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24th January 2020, 06:26 #1
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Freq computation without DIV instruction for 8bit MCU
I am using 8bit MCU.
Configuration: Timer Capture to measure pulse width.
S/W Computation:  Using pulse width calculate Frequency using formula  (Clock frequency/pulse width) << 2
Problem: The division instruction is taking 40usec which is too much overhead for the kind of application I am working on.
Probable Remedy: I was thinking to work out any alternative where I should not use DIV instruction and yet should be able to compute the frequency.
Mandatory/What can't be changed: MCU and timer capture is fixed. the clock source can be modified with prescalar.
Any help?

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24th January 2020, 08:11 #2
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Re: Freq computation without DIV instruction for 8bit MCU
Hi,
40us?? Why that much time?
Floating point? > try integer solutions (what input values range, what expected calculation resolution and accuracy?)
What microcontroller, what clock frequency, what language, libraries and compiler do you use?
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24th January 2020, 11:01 #3
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Re: Freq computation without DIV instruction for 8bit MCU
Consider creating a lookup table. Say you want two digits of precision... Then you need an array of 100 elements. Exponents are a simple matter of reversing the sign (positive to negative, negative to positive).
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24th January 2020, 13:14 #4
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24th January 2020, 13:15 #5
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Re: Freq computation without DIV instruction for 8bit MCU
Thought of doing that too.... but since it is low end 8bit MCU... the available RAM is only 2K... I can't afford to use LUT.

24th January 2020, 20:28 #6
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Re: Freq computation without DIV instruction for 8bit MCU
Hi,
LUT does not neccessarily be in SRAM.
But an integer division should be fast. But without ranges...accuracy, resolution...
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25th January 2020, 01:48 #7
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Re: Freq computation without DIV instruction for 8bit MCU
Mandatory/What can't be changed: MCU and timer capture is fixed. the clock source can be modified with prescalar.
For example, by setting timer period as 1,024s, the division operation can be made by rightshifting 10 digits, so if e.g 512 pulses are accounted during this time, the computed frequency is 500Hz with a reasonable resolution.
Needless to say that a timer with the largest magnitude should be choosen; many 8 bit MCU's have either 8 and 16 bits timers available.
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25th January 2020, 11:49 #8
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Re: Freq computation without DIV instruction for 8bit MCU
For example, by setting timer period as 1,024s, the division operation can be made by rightshifting 10 digits
Although the specification is incomplete (didn't mention measurement rate, resolution, minimal input frequency), I presume long unsigned (uint32) arithmetic is used here. Thus tables are hardly an option. However, if you choose a rate equal to the maximum input frequency of 200 Hz, what's the problem with spending 40 µs for the calculation?
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