lookuptable help

[rems]

thermisor value is calucte using data from the datasheet

resistance =12k at 25 degree(according to datasheet
thermistor value=resitance at 25* Rt/R25
i dont know its correct or not
for example 12k*1.9=2280 (resistance at 10 degree according to datasheet).this is how i calculated.
as this is my first design and program ,iam totally a beginner in this field.

 

[Ian Rogers]

Look at that PDF on page 3.... If you hook your NTC in parallel to a 56kΩ then use a Rs of 5.6kΩ ... It linearises it, making reading the temperature easier..

 

[ericgibbs]

hi,
For reference only, this is the ADC sim I commented on.

 

[Ian Rogers]

hi,
For reference only, this is the ADC sim I commented on.

10μS isn't a great deal then... I thought that after a read on the ADC the internal hold cap would be pretty much used up... I expected worse.

Live and learn as they say...

 

[ericgibbs]

hi rems,
Checked your Thermistor pdf tables, our values are very close.

 

[rems]

ok.thanks.the thermistor which i use is 5kohm.and beta value is 3760.EPCOS - B57276K123A30 .So can i use this lookup table.could you please check my code to set up the ADC value.iam attaching my schematic also.

 

[ericgibbs]

ok.thanks.the thermistor which i use is 5kohm.and beta value is 3760.EPCOS - B57276K123A30 .So can i use this lookup table.could you please check my code to set up the ADC value.iam attaching my schematic also.

hi rems,
These are plots for your thermistor, using 12K Rt1 and also a linearised version.

Ian will help with your code, I dont use 'C', sorry

EDIT:
Larger plot, you should be able to scale from this OK.

 

[rems]

ok