Manual Calibration without NTP

This section describes how to calibrate the clock to an accuracy of order tenths of a second. This procedure was found useful when ntpd was used on a variable speed processor and upset the drift correction.

Linux

The aim is to calibrate the kernel frequency adjustment variable(s) stored in /etc/adjtime. This following is a translation of Laurent Picouleau's page. Many thanks to Mike Cook for the above explanation and supplying this link.

The following steps are to be done as root.

• Day 1
  • Set the CMOS clock to "approximately" UTC using the BIOS (1).
  • Install hwclock and adjtimex
  • Run hwclock --set --date="MM/DD/YY hh:mm:00" --utc (2) --debug on the fourth beep (radio, I presume) (3)
• Day 2
  • Run hwclock --set --date="MM/DD/YY hh:mm:00" --utc --debug
  • If the first value of the first line of /etc/adjtime is greater than 8 then
    • run adjtimex -u -c
    • record the suggested value for "tick"
    • run adjtimex -u -t suggested_value
  • Run adjtimex -u -l=/var/log/clocks.log
• Day 7
  • Run adjtimex -u -l=/var/log/clocks.log
  • Run hwclock --set --date "MM/DD/YY hh:mm:00" --utc --debug
  • Run adjtimex -u -l=/var/log/clocks.log
  • Run hwclock --utc --hctosys
  • Run adjtimex -u -l=/var/log/clocks.log
  • Run adjtimex -u -c=22 -i=100
  • Calculate the average of the 20 values given for "freq"
  • Run adjtimex -u -f calculated_average
  • Run adjtimex -u -l=/var/log/clocks.log

Phew! Your clock system is now calibrated to your reference and will drift by about only one tenth of a second per day.

Footnotes:

1. Or any other method

2. Note care the date format: month/day/year

3. Use local time, don't use GMT.

-- ParkerJones - 25 Nov 2004