A/D Conversion Clock Select bits confussion ..

[gunayburak]

Hello everyone ..

For a long time I've been struggling with these settings ... When one would like to perform an ADC reading what kind of settings he must add into his code ?

As far as I read the datasheet of 16F1827 , I see the following details about this setting

ADCS<2:0>:A/D Conversion Clock Select bits
000=FOSC/2
001=FOSC/8
010=FOSC/32
011=FRC(clock supplied from a dedicated RC oscillator)
100=FOSC/4
101=FOSC/16
110=FOSC/64
111=FRC(clock supplied from a dedicated RC oscillator)

So , If I'm running my PIC at 32 MHZ internal oscillator , the setting should be 011 or 010 ?

and the SECOND QUESTION :

What is the maximum ADC conversion time at 32MHZ for the PIC16F1827 ...
When I look at the conversion tables that Microchip has published for this PIC , I can't understand which is the exact time at the selected OSC frequency that takes for a complete ADC conversion time ..

Thanks in advance ...

[richard]

010=FOSC/32 tad=1.0uS
011=FRC tad=1.0uS to 6uS (vcc and temperature dependant)
110=FOSC/64 tad=2.0uS

a 10 bit adc takes 11.5 tads

any divider less than fosc/32 @32mhz is out of reliable conversion range

[elcrcp]

FOSC is Frequency of OSCillator, so if you are using 32MHz internal osc, your fosc is 32MHz if you are using 8MHz external osc then your fosc is 8 MHz and if you are using 4 MHz external with PLL then your FOSC is 16MHz.
011 and 111 settings are for an internal dedicated oscillator with 1.6us period if I remember correct...
You can always use fosc/2 timing since faster is generally better. But again, it depends on your timing need. If you don't need a special timing, just choose whatever you like.

Edit: I'm not sure minimum conversion time but I believe it has to be larger then 10us

[gunayburak]

Thanks for your answers but I still do not understand something .. Which one of these oscillator settings should be chosen for a robust operation .. Or in other words , for a "classic proper" operation ... What I understand from your explanations is that I can choose any of these settings non regarding if the OSC is internal or external ...

As far as I know the Frc stands for internal oscillator ... Please correct me If I'm wrong at this point .. So If I choose 4MHZ with PLL operation , I'm running it with 32 MHZ internal oscillator which is also called the internal RC osc block , so in this case I should go with Frc for a more accurate conversion ... or with Fosc/32 or Fosc/2 for a faster conversion ... Right ?

[aratti]

32 MHz is a too high frequency to obtain a good ADC reading. The minimum TAD is 1.6 us and If you set fosc/64 you will get a TAD of 2 us and you are barely inside the correct timing. Since you will be in the low end side as far as the TAD value is concerned. Dropping your fosc to 16 MHz with the same setting you will have a TAD of 4 us which is much better than 2 us.

Cheers
Al.

[gunayburak]

Thanks Al .. You may be right ...
Well the question is how to set the define adc sampleus parameter in pbp ? I should adjust it according to the decided Fosc ?

[aratti]

The sampleus parameter is the time given for the acquisition ( changing the sample & hold capacitor). This timing is critical since you must charge the capacitor no more than 1T..(up to 1T the charging curve is quasi linear) Now If you have decided to use fosc = 16 MHz than the minimum pauseus you can get is 5 us so a Define adc_sampleus 15 should give you good results.

Cheers

Al.

[gunayburak]

Should we not calculate it 1.6x11 Tad = 17.6 uS ? Thus is it not better to set it to 20 uS ?

[aratti]

The 11 TADs is the time taken by the MCU for the 10 bits convertion! Please do not confuse acquisition with convertion. The setting suggested is the time you will give to the sample & hold capacitor to charge with your input dc value. This time should be enough to charge the cap not beyond the 1T, because beyond this point the charging function is no longer quasi-linear, but totaly logaritmic.

You can set it to 20 us and see If your ADC reading are consistent and adjust this value to suite your need. (Remember that the best acquisition time will depend by the hardware setting, so every circuit has his own best acquisition time)

Cheers
Al.

[gunayburak]

Grazie tante per la risposta cara aratti ..

