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Detecting voltage

 
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pom



Joined: 25 Nov 2004
Posts: 30
Location: Germany

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Detecting voltage
PostPosted: Thu Nov 25, 2004 10:59 am     Reply with quote
Hello,

I want to detect voltage by a 18F258. I know how to read the AD-converter and by this I get a value among 0 and 255 (8 Bit). In default, the 255 represents a voltage of 5V and the 0 represents a voltage of 0V. By changing the reference, the 255 might represent the voltage level of AN3 (2,5V in this case).

Is it also possible to change the 0 level? My aim is to have the 0 representing the voltage of AN2 (1,2 V in this case). My measurement would be delimited among 1,2V (read as 0) and 2,5V (read as 255).

I hope, you understand the question. My english is not that good. If there are problems in understanding, please ask me.

I am looking forward to your answers. Thank you.
newguy



Joined: 24 Jun 2004
Posts: 1907

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PostPosted: Thu Nov 25, 2004 11:51 am     Reply with quote
From the 18F258's data sheet, page 245:

Quote:
The analog reference voltage is software selectable to
either the device’s positive and negative supply voltage
(VDD and VSS) or the voltage level on the RA3/AN3/
VREF+ pin and RA2/AN2/VREF- pin.


So yes, it's definitely possible.

From the 18f258.h file:

Code:
////////////////////////////////////////////////////////////////// ADC
// ADC Functions: SETUP_ADC(), SETUP_ADC_PORTS() (aka SETUP_PORT_A),
//                SET_ADC_CHANNEL(), READ_ADC()
// Constants used in SETUP_ADC_PORTS() are:
#define NO_ANALOGS             0x86         // None
#define ALL_ANALOG             0x80         // A0 A1 A2 A3 A5 E0 E1 E2 Ref=Vdd
#define ANALOG_RA3_REF         0x81         // A0 A1 A2 A5 E0 E1 E2 Ref=A3
#define A_ANALOG               0x82         // A0 A1 A2 A3 A5 Ref=Vdd
#define A_ANALOG_RA3_REF       0x83         // A0 A1 A2 A5 Ref=A3
#define RA0_RA1_RA3_ANALOG     0x84         // A0 A1 A3 Ref=Vdd
#define RA0_RA1_ANALOG_RA3_REF 0x85         // A0 A1 Ref=A3
#define ANALOG_RA3_RA2_REF              0x88   // A0 A1 A5 E0 E1 E2 Ref=A2,A3
#define ANALOG_NOT_RE1_RE2              0x89   // A0 A1 A2 A3 A5 E0 Ref=Vdd
#define ANALOG_NOT_RE1_RE2_REF_RA3      0x8A   // A0 A1 A2 A5 E0 Ref=A3
#define ANALOG_NOT_RE1_RE2_REF_RA3_RA2  0x8B   // A0 A1 A5 E0 Ref=A2,A3
#define A_ANALOG_RA3_RA2_REF            0x8C   // A0 A1 A5 Ref=A2,A3
#define RA0_RA1_ANALOG_RA3_RA2_REF      0x8D   // A0 A1 Ref=A2,A3
#define RA0_ANALOG                      0x8E   // A0
#define RA0_ANALOG_RA3_RA2_REF          0x8F   // A0 Ref=A2,A3


From this, the setups that would suit you are:
ANALOG_RA3_RA2_REF
ANALOG_NOT_RE1_RE2_REF_RA3_RA2
A_ANALOG_RA3_RA2_REF
RA0_RA1_ANALOG_RA3_RA2_REF
RA0_ANALOG_RA3_RA2_REF

If you're only measuring one analog input, then the last choice, RA0_ANALOG_RA3_RA2_REF, would suit you best.
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