Low frequency audio level detector using PIC ADC?

[HankMcSpank]

Ok, so in the light of your point about taking longer in the interrupt routine, I went back to basics...

ADC_Sample:
@ INT_DISABLE  TMR2_INT     ; enable Timer 0 interrupts
    toggle PortC.3
@ INT_ENABLE  TMR2_INT     ; enable Timer 0 interrupts
@ INT_RETURN

This gives me an interrupt frequency of 11.44khz (ie 5.72khz on the scope)...so my interrupt routine is severely skewing my interrupt! (told you I sucked!)

It's late here in the UK...need to sleep on this one - many thanks for your help.

[HankMcSpank]

Having slept on this....think I'm gonna have to give this one much more thought.

Not the interrupt frequency problem (btw - Bert, I think you were right...put the peak detect code in the main loop!), but I noticed late last night that for the above code 'trapped' the peak of a nice sigen sine wave very well, a real life guitar signal caused problems. Basically, a guitar signal is full of harmonics...so you've not just got a fundamental going on, you've got strong second harmonic immediately after a string has been plucked - these harmonics are lower in signal magnitude & worse still, force the signal to cross zero too - which plays havoc with the larger fundamental peak detect bit.

I guess what I really need is a slick bit of code that acts like a diode with a smoothing cap following on - there's probably a role to play with DTs averaging routines here!

Something along these lines...

1. Is present ADC sample larger than last ...if so then store it (that'll be the diode bit)

2. When cycle crosses zero, then largest sample from the logic above is the peak.

3. Have a rolling average for all the peaks (that'll be the capactor/LPF bit)


One to mull.

[cncmachineguy]

This gives me an interrupt frequency of 11.44khz (ie 5.72khz on the scope)

Hank, this still bothers me.

Can you try this:

BMAIN:  'make this your new temporary main for this test
 LATC = LATC ^ %00001000 ' this assumes you can use portc.4 as an output.
 GOTO BMAIN

 'new ISR
LATC = LATC ^ %00000100 ' this will toggle portc.3

the point here is main will just toggle portc.4 and the ISR will toggle portc.3. When you scope these, we will see the speed at which you are running. We expect main to be ~4 instructions, maybe 6. so that would give C.4 a frequency of .5M (.25M) and the ISR should interupt every 7 or 8th time through main.

[HankMcSpank]

Hi Bert,

Sure, I will certainly try that (it troubles me & I'd like to understand why I'm not getting the interrupt frequency that I should), but it won't be until tonight after the kids have been put to bed! I'm in the UK, so it'll probably be about 5.00pm EST when I have some results.

[languer]

I am no fan of doing the RMS thing on the PIC itself, but in any case you probably be better served by having the CCP module trigger the ADC conversion. That way you get a very precise sampling rate (look at section 15.2.5 SPECIAL EVENT TRIGGER of the datasheet).

[cncmachineguy]

Oh Thats a pretty sneaky way to do it. (BTW, section 16.2.5 not 15.)

@Hank, are you by any chance using this in the same applaction as the phase do-hicky you were working on a little while back? Seems if so, you may already have the parts in place for the trigger.

[HankMcSpank]

languer - tks, I'd actually seen that previously in the datasheet, but couldn't immediately work out how to apply it, then I forgot about it.

Bert - phase project has been parked up.

Meanwhile, this ongoing problem is going to turn out so easy that I'll never be able to show my face on here again.

Ok, the latc didn't work, so I'm using a basic toggle, here the stripped back code...

