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FCP
Joined: 19 May 2010
Posts: 13
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| PIC18LF45K80 ADC problem |
 Posted: Fri Feb 22, 2013 3:36 pm |
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I need help with the ADC of a PIC18LF45K80, which Microchip has created an errata related to it.
The problem is that the results of the conversion do not match the input voltage and overall the behavior is not consistent (or linear) with changes in the input signal.
| Code: |
/****************************************************************************************************/
/* ADC configuration */
/* Channels 0, 1, & 2 for sensor IC #1 */
/* Channels 5, 6, & 7 for sensor IC #2 */
/* Reference voltage Vref+ set from channel 3 at +3.3V; Vref- set to AVss */
/* Analog Negative channel is 00 (AVss) */
/* AD result is right justified */
/* AD acquisition time set to 20 TAD; AD conversion clock set at Fosc/64 */
/****************************************************************************************************/
#byte ADCON0 = 0xfc2 // AD Converter Control Register 0
#byte ADCON1 = 0xfc1 // AD Converter Control Register 1
#byte ADCON2 = 0xfc0 // AD Converter Control Register 2
#byte ANCON0 = 0xf5d // AD Converter Port Configuration Register 0
#byte ANCON1 = 0xf5c // AD Converter Port Configuration Register 1
#byte ADRESH = 0xfc4 // AD Converter Result High Byte Register
#byte ADRESL = 0xfc3 // AD Converter Result Low Byte Register
#define ADCSET1 0x00 // ADC Setup for ADCON0
#define ADCSET2 0x10 // ADC Setup for ADCON1
#define ADCSET3 0xbe // ADC Setup for ADCON2
#define ADCSET4_1 0x0f // ADC Setup for ANCON0 with 3 channels active
#define ADCSET4_2 0xef // ADC Setup for ANCON0 with 6 channels active
#define ADCSET5 0x00 // ADC Setup for ANCON1
#bit ADON = ADCON0.0 // ADC on/off bit
#bit ADGO = ADCON0.1 // ADC start conversion bit; conversion flag
#bit CHS0 = ADCON0.2 // ADC channel selection bits for conversion
#bit CHS1 = ADCON0.3
#bit CHS2 = ADCON0.4
#bit CHS3 = ADCON0.5
#bit CHS4 = ADCON0.6
#bit VCFG0 = ADCON1.4 // ADC Vref+ selection bits for conversion
#bit VCFG1 = ADCON1.5
void device_config(void){
SCS0 = 0; // Setup primary oscillator
SCS1 = 0;
TXSTA1 = TXCOM1; // Setup UART1
RCSTA1 = RXCOM1;
SPBRG1 = BRCOM1;
BAUDCON1 = BRSCOM1;
set_tris_a(0x2f); // RA6, RA7 OSC; rest inputs
set_tris_b(0xc0); // High address port, inputs; rest outputs
port_b_pullups(TRUE); // Enable pull-ups for high address port
set_tris_c(0x81); // xBEE_TX, SOSC0 inputs; rest outputs
set_tris_d(0xFF); // All inputs
port_d_pullups(TRUE); // Enable pull-ups for Address port
#ifdef USES_MAG2
set_tris_e(0x0F); // ADC inputs for second magnetic IC
#else
set_tris_e(0x08);
#endif
//lastRXchar = RCREG1;
ANCON0 = ADCSET4_1; // Setup of ADC
#ifdef USES_MAG2
ANCON0 = ADCSET4_2;
#endif
ANCON1 = ADCSET5;
ADCON0 = ADCSET1;
ADCON1 = ADCSET2;
ADCON2 = ADCSET3;
T1CON = TMR1SET; // Setup Timer1
set_timer1(TMR1VAL_5SEC); // Start Timer1
}
void measure_voltage(void){
unsigned int16 ADCresult, ADCdata;
unsigned int8 i;
ADON = ON; // Enable ADC
CHS0 = 0; // Vdd core as ADC input
CHS1 = 0;
CHS2 = 0;
CHS3 = 0;
CHS4 = 0;
ADCdata = 0;
for( i = 0; i < 4; i++){
delay_us(50);
ADGO = ON; // Start sampling
while(ADGO); // Wait until conversion is finish
ADCresult = ADRESH << 8;
ADCresult += ADRESL;
ADCdata += ADCresult;
}
ADON = OFF; // Disable ADC
sensor_state.voltage = (ADCdata >> 2); // Average 4 samples
}
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Before I change microcontroller I want to review if anything can be done to get the ADC working correctly.
Thank you. |
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gpsmikey
Joined: 16 Nov 2010
Posts: 588
Location: Kirkland, WA
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| Posted: Sat Feb 23, 2013 1:41 am |
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I have not used that one, but often times, strange readings show up when either there is noise on the input or the input impedance is too high. Make sure the input impedance is low enough compared to the data sheet for that chip. One quick test for noise is - do you get the same (wrong) readings each time for a given input voltage or does it wander around for a given input voltage (which would tend to indicate noise problems) ?
_________________
mikey
-- you can't have too many gadgets or too much disk space !
old engineering saying: 1+1 = 3 for sufficiently large values of 1 or small values of 3 |
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Ttelmah
Joined: 11 Mar 2010
Posts: 19513
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| Posted: Sat Feb 23, 2013 2:28 am |
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1) Use the CCS functions.
2) The problems with the ADC on this chip, mean that it works 'OK' if you accept it as a 10bit ADC, but with a bit of offset (this comes down to applying a calibration). No point in fiddling round with the ADC code, just divide the results by 4, and store an offset calibration value for the chip (take a reading of a known voltage, and store this, then use this to give the offset required).
3) If you do have to go 'DIY', then use the compiler functions to help. Make16, is several instructions more efficient than rotating and adding, to form a 16bit value. etc..
4) As a comment, looking at the code, you are selecting an external Vref. Have you got this connected?.
Best Wishes |
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FCP
Joined: 19 May 2010
Posts: 13
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| Posted: Mon Feb 25, 2013 9:10 am |
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Hi,
Thank you for your comments.
I do have the reference voltage present and is constant, it comes form a +3.3V charge pump IC. Which I will use for the base value to get the offset voltage for adjusting the reading.
As for having a 10-bit data, we selected this particular device because of the 12-bit ADC, so I either make it "work" or change the device and add an external ADC. |
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Ttelmah
Joined: 11 Mar 2010
Posts: 19513
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| Posted: Mon Feb 25, 2013 10:03 am |
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As it stands it won't give a reliable 12bit ADC. Lesson is to read the errata when selecting a part.
Talk to Microchip. Ask explicitly how long it will be before a fixed version is available. You may well find they have engineering samples already, since this is a 'major' failure on the chip.
There are other chips like the 67J93, which have a fully working 12bit ADC. It has a couple of minor issues with the MSSP, but otherwise is 99% 'bug free'. You might see if this has the other features you want.
Best Wishes |
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FCP
Joined: 19 May 2010
Posts: 13
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| Posted: Mon Feb 25, 2013 3:30 pm |
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Hi,
On the IC selection side, I had no input, right now I am trying to fix the issues that the design has.
So far, I have been able to correct the issues with the data conversion up to satisfactory levels.
Nevertheless, I still need to get the actual supply voltage to the IC to judge when the voltage coming into the system is no longer having the adequate level.
Thank you. |
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