[richard]

it might be interesting to connect the "dummy conversion" pin to ground and then in another pass vcc , and give it a long acquisition and then take your reading . I would expect the real effect of a too short acquisition time would become more evident (ie the previous reading sample cap charge will have a effect on the current reading)

[tumbleweed]

I wondered about that too. As you say, connecting it to a floating input would probably just leave the charge on Chold since all you have is the leakage current to discharge it.

I noticed in figure 19-4 that they show a 2TAD "discharge" time between measurements, and they say this:

19.2.3 DISCHARGE
The discharge phase is used to initialize the value of
the capacitor array. The array is discharged after every
sample. This feature helps to optimize the unity-gain
amplifier, as the circuit always needs to charge the
capacitor array, rather than charge/discharge based on
previous measure values.

The PIC18 reference manual has the same sort of text, but is also has the statement

The charge holding capacitor (CHOLD) is not discharged after each conversion

so it sounds like they're talking about something to do with the internal ADC structure and not Chold.

It WOULD be interesting to see the same measurements without any averaging and with alternating input voltages for the in between "idle" cycles.

[HenrikOlsson]

Hi guys,
OK, doing a dummy conversion on a floating input wasn't the greatest idea....

I took out the averaging and set up my arb gen to generate 3.3Vpp noise and injected that into AN1 (the dummy channel).

25.6mV input, 10k source resistor, 3.3V noise on "dummy channel":
FOSC/2     2 TAD      Result: 512
FOSC/8     2 TAD      Result: 120
FOSC/32    2 TAD      Result: 23
FRC        2 TAD      Result: 6
FOSC/4     2 TAD      Result: 184
FOSC/16    2 TAD      Result: 54
FOSC/64    2 TAD      Result: 7
FOSC/2     4 TAD      Result: 190
FOSC/8     4 TAD      Result: 59
FOSC/32    4 TAD      Result: 14
FRC        4 TAD      Result: 13
FOSC/4     4 TAD      Result: 117
FOSC/16    4 TAD      Result: 21
FOSC/64    4 TAD      Result: 14
FOSC/2     6 TAD      Result: 127
FOSC/8     6 TAD      Result: 32
FOSC/32    6 TAD      Result: 11
FRC        6 TAD      Result: 11
FOSC/4     6 TAD      Result: 74
FOSC/16    6 TAD      Result: 7
FOSC/64    6 TAD      Result: 10
FOSC/2     8 TAD      Result: 512
FOSC/8     8 TAD      Result: 15
FOSC/32    8 TAD      Result: 15
FRC        8 TAD      Result: 12
FOSC/4     8 TAD      Result: 56
FOSC/16    8 TAD      Result: 10
FOSC/64    8 TAD      Result: 6
FOSC/2     12 TAD     Result: 512
FOSC/8     12 TAD     Result: 11
FOSC/32    12 TAD     Result: 16
FRC        12 TAD     Result: 13
FOSC/4     12 TAD     Result: 31
FOSC/16    12 TAD     Result: 11
FOSC/64    12 TAD     Result: 14
FOSC/2     16 TAD     Result: 512
FOSC/8     16 TAD     Result: 13
FOSC/32    16 TAD     Result: 11
FRC        16 TAD     Result: 11
FOSC/4     16 TAD     Result: 18
FOSC/16    16 TAD     Result: 11
FOSC/64    16 TAD     Result: 9
FOSC/2     20 TAD     Result: 49
FOSC/8     20 TAD     Result: 15
FOSC/32    20 TAD     Result: 10
FRC        20 TAD     Result: 8
FOSC/4     20 TAD     Result: 14
FOSC/16    20 TAD     Result: 10
FOSC/64    20 TAD     Result: 13

***************************************

3.285V, 10k source resistor 3.3V noise on "dummy channel":

