------very clever.
here are the other dividers: other src and hex
