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I have added a couple of accumulators and counters to the interrupt routine.
I put in two 24 bit accumulators. Every time I read an A/D I add it to one of the accumulators and count. When I have taken 148 samples (74 in each accumulator) I transfer the totals to memory, reset the accumulators, set a flag, and count 60ths of a second down to 1 sec.
The Basic program running in the background sends the data to a serial port so I can see it on my PC. It cannot send all the data it sees so it just sends one set of data per second. That means that 59 samples out of 60 are being discarded for every one that is displayed. I can see about 30 seconds worth of data on the screen. There are two columns of numbers one column reads about 16420 and the other is 7880.
If you take the square root of the sum of the squares of those two numbers, then divide by 74, and multiply by 5 volts/1024 you get the peak voltage of the signal I am reading. Double that for the peak to peak.
If you take the arctan(7880/16420) you get about 26 degrees. At 152 usec per cycle that is 11 usec. That means that the A/D conversion is beginning about 11 usec after the interrupt.
When I first set this up I was seeing sudden jumps of over 100 in the data, but that turned out to be because of a timer that Basic has running. It is a low priority interrupt but apparently it keeps all other interrupts masked until it finishes. So that was delaying my interrupts and causing spikes in the samples. After disabling that clock, the noise dropped by more than a factor of ten. Now I typically see peak to peak noise of 10 or less. That is in a sum of 74 samples so reflecting it back to A/D I am seeing 10/74 or about 1/8 of the LSB. This is better than I expected. Also, if I put a target near the coil and move it by small amounts I can see the average values change by about 10. So I am also seeing a resolution of about 1/8 LSB. Though I may find voltage ranges where it wants to jump in larger increments than that.
The interrupt routine now takes about 8 usec for no A/D, 13 usec with an A/D and a maximum of 19 usec every 60th of a sec when it stores the accumulated data. So it looks like there is still room in there for a filter. With the clean signal that I am reading now, the filter is not necessary. But when I start reading real data from the receive winding I expect it to be a lot noisier.
Robert Hoolko
I put in two 24 bit accumulators. Every time I read an A/D I add it to one of the accumulators and count. When I have taken 148 samples (74 in each accumulator) I transfer the totals to memory, reset the accumulators, set a flag, and count 60ths of a second down to 1 sec.
The Basic program running in the background sends the data to a serial port so I can see it on my PC. It cannot send all the data it sees so it just sends one set of data per second. That means that 59 samples out of 60 are being discarded for every one that is displayed. I can see about 30 seconds worth of data on the screen. There are two columns of numbers one column reads about 16420 and the other is 7880.
If you take the square root of the sum of the squares of those two numbers, then divide by 74, and multiply by 5 volts/1024 you get the peak voltage of the signal I am reading. Double that for the peak to peak.
If you take the arctan(7880/16420) you get about 26 degrees. At 152 usec per cycle that is 11 usec. That means that the A/D conversion is beginning about 11 usec after the interrupt.
When I first set this up I was seeing sudden jumps of over 100 in the data, but that turned out to be because of a timer that Basic has running. It is a low priority interrupt but apparently it keeps all other interrupts masked until it finishes. So that was delaying my interrupts and causing spikes in the samples. After disabling that clock, the noise dropped by more than a factor of ten. Now I typically see peak to peak noise of 10 or less. That is in a sum of 74 samples so reflecting it back to A/D I am seeing 10/74 or about 1/8 of the LSB. This is better than I expected. Also, if I put a target near the coil and move it by small amounts I can see the average values change by about 10. So I am also seeing a resolution of about 1/8 LSB. Though I may find voltage ranges where it wants to jump in larger increments than that.
The interrupt routine now takes about 8 usec for no A/D, 13 usec with an A/D and a maximum of 19 usec every 60th of a sec when it stores the accumulated data. So it looks like there is still room in there for a filter. With the clean signal that I am reading now, the filter is not necessary. But when I start reading real data from the receive winding I expect it to be a lot noisier.
Robert Hoolko