FOSC/2     2 TAD      Result: 511
FOSC/8     2 TAD      Result: 861
FOSC/32    2 TAD      Result: 1008
FRC        2 TAD      Result: 1022
FOSC/4     2 TAD      Result: 812
FOSC/16    2 TAD      Result: 963
FOSC/64    2 TAD      Result: 1014
FOSC/2     4 TAD      Result: 511
FOSC/8     4 TAD      Result: 964
FOSC/32    4 TAD      Result: 1021
FRC        4 TAD      Result: 1023
FOSC/4     4 TAD      Result: 888
FOSC/16    4 TAD      Result: 1004
FOSC/64    4 TAD      Result: 1015
FOSC/2     6 TAD      Result: 511
FOSC/8     6 TAD      Result: 994
FOSC/32    6 TAD      Result: 1019
FRC        6 TAD      Result: 1023
FOSC/4     6 TAD      Result: 911
FOSC/16    6 TAD      Result: 1018
FOSC/64    6 TAD      Result: 1019
FOSC/2     8 TAD      Result: 511
FOSC/8     8 TAD      Result: 1006
FOSC/32    8 TAD      Result: 1019
FRC        8 TAD      Result: 1021
FOSC/4     8 TAD      Result: 964
FOSC/16    8 TAD      Result: 1022
FOSC/64    8 TAD      Result: 1016
FOSC/2     12 TAD     Result: 511
FOSC/8     12 TAD     Result: 1015
FOSC/32    12 TAD     Result: 1019
FRC        12 TAD     Result: 1019
FOSC/4     12 TAD     Result: 991
FOSC/16    12 TAD     Result: 1023
FOSC/64    12 TAD     Result: 1015
FOSC/2     16 TAD     Result: 511
FOSC/8     16 TAD     Result: 1014
FOSC/32    16 TAD     Result: 1018
FRC        16 TAD     Result: 1018
FOSC/4     16 TAD     Result: 1005
FOSC/16    16 TAD     Result: 1016
FOSC/64    16 TAD     Result: 1023
FOSC/2     20 TAD     Result: 983
FOSC/8     20 TAD     Result: 1020
FOSC/32    20 TAD     Result: 1017
FRC        20 TAD     Result: 1023
FOSC/4     20 TAD     Result: 1010
FOSC/16    20 TAD     Result: 1017
FOSC/64    20 TAD     Result: 1019

Then I connected the dummy channel to GND:

25.6mV input, 10k source resistor, "dummy channel" connected to GND:

FOSC/2     2 TAD      Result: 8
FOSC/8     2 TAD      Result: 6
FOSC/32    2 TAD      Result: 8
FRC        2 TAD      Result: 7
FOSC/4     2 TAD      Result: 6
FOSC/16    2 TAD      Result: 7
FOSC/64    2 TAD      Result: 7
FOSC/2     4 TAD      Result: 8
FOSC/8     4 TAD      Result: 7
FOSC/32    4 TAD      Result: 8
FRC        4 TAD      Result: 7
FOSC/4     4 TAD      Result: 6
FOSC/16    4 TAD      Result: 7
FOSC/64    4 TAD      Result: 7
FOSC/2     6 TAD      Result: 8
FOSC/8     6 TAD      Result: 7
FOSC/32    6 TAD      Result: 7
FRC        6 TAD      Result: 7
FOSC/4     6 TAD      Result: 6
FOSC/16    6 TAD      Result: 7
FOSC/64    6 TAD      Result: 7
FOSC/2     8 TAD      Result: 8
FOSC/8     8 TAD      Result: 7
FOSC/32    8 TAD      Result: 7
FRC        8 TAD      Result: 7
FOSC/4     8 TAD      Result: 7
FOSC/16    8 TAD      Result: 8
FOSC/64    8 TAD      Result: 7
FOSC/2     12 TAD     Result: 8
FOSC/8     12 TAD     Result: 7
FOSC/32    12 TAD     Result: 7
FRC        12 TAD     Result: 7
FOSC/4     12 TAD     Result: 8
FOSC/16    12 TAD     Result: 7
FOSC/64    12 TAD     Result: 7
FOSC/2     16 TAD     Result: 8
FOSC/8     16 TAD     Result: 7
FOSC/32    16 TAD     Result: 7
FRC        16 TAD     Result: 7
FOSC/4     16 TAD     Result: 8
FOSC/16    16 TAD     Result: 7
FOSC/64    16 TAD     Result: 7
FOSC/2     20 TAD     Result: 8
FOSC/8     20 TAD     Result: 7
FOSC/32    20 TAD     Result: 7
FRC        20 TAD     Result: 7
FOSC/4     20 TAD     Result: 8
FOSC/16    20 TAD     Result: 8
FOSC/64    20 TAD     Result: 7

*******************************************************

3.285V input, 10k source resistor, "dummy" channel connected to GND:

