02-12-2021, 10:27 AM
GPS is a bad idea for anything not needing navigation. It needs reception and is vulnerable. It's stupidly used for loads of commercial applications simply to save less €200 on a €10K+ system so next decent Solar Flare like in 19th C there will be no Mobile, DTT or DAB most places.
NTP is a bad idea except to update occasionally as it needs the Internet, and thus has at least two vulnerabilities.
There are also UK & German LW radio time signals. Reception of the UK one is erratic here and can be blocked by an LED lamp or other SMPSU.
Local Time sources, High to Low accuracy:
Atomic Clock (now affordable)
Oven crystal based clock (as low as €100). Typically large 5MHz or 10MHz module
TCXO module (as low as €5) Now nearly as good as older ovens. Can be as small as a regular crystal.
Trimmed 32.768 kHz tuning fork crystal on your wrist (Good watches are trimmed for just below body heat)
Trimmed 32.768 kHz on a cpu with no temperature stability, assuming ambient is indoors.
Trimmed 32.768 kHz on a cpu with no temperature stability, assuming outdoors.
Untrimmed CPU crystal of any type depends on temperature for stability but frequency could be significantly out.
A PIC micro with an adjustable C on the crystal is quite good. See JAL and JAL libraries. Or for HW use a £2.50 quartz alarm clock. The actuator pulses are 1 Hz. Add HW divider, or as mentioned use the trick of using the minute hand directly. The alarm uses a contact on a disc moved by alarm setting knob.
NTP is a bad idea except to update occasionally as it needs the Internet, and thus has at least two vulnerabilities.
There are also UK & German LW radio time signals. Reception of the UK one is erratic here and can be blocked by an LED lamp or other SMPSU.
Local Time sources, High to Low accuracy:
Atomic Clock (now affordable)
Oven crystal based clock (as low as €100). Typically large 5MHz or 10MHz module
TCXO module (as low as €5) Now nearly as good as older ovens. Can be as small as a regular crystal.
Trimmed 32.768 kHz tuning fork crystal on your wrist (Good watches are trimmed for just below body heat)
Trimmed 32.768 kHz on a cpu with no temperature stability, assuming ambient is indoors.
Trimmed 32.768 kHz on a cpu with no temperature stability, assuming outdoors.
Untrimmed CPU crystal of any type depends on temperature for stability but frequency could be significantly out.
A PIC micro with an adjustable C on the crystal is quite good. See JAL and JAL libraries. Or for HW use a £2.50 quartz alarm clock. The actuator pulses are 1 Hz. Add HW divider, or as mentioned use the trick of using the minute hand directly. The alarm uses a contact on a disc moved by alarm setting knob.
