How to get a millisecond precision uptime from user-space in Linux?

Question

I'm working on a Raspberry Pi based project that has a GPS module which my boss wants me to get the time from for the system clock. However we also need to take readings on different sensors whilst the GPS may not have a fix, and we need to know to the millisecond precision (tolerance of 50-100ms is fine) when these readings were taken.

Personally I want a hardware RTC for this, but I've been instructed to work around it. My idea is to mark each reading with a relative time from system boot, the system time is not reliable, and is updated by NTP/Satellite time when available (I can then fix-up the records when a synchronized time is available using the relative time).

So, how can I get a millisecond precise uptime in Linux from user-space C code? Something like the jiffies value available in the kernel would be perfect.

Answer 1

I think you have to check the main controller(CPU) on your board. Usually, there will be a hardware timer module integrated into the CPU, or decrementer register implementation in the CPU core.

If there is a hardware timer or DEC register on your CPU, then use it to implement a periodical interrupt(the frequency can be 1000HZ or else). The interrupt handler can notify/wakeup the user-space process to do the necessary real-time work.