FOSC/2     2 TAD      Result: 511
FOSC/8     2 TAD      Result: 714
FOSC/32    2 TAD      Result: 996
FRC        2 TAD      Result: 1019
FOSC/4     2 TAD      Result: 610
FOSC/16    2 TAD      Result: 892
FOSC/64    2 TAD      Result: 1018
FOSC/2     4 TAD      Result: 511
FOSC/8     4 TAD      Result: 862
FOSC/32    4 TAD      Result: 1018
FRC        4 TAD      Result: 1018
FOSC/4     4 TAD      Result: 687
FOSC/16    4 TAD      Result: 991
FOSC/64    4 TAD      Result: 1019
FOSC/2     6 TAD      Result: 511
FOSC/8     6 TAD      Result: 955
FOSC/32    6 TAD      Result: 1020
FRC        6 TAD      Result: 1019
FOSC/4     6 TAD      Result: 761
FOSC/16    6 TAD      Result: 1010
FOSC/64    6 TAD      Result: 1019
FOSC/2     8 TAD      Result: 511
FOSC/8     8 TAD      Result: 983
FOSC/32    8 TAD      Result: 1020
FRC        8 TAD      Result: 1018
FOSC/4     8 TAD      Result: 836
FOSC/16    8 TAD      Result: 1018
FOSC/64    8 TAD      Result: 1019
FOSC/2     12 TAD     Result: 511
FOSC/8     12 TAD     Result: 1010
FOSC/32    12 TAD     Result: 1020
FRC        12 TAD     Result: 1019
FOSC/4     12 TAD     Result: 944
FOSC/16    12 TAD     Result: 1019
FOSC/64    12 TAD     Result: 1020
FOSC/2     16 TAD     Result: 511
FOSC/8     16 TAD     Result: 1017
FOSC/32    16 TAD     Result: 1019
FRC        16 TAD     Result: 1018
FOSC/4     16 TAD     Result: 981
FOSC/16    16 TAD     Result: 1020
FOSC/64    16 TAD     Result: 1019
FOSC/2     20 TAD     Result: 511
FOSC/8     20 TAD     Result: 1017
FOSC/32    20 TAD     Result: 1019
FRC        20 TAD     Result: 1019
FOSC/4     20 TAD     Result: 998
FOSC/16    20 TAD     Result: 1021
FOSC/64    20 TAD     Result: 1020

And finally, dummy channel to Vdd (3.3V):

25.6mV input, 10k source resistor, "dummy channel" connected to Vdd:

FOSC/2     2 TAD      Result: 512
FOSC/8     2 TAD      Result: 344
FOSC/32    2 TAD      Result: 29
FRC        2 TAD      Result: 8
FOSC/4     2 TAD      Result: 432
FOSC/16    2 TAD      Result: 192
FOSC/64    2 TAD      Result: 9
FOSC/2     4 TAD      Result: 512
FOSC/8     4 TAD      Result: 262
FOSC/32    4 TAD      Result: 9
FRC        4 TAD      Result: 8
FOSC/4     4 TAD      Result: 383
FOSC/16    4 TAD      Result: 46
FOSC/64    4 TAD      Result: 8
FOSC/2     6 TAD      Result: 512
FOSC/8     6 TAD      Result: 180
FOSC/32    6 TAD      Result: 8
FRC        6 TAD      Result: 7
FOSC/4     6 TAD      Result: 350
FOSC/16    6 TAD      Result: 17
FOSC/64    6 TAD      Result: 7
FOSC/2     8 TAD      Result: 512
FOSC/8     8 TAD      Result: 99
FOSC/32    8 TAD      Result: 8
FRC        8 TAD      Result: 7
FOSC/4     8 TAD      Result: 312
FOSC/16    8 TAD      Result: 10
FOSC/64    8 TAD      Result: 7
FOSC/2     12 TAD     Result: 512
FOSC/8     12 TAD     Result: 25
FOSC/32    12 TAD     Result: 7
FRC        12 TAD     Result: 8
FOSC/4     12 TAD     Result: 229
FOSC/16    12 TAD     Result: 8
FOSC/64    12 TAD     Result: 7
FOSC/2     16 TAD     Result: 512
FOSC/8     16 TAD     Result: 12
FOSC/32    16 TAD     Result: 7
FRC        16 TAD     Result: 8
FOSC/4     16 TAD     Result: 147
FOSC/16    16 TAD     Result: 7
FOSC/64    16 TAD     Result: 7
FOSC/2     20 TAD     Result: 512
FOSC/8     20 TAD     Result: 9
FOSC/32    20 TAD     Result: 7
FRC        20 TAD     Result: 8
FOSC/4     20 TAD     Result: 66
FOSC/16    20 TAD     Result: 7
FOSC/64    20 TAD     Result: 7