@ __CONFIG _CONFIG1, _FCMEN_OFF & _FOSC_INTOSC & _WDTE_OFF & _MCLRE_OFF & _CP_OFF & _IESO_OFF & _BOREN_OFF & _PWRTE_OFF & _LVP_OFF & _CLKOUTEN_OFF 
@ __CONFIG _CONFIG2, _LVP_OFF & _PLLEN_OFF


INCLUDE "DT_INTS-14.bas" ' Base Interrupt System
INCLUDE "ReEnterPBP.bas" ' Include if using PBP interrupts

Osccon = %01110010      'Osc 8Mhz

DEFINE  OSC 8
DEFINE  NO_CLRWDT 1   ' PBP doesn't clear WDT automatically

'16F1828 Pin Connections*********************************************
TRISC.4 = 0     '6   RC4      Main Output         
'**********************************************************************
CM1CON0 = 0   ' COMPARATORS OFF
CM2CON0 = 0   ' COMPARATORS OFF

ADCON0 = 0
ADCON1 = 0

ANSELA     = 0          ; All Port A ports to be digital
ANSELB     = 0          ; All Port B ports to be digital
ANSELC     = 0          ; All Port B ports to be digital

txsta = %10100100  'needed to turn on pin 10 as valid HSerout pin. 

WPUA = 0        'weak pullups disabled.
WPUB = 0        'weak pullups disabled.
WPUC = 0        'weak pullups disabled.

'Timer2 Registers:
'Prescaler=1:1; TMR2 PostScaler=1:1; PR2=66 - Freq = 30,303.0303Hz - Period = 33,000 ns
T2CON.6  = 0 ' bits 6-3 Post scaler 1:1 thru 1:16
T2CON.5  = 0
T2CON.4  = 0
T2CON.3  = 0
T2CON.2  = 1 ' Timer2 on bit: 1=Timer2 is on;
T2CON.1  = 0 ' bits 1-0  Prescaler Rate Select bits
T2CON.0  = 0 
PR2 = 66            ' PR2 (Timer2 Match value) 


ASM
INT_LIST  macro ; IntSource,    Label,         Type, ResetFlag?
    INT_Handler   TMR2_INT,  _Bert_ISR,   PBP,  yes
    endm
    INT_CREATE       ; Creates the interrupt processor
ENDASM

@ INT_ENABLE  TMR2_INT     ; enable Timer 0 interrupts
 
;----[Main Program Loop]------------------------------------------------------------------
BMAIN:  'make this your new temporary main for this test
 toggle portc.4   'pin 6   (blue scope trace)
 'LATC = LATC ^ %00001000 ' this assumes you can use portc.4 as an output.

 GOTO BMAIN
 
 '---[TMR2 - interrupt handler]--------------------------------------------------
Bert_ISR:
@ INT_DISABLE  TMR2_INT     ; enable Timer 0 interrupts
    toggle PortC.3   'pin 7 (yellow scope trace)
    'LATC = LATC ^ %00000100 ' this will toggle portc.3

@ INT_ENABLE  TMR2_INT     ; enable Timer 0 interrupts
@ INT_RETURN

end

& here's the result....

green trace is the main loop toggle (port C.4) with a frequency of just 1kHz(!!!) & the yellow trace is the ISR toggle (port C.3)with a frequency that's higher at 12Khz(!!!) , I have not got one clue what is going on here - it feels like 'week one - intro to PICs' all over again!

[cncmachineguy]

Hank, 2 things to try. First turn off use PBP in the int setup. check the output. then change pr=200. no reason for the value, just much larger. You don't need to post the Scope unless you want. I am happy with just the numbers right now.

INT_Handler   TMR2_INT,  _Bert_ISR,   PBP,  yes

becomes

INT_Handler   TMR2_INT,  _Bert_ISR,   ASM,  yes

[HankMcSpank]

Hi Bert,

You're onto something!

Firstly, this doesn't stack up, but when I disable interrupts my Main loop sees a frequency of 142Khz on my scope (therefore double that at 284khz?!)

Ok, now when I enable interrupts with the 'pbp' entry ....much baddo, my main loop toggle shows just 498Hz on my scope (therefore about 1khz)

change the bit you outlined to it so asm, I once again get the main running 142khz (albeit choppy - I guess the interrupts are stopping a nice clean toggle in the main) & the interrupt toggle showing about 15khz on my scope (30khz)

changing PR=200, changes the interrupt toggle rate to about 5khz on my scope (therefore 10khz)

it's after 1.30am here in London.....and so to bed, g'night.