Low frequency audio level detector using PIC ADC?

[HankMcSpank]

Excellent.

With PBP and main loop at 1K, what was ISR frequency?

On my scope, with main loop @1Khz, the ISR frequency shows as 5.98Khz (therefore 12Khz)

[cncmachineguy]

Ok, here's what we know. Type=ASM, we get the timing we expect. Now if you lower PR until the main loop frequency starts to fall off, we will know the max rate for a clean interrupt. Well actually you can tell by the "choppy" main signal. Is each cycle choppy, every other 1, every 3rd? If the main loop takes 7 instructions, then we would expect it to occur 66/7 times before interruption. Well minus the time the ISR takes. Using PR with TMR2, the clock keeps ticking. So you get a more stable interrupt as the ISR also adds to the count. Unless it is turned off and on.

Thats enough for now, I may be rambling.

[HankMcSpank]

Hi Bert, no you're not rambling...this is a good stuff for those of us, that aren't used to getting deep down 'n dirty with instructions/clock timings (that'll be me)....I'll have a tweak with the PR.

That said, I'm travelling to visit relatives in the North of England tonight..so no more updates until I can get back & fire the PIC up (Monday night) - but many thanks for your input thus far.

[cncmachineguy]

Have a fun and safe trip Hank

[HankMcSpank]

Ok, here's what I see with a PR value of 200....

(top green trace is main loop 'toggle', the lower yellow trace is interrupt 'toggle ....oh btw, ignore the spikes sitting on the square waves!)



now here it is with a PR value of 66....



Now, whilst I can understand the need to get a clean main loop (& establish what PR2 value yields that), I'm wondering here what's going on with the PR2 value ....does higher mean less interrupts? (ie should I be taking the PR2 value higher or lower)...cos whichever way I quickly experimented with I got an uneven main toggle trace!

oh & btw, how did you determine the port toggle took 7 instruction cycles a couple of posts ago?!!


I've pondered the end goal a little bit - an anlogue peak detect circuit emulator.

What's going on with a peak detector is that a cap charges up to reflect the value an AC signal....a diode stops the cap charge decrementing when the waveform traverses back towards zero - meanwhile there's another resitor in parallel across the cap that disharges the cap (at a t rate chosen by the circuit designer).

So I guess I need some code like this...

1. Take an ADC Sample (via a timer ...or special event trigger etc)
2. Is the present signal sample bigger than the last? ......if yes, then update a variable with the latest value, if not then do nothing.

Have another timer decrement the same variable (this is the discharge time)


Ought to be simple enough (once I get a grip on this interrupt frequency melarkey!).....and would stop the strong second harmonic presence messing up the overall results as seen when I put a guitar into the circuit.

[cncmachineguy]

Ok hank, since you choose to swallow the red pill, here goes:

PR works opposite from the timer's. Timers ALWAYS count up and generate interrupt (if enabled) on overflow, So for a given count, we preload the timer to start somewhere other then zero. PR on the other hand, is a match for the timer. So if PR=200, the timer will start counting (from zero). When it gets to 200 (TMR = PR), timer is reset and starts again. If there is no PR postscaler, interrupt will be set.

So a higher PR will yeild less interrupts. I would like to take back my statement about a "clean" main. You can NEVER acheive that in the literial sense if using interrupts by definiation it will be interrupted. On the other hand, lets say the main is 7 instructions, you would never expect main toggle to be less then 1/7th of its uninterupted rate. Do you see why?

Make sense?

How did I know 7 insctructions? Its just math. you are running a 8 Mhz clock. that becomes 2MIPS. (2 million insctructions per second). 1 instruction takes .0000005 sec * 7 = .0000035 seconds. 1/.0000035 = ~285KHz. Ok really this is the proof, but go backwards to get the number of instructions.

[HankMcSpank]

Ok, yes, I'm getting there (thanks), but I come back to how did you know the main took 7 instructions, once again, here's the main...

BMAIN:  'make this your new temporary main for this test
 toggle portc.4   'pin 6   (green scope trace)
GOTO BMAIN

So, to my eyes, there are two instructiions in there ...the toggle & the goto ...do these add upto to the aforementioned 7?