*********************************************


3.285V input, 10k source resistor, "dummy channel" connected to Vdd:

FOSC/2     2 TAD      Result: 1022
FOSC/8     2 TAD      Result: 1021
FOSC/32    2 TAD      Result: 1020
FRC        2 TAD      Result: 1020
FOSC/4     2 TAD      Result: 1022
FOSC/16    2 TAD      Result: 1020
FOSC/64    2 TAD      Result: 1019
FOSC/2     4 TAD      Result: 1022
FOSC/8     4 TAD      Result: 1019
FOSC/32    4 TAD      Result: 1019
FRC        4 TAD      Result: 1019
FOSC/4     4 TAD      Result: 1019
FOSC/16    4 TAD      Result: 1021
FOSC/64    4 TAD      Result: 1020
FOSC/2     6 TAD      Result: 1023
FOSC/8     6 TAD      Result: 1021
FOSC/32    6 TAD      Result: 1019
FRC        6 TAD      Result: 1020
FOSC/4     6 TAD      Result: 1020
FOSC/16    6 TAD      Result: 1020
FOSC/64    6 TAD      Result: 1019
FOSC/2     8 TAD      Result: 1022
FOSC/8     8 TAD      Result: 1020
FOSC/32    8 TAD      Result: 1019
FRC        8 TAD      Result: 1019
FOSC/4     8 TAD      Result: 1020
FOSC/16    8 TAD      Result: 1021
FOSC/64    8 TAD      Result: 1019
FOSC/2     12 TAD     Result: 1022
FOSC/8     12 TAD     Result: 1020
FOSC/32    12 TAD     Result: 1019
FRC        12 TAD     Result: 1020
FOSC/4     12 TAD     Result: 1020
FOSC/16    12 TAD     Result: 1021
FOSC/64    12 TAD     Result: 1020
FOSC/2     16 TAD     Result: 1021
FOSC/8     16 TAD     Result: 1021
FOSC/32    16 TAD     Result: 1019
FRC        16 TAD     Result: 1021
FOSC/4     16 TAD     Result: 1021
FOSC/16    16 TAD     Result: 1019
FOSC/64    16 TAD     Result: 1019
FOSC/2     20 TAD     Result: 1020
FOSC/8     20 TAD     Result: 1021
FOSC/32    20 TAD     Result: 1019
FRC        20 TAD     Result: 1021
FOSC/4     20 TAD     Result: 1018
FOSC/16    20 TAD     Result: 1019
FOSC/64    20 TAD     Result: 1020

Henrik.

[tumbleweed]

Thanks Henrik. So again, removing the ones where the ADCS setting is invalid/too fast gives:

25.6mV input, 10k source resistor, 3.3V noise on "dummy channel":

FRC        2 TAD      Result: 6
FOSC/64    2 TAD      Result: 7
FRC        4 TAD      Result: 13
FOSC/64    4 TAD      Result: 14
FRC        6 TAD      Result: 11
FOSC/64    6 TAD      Result: 10
FRC        8 TAD      Result: 12
FOSC/64    8 TAD      Result: 6
FRC        12 TAD     Result: 13
FOSC/64    12 TAD     Result: 14
FRC        16 TAD     Result: 11
FOSC/64    16 TAD     Result: 9
FRC        20 TAD     Result: 8
FOSC/64    20 TAD     Result: 13

*********************************************

3.285V, 10k source resistor 3.3V noise on "dummy channel":

FRC        2 TAD      Result: 1022
FOSC/64    2 TAD      Result: 1014
FRC        4 TAD      Result: 1023
FOSC/64    4 TAD      Result: 1015
FRC        6 TAD      Result: 1023
FOSC/64    6 TAD      Result: 1019
FRC        8 TAD      Result: 1021
FOSC/64    8 TAD      Result: 1016
FRC        12 TAD     Result: 1019
FOSC/64    12 TAD     Result: 1015
FRC        16 TAD     Result: 1018
FOSC/64    16 TAD     Result: 1023
FRC        20 TAD     Result: 1023
FOSC/64    20 TAD     Result: 1019

*********************************************

25.6mV input, 10k source resistor, "dummy channel" connected to GND:

