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Beenabout
Joined: 19 Jan 2010
Posts: 7
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| Using 0.6V and 1.2V internal Vref on PIC12F615 |
 Posted: Tue Jan 19, 2010 8:53 am |
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I am using PCM 4.093.
I want to enable and then sample the internal reference on the 12F615 before sampling a pin with a sensor connected to it.
The H file only gives an option to enable the 0.6V reference:
#define VREF_6th 0x10 // Turn on .6V ref
The 1.2V reference stated in the datasheet is not mentioned anywhere in the H file.
It is also not clear how I can point the ADC to read an internal VREF as there is no sAN channel number associated with either in the H file.
I have tried setting the CHS bits in the ADCON directly as follows but still get no value for the VREF chosen (ADC reads 0):
| Code: |
#byte O_SIX_VREF = 0x14 //0.6V ref
byte data;
void ADCONVREF(){
setup_vref(VREF_6th); //Enable internal 0.6V VREF
setup_ADC(ADC_CLOCK_INTERNAL); //Use internal ADC clock
SETUP_ADC_PORTS( sAN3 | VSS_VDD);//Set AN3 (GP4) to analogue and use VDD as reference
data = ADCON && !O_SIX_VREF; //clear CHS bits in #byte variable data
ADCON = data || O_SIX_VREF; //set CHS bits ADCON for required VREF to select VREF channel
delay_us(10);
}
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Any ideas? |
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Ttelmah
Guest
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| Posted: Tue Jan 19, 2010 3:22 pm |
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First, setup_vref, controls the Vref setting to the comparator. The comparator doesn't support the 1.2v reference, hence no constant for this.
Don't use this.
On the ADC, the Vref is available as another input. The channel configuration constants supplied, don't offer this, and are completely wrong (SAN0, for example, actually requires '0' put into the CS bits, the supplied constants, put in '1'.....).
Simply use the following constants and setup:
| Code: |
#define sAN0 0 //| GP0
#define sAN1 1 //| GP1
#define sAN2 2 //| GP2
#define sAN3 3 //| GP4
#define CVREF 4
#define VREF06 5
#define VREF12 6
setup_ADC(ADC_CLOCK_INTERNAL);
SETUP_ADC_PORTS(VSS_VDD);
set_adc_channel(VREF06);
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This chip doesn't have separate 'selection', and 'setup', you just set the channel you want to read, and provided it is set as an input, it is connected to the multiplexer.
Best Wishes |
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Beenabout
Joined: 19 Jan 2010
Posts: 7
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| Posted: Wed Jan 20, 2010 4:05 am |
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Many thanks for that reply - its all a little clearer now. It makes sense when read looking at the ADC diagram in the datasheet depicting the CHS channels as a block.
Only problem is that although selecting channel 6 as indicated I do get a value corresponding to 1.19V (1.2Vref), selecting channel 5 results in a value of zero returned. But I can make do with just the 1.2V reference.
I take it that both references (the 0.6V ref is presumably the 1.2V ref divided by two mosfet resistors) are always on and that there is no way of turning them off to save power? My application is low power. |
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Ttelmah
Guest
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| Posted: Wed Jan 20, 2010 6:23 am |
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Look for the FVREN enable bit. Figure 9.6.
I don't know which CCS code controls this, I'd suggest just defining it as a #bit, and turning it on/off as needed.
Note also the oscillator selection effect on the ADC Vref connection. hence when you select the low speed oscillator, or sleep, the VREF always goes off... 
Best Wishes |
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Beenabout
Joined: 19 Jan 2010
Posts: 7
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| Posted: Thu Jan 21, 2010 4:32 am |
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| Thanks again! |
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