Hi, I’m developing a system basen on FreeRTOS and schedulling periodic task with software timers. I have a tick off 100Hz generated with a 8MHz on the LIRC. The timers have a period of 30s and a total count of 15 mins for resume a task, but every 15 minutes it delays 4 more seconds mor or less. How could I increase the precision of the timer?
Four seconds in fifteen minutes would be a 0.4% inaccuracy. How accurate is the LIRC? You have not said which chip you are using or what the LIRC is – but some low power clocks or clocks generated internally in the MCU rather than using a precision crystal are comparatively inaccurate. Have you measured the tick frequency with a scope or frequency meter to see how close to 100Hz it is?
Hi, thank you for your reply The MCU It’s the MKL43Z256VLH4 of NXP, In the data sheet saids that the IRLC has a 3% acuracy. The tick has a frecuency of 99.717 measured with a scope.
In which case it looks like you are actually exceeding the accuracy requirements of the chip ;o)
It’s seems, so do you think to reach the acuracy I need trough the 15 minutes, an RTC based on a 32.768kHz crystal is needed?
I don’t think the resolution of such a slow crystal would give you exactly the tick frequency you need. If you need high accuracy, you will need a more expensive crystal.
just to be clear, it sounds like you are saying that you are geneating a 15 minute delay by counting 30 30 second delays, and are finding the timing to be off by 4 second in 15 minutes (a 0.4% error) but your timer is specifiec at only 3%. A 32,768 Hz crystal would have much better accuracy, but being an ‘odd’ frequency, you likely would want to set the dividers to generate something like a 128 Hz tick (and only do conversions using the tick-rate-hz value and not ms per tick which will be rounded. If you dropped the tick rate to 64 Hz, then you could ask for a 900 second delay as 57,600 ticks which is within a 16 bit number for ticks,
KL43 : the reference says that the internal 48MHz reference clock has a precision of 0.5%, which is better than the 3 % you have now. I think it is possible to connect an external crystal to get a better precision. The precision of crystals is expressed in PPM ( Parts Per Million ). A PPM of 10 or lower is good, but that may still deviate up to 5 minutes per year. The difficulty is that the actual frequency depends on the temperature. I sometimes use an external 32,768 Hz crystal just to feed an RTC, which keeps track of date and time. A frequency of 32768 is odd indeed, you can not create a 100 or a 1000 Hz clock. For an RTC, it will normally be divided by 32768 to get a 1 Hz clock: a second counter.
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