ADC sampling time

[flotulopex]

Hello,

I was wondering why the time I measure(1) for one ADC sampling channel is that much greater than the one stated on the datasheet?

The oscillo sais around 113µs.
<img src="http://www.picbasic.co.uk/forum/attachment.php?attachmentid=2433&stc=1&d=120619236 8">

My PIC has a 4MHz XTal and Fosc is set to 16. Shouldn't I get something like 4µs (green zone)?
<img src="http://www.picbasic.co.uk/forum/attachment.php?attachmentid=2434&stc=1&d=120619166 4">

(1): my oscillo triggers a port that is being set before and cleared after the ADC process. Is it not a valid measurement?

[Acetronics2]

Hi, Lazy Roger

Go on your datasheet reading to $ 9.1.5 ...

Hé, oui ... faut TOUT lire ...

Alain

[dhouston]

From datasheet 9.1.4...

"The time to complete one bit conversion is defined as
TAD. One full 10-bit conversion requires 11 TAD periods
as shown in Figure 9-2."

[flotulopex]

Not always lazy: most of the time, I'm hungry... of knowledge

Okay, looking at tables "17-18" and "17-16" and if my (too) modestly developped brain understands what seems to be so obvious, I would calculate the aquisition time like this:

- aquisition time (param. 132) is 11,5µs;
- converstion time (param. 131) is 11Tad (param. 130) @ 3µs each(full range), makes 33µs.

All in all the total calculated time gives 11,5µs + 33µs = 44,5µs.

I'm still missing something, or at least 113µs - 44,5µs = 68,5µs.

Is my PIC getting as rest???

[skimask]

Is my PIC getting as rest???

Your code is probably setting a few bank select registers, doing some gosub'ing and return'ing during all of this. But that would still only account for a handful of the 'missing' cycles.
Post the code that's running this. Maybe those missing cycles are laying around in there...

[flotulopex]

Here it is:

' PIC 16F690 Fuses
@ DEVICE FCMEN_OFF
@ DEVICE IESO_OFF
@ DEVICE BOD_ON
@ DEVICE CPD_OFF
@ DEVICE PROTECT_OFF
@ DEVICE MCLR_OFF
@ DEVICE PWRT_OFF
@ DEVICE WDT_OFF
@ DEVICE HS_OSC

'-------------------------------------------------------------------------------
' Registers   76543210
OPTION_REG = %10000101 'PORT A&B Pull-Ups disabled (look WPUA & WPUB)
ANSEL      = %00000001 'Select analog inputs Channels 0 to 7
ANSELH     = %00000000 'Select analog inputs Channels 8 to 11
ADCON0     = %10000010 'A/D Module is ON (Bit5:2 select channels 0:11)
ADCON1     = %00010000 'A/D control register
CM1CON0    = %00000000 'Comparator1 Module is OFF
CM2CON0    = %00000000 'Comparator2 Module is OFF
INTCON     = %00000000 'INTerrupts CONtrol (TMR0 ON)
TRISA      = %00000001 'Set Input/Output (0 to 5)
PORTA      = %00000000 'Ports High/Low (0 to 5)
TRISB      = %00000000 'Set Input/Output (4 to 7)
PORTB      = %00000000 'Ports High/Low (4 to 7)
TRISC      = %00000000 'Set Input/Output (0 to 7)
PORTC      = %00000000 'Ports High/Low (0 to 7)

'-------------------------------------------------------------------------------
' Defines
DEFINE ADC_BITS 10     'Number of bits in ADCIN result

'-------------------------------------------------------------------------------
' Variables
Volt    var word
Check   var PORTB.6 'I use this one to measure the ADC start/end

'-------------------------------------------------------------------------------
' Program
MAIN:
    check = 1
    adcin 0, volt
    check = 0
    goto MAIN
end

NB: is there a way to make the code appear with the same text formatting as I have in MCS?