Fine-grained clock resolution using low and high frequency clock sources in a low-power system
A periodic output generator has a first clock source coupled to a first counter and a second clock source with a frequency greater than the first clock source, the second clock source coupled to a second counter, the first clock source operating continuously, the second clock source enabled when the first clock source reaches a count C 1 . The second clock source generates an output when a count C 2 is reached, and the counters are reset and the process repeats. In another example, a timestamp generator has a high speed clock and a real time clock operative on a low speed clock. The timestamp generator receives an external event, turns on the high speed clock generator and counts high speed clock cycles C until the arrival of the next time stamp, and computes an event timestamp as the next timestamp less c/f, less the startup time of the high speed clock.
1. An event timestamp generator forming a timestamp from an external event, the event timestamp generator comprising:
a real time clock generating timestamps which are updating at a first time interval T RTC ;
a clock generator enabled by said external event, said clock generator having a frequency f 2 greater than 1/T RTC and a startup time Ts;
a fine interval controller receiving said real time clock input updates and said external event input, said fine interval controller counting a number of high speed clock cycles N 2 from a current timestamp value until a next timestamp TS 2 ;
the fine interval controller thereafter providing an output timestamp computed as TS 2 −Ts−N 2 /f 2 .
2. An event timestamp generator of claim 1 where said clock generator is a ring oscillator or an RC oscillator.
3. An event timestamp generator of claim 1 where said clock generator has a frequency which is at least 100 times the rate of updates of the timestamp generated by the real time clock.
4. An event timestamp generator of claim 1 where said Ts and f 2 are periodically calibrated using a real time clock interval.
5. An event timestamp generator of claim 1 where said Ts and said f 2 are calibrated by counting the number of f 2 clock cycles in a timestamp interval when the high speed clock is first enabled and counting the number of f 2 clock cycles in a timestamp interval after the high speed clock is enabled.
6. A method for determining a timestamp from an external event, the method operative on a controller, the method comprising:
updating a timestamp at periodic intervals T ST from a current value TS 1 to a next value TS 2 ;
upon receipt of an external event, applying power to a high speed clock having a startup time Ts and a frequency Fhs, where Fhs>1/T ST ;
counting n high speed clock cycles until the next time value TS 2 ;
outputting an external event timestamp computed from TS 2 −Ts−n*1/Fhs.
7. The method of claim 6 where the external event comprises an electrical input to the controller.
8. The method of claim 6 where the external event timestamp is output on an electrical interface.
9. The method of claim 6 where the updating of the timestamp is derived from a counter operative with a ring oscillator or RC oscillator.
10. The method of claim 6 where the high speed clock is powered down until the external event is detected.
11. The method of claim 6 where the high speed clock frequency Fhs is at least 100 times greater than the timestamp rate 1/T ST .
12. The method of claim 6 where the timestamp update rate is periodically calibrated by counting the number of high speed clock cycles during a timestamp update cycle.
13. The method of claim 6 where a clock source for the high speed clock has a greater precision than a clock source for the timestamp update.