Low frequency audio level detector using PIC ADC?

[HankMcSpank]

The confusion comes from my exclusion. When I am talking about number of instructions, I mean to say ASM instructions. This just shows the beauty of PBP. I prolly should use the term fosc/4, but I have always just thought about it as instructions. BTW, some take 1(fosc/4) and some take 2. GOTO (in ASM) takes 2 while BCF (clears a bit) takes 1

thanks bert...ok, so taking PR2 = 8 yields an interrupt toggle of 22khz, therefor 44khz (I took PR2 down to 6, but the interrupt frequency on my scope stayed the same?!)

I'm thinking about putting the 'capacitor discharge' emulation into the same interrupt, so something high level like this...

(I will use a variable called previous_sample to store highest ADC sample)

1. does present incoming ADC sample equal (or is greater than) previous_sample.

i)if so previous_sample = present incoming sample .....& retrun

ii) if not, decrement previous_sample by '1' & return (the 1 here can be altered depending on how fast you want the cap emulation to dischare)

with an ADC sample of 8 bits, the maximum signal sample is 255.....the maximum pseudo discharge time will be the interrupt rate 22.27uS (44khz) * 255, therefore about 6ms (which is nicely tied in with the lowest frequency 'half cycle' on a guitar @82Hz)....sound about right?

[cncmachineguy]

thanks bert...ok, so taking PR2 = 8 yields an interrupt toggle of 22khz (I took PR2 down to 6, but the interrupt frequency on my scope stayed the same so it looks like I'll have to stick with that?!)

This doesn't sound to good. lets see if it makes sense: PR2=8 means every time TMR2 counts to 8, an interrupt should fire and TMR2 gets reset. with PR2=66, you had ~30K. So remember the main thats approx 7 clock ticks(clock ticks = instructions, so as not to continue confusing everyone). The math remains the same, for TMR to count to 8 thats 4uSec! 1/.000004 = 250K, not 22K. So my guess is you are going too fast. what was mains frequency? start back at PR2=66 and increase it. see if the INT gets slower, until it is 20K. At PR2=200, I think you were at 15K? so just guessing, maybe a value of ~130?

Now about that ADC time. I haven't read the errata, but did it say not to run faster then 8mHz or not to run the ADC faster? I ask because I saw something about the speed of the ADC implying there is a prescaler for that so maybe you could run at 32mHz?

I'm thinking about putting the 'capacitor discharge' emulation into the same interrupt, so something high level like this...

(I will use a variable called previous_sample to store highest ADC sample)

1. does present incoming ADC sample equal (or is greater than) previous_sample.

i)if so previous_sample = present incoming sample .....& retrun

ii) if not, decrement previous_sample by '1' & return

with an ADC sample of 8 bits, the maximum signal sample is 255.....the maximum pseudo discharge time will be the interrupt rate (22khz) * 255, therefore about 12ms (which is nicely tied in with the lowest frequency 'cycle' on a guitar @82Hz)....sound about right?

I don't know much about this, I will have to think on it tonight when I get home from work.

[HankMcSpank]

thanks bert...slight oversight in my previous (noe edited)...the toggle frequency on my scope is HALF the interrupt frequency!

Re the pseudo cap - a peak detect cicuit (the end goal here)....it's basically a diode followed by a cap...when the incoming ac signal is higher in magnitude in voltage than that across the cap the diode allows it through...and the cap's voltage increases......when the incoming signal is lower the diode switches off....the cap holds the voltage, but for this circuit to be useful, we actually add in a resistor to discharge it at the designer's choice.....hence wanting to decrement the ADCsample variable when the incoming signal is lower.

http://hades.mech.northwestern.edu/i...#Peak_Detector

[HankMcSpank]

ok, despite far too loong trying, i can't get into the interrupt, do my stuff & get out fast enough (for the sample rate I need)....so i'm officially parking the interrupt method of taking ADC samples at a high frequency.

So back to what languer recommended to me earlier - using the special event trigger to do ADC in background hw.

this from the data sheet (http://ww1.microchip.com/downloads/e...Doc/41419A.pdf page 216 23.2.4 SPECIAL EVENT TRIGGER )...

"When Special Event Trigger mode is chosen(CCPxM<3:0> = 1011), the CCPx module does the following:
• Resets Timer1
• Starts an ADC conversion if ADC is enabled
The CCPx module does not assert control of the CCPxpin in this mode.
The Special Event Trigger output of the CCP occursimmediately upon a match between the TMR1H,TMR1L register pair and the CCPRxH, CCPRxL register pair. The TMR1H, TMR1L register pair is not reset until the next rising edge of the Timer1 clock. The Special
Event Trigger output starts an A/D conversion (if the A/D module is enabled). This allows the CCPRxH, CCPRxL register pair to effectively provide a 16-bit programmable
period register for Timer1."

After an hour of dicking about - I've got it working but would like confirmation how to set the sampling frequency - So I'm back to maths again!

Ok, with an 8Mhz clock, I now know that we get 2MIPs.....therefore will the Timer1 increment with each instruction clock cycle? so - 1/2,000000? ...that's every 500ns?). If so, to get a sampling frequency of say 40khz, this would mean an ADC sample needed every 25uS, therefore every (25us/500ns) 50 instruction cycles. is it just a matter then of setting CCPR4H = 0 and CCPR4H = 50 for the Timer1H & Timer1L registers to match ???

[cncmachineguy]

YES, See how easy it is

[HankMcSpank]

YES, See how easy it is

thanks for bearing with me Bert ...it seems to be working a treat - ie Im feeding an AC signal of 5V peak to peak into an analogue pin...and the PIC is extracting the signal's peak level in real time.....just like a hardware peak detector (but better as the 'charge time' is essentially instant, and the 'discharge time' linear & changeable on the fly).

FWIW, I'm also reversing the ADC reading from the negative half of the incoming AC signal (ie readings below 128) to give a smoother overall reading (essentially full wave rectification in software)

the special event trigger was absolutely key in getting this to work without using interrupts ....a hearty thanks to languer too :-)

[Sneaky-geek]

Hi Hank,
That is great that you got it to work! Is there a chance that you could post a working version in the code examples section for the rest of us? It is still a little foggy to me.......

Thank you,
Sneaky-geek