FRC        2 TAD      Result: 7
FOSC/64    2 TAD      Result: 7
FRC        4 TAD      Result: 7
FOSC/64    4 TAD      Result: 7
FRC        6 TAD      Result: 7
FOSC/64    6 TAD      Result: 7
FRC        8 TAD      Result: 7
FOSC/64    8 TAD      Result: 7
FRC        12 TAD     Result: 7
FOSC/64    12 TAD     Result: 7
FRC        16 TAD     Result: 7
FOSC/64    16 TAD     Result: 7
FRC        20 TAD     Result: 7
FOSC/64    20 TAD     Result: 7

*********************************************

3.285V input, 10k source resistor, "dummy" channel connected to GND:

FRC        2 TAD      Result: 1019
FOSC/64    2 TAD      Result: 1018
FRC        4 TAD      Result: 1018
FOSC/64    4 TAD      Result: 1019
FRC        6 TAD      Result: 1019
FOSC/64    6 TAD      Result: 1019
FRC        8 TAD      Result: 1018
FOSC/64    8 TAD      Result: 1019
FRC        12 TAD     Result: 1019
FOSC/64    12 TAD     Result: 1020
FRC        16 TAD     Result: 1018
FOSC/64    16 TAD     Result: 1019
FRC        20 TAD     Result: 1019
FOSC/64    20 TAD     Result: 1020

*********************************************

25.6mV input, 10k source resistor, "dummy channel" connected to Vdd:

FRC        2 TAD      Result: 8
FOSC/64    2 TAD      Result: 9
FRC        4 TAD      Result: 8
FOSC/64    4 TAD      Result: 8
FRC        6 TAD      Result: 7
FOSC/64    6 TAD      Result: 7
FRC        8 TAD      Result: 7
FOSC/64    8 TAD      Result: 7
FRC        12 TAD     Result: 8
FOSC/64    12 TAD     Result: 7
FRC        16 TAD     Result: 8
FOSC/64    16 TAD     Result: 7
FRC        20 TAD     Result: 8
FOSC/64    20 TAD     Result: 7

*********************************************

3.285V input, 10k source resistor, "dummy channel" connected to Vdd:

FRC        2 TAD      Result: 1020
FOSC/64    2 TAD      Result: 1019
FRC        4 TAD      Result: 1019
FOSC/64    4 TAD      Result: 1020
FRC        6 TAD      Result: 1020
FOSC/64    6 TAD      Result: 1019
FRC        8 TAD      Result: 1019
FOSC/64    8 TAD      Result: 1019
FRC        12 TAD     Result: 1020
FOSC/64    12 TAD     Result: 1020
FRC        16 TAD     Result: 1021
FOSC/64    16 TAD     Result: 1019
FRC        20 TAD     Result: 1021
FOSC/64    20 TAD     Result: 1020

Probably the ones of most interest are where Chold has to swing the full range between samples:

3.285V input, 10k source resistor, "dummy" channel connected to GND:

FRC        2 TAD      Result: 1019
FOSC/64    2 TAD      Result: 1018
FRC        4 TAD      Result: 1018
FOSC/64    4 TAD      Result: 1019
FRC        6 TAD      Result: 1019
FOSC/64    6 TAD      Result: 1019
FRC        8 TAD      Result: 1018
FOSC/64    8 TAD      Result: 1019
FRC        12 TAD     Result: 1019
FOSC/64    12 TAD     Result: 1020
FRC        16 TAD     Result: 1018
FOSC/64    16 TAD     Result: 1019
FRC        20 TAD     Result: 1019
FOSC/64    20 TAD     Result: 1020

*********************************************

25.6mV input, 10k source resistor, "dummy channel" connected to Vdd:

FRC        2 TAD      Result: 8
FOSC/64    2 TAD      Result: 9
FRC        4 TAD      Result: 8
FOSC/64    4 TAD      Result: 8
FRC        6 TAD      Result: 7
FOSC/64    6 TAD      Result: 7
FRC        8 TAD      Result: 7
FOSC/64    8 TAD      Result: 7
FRC        12 TAD     Result: 8
FOSC/64    12 TAD     Result: 7
FRC        16 TAD     Result: 8
FOSC/64    16 TAD     Result: 7
FRC        20 TAD     Result: 8
FOSC/64    20 TAD     Result: 7

For no averaging those numbers seem pretty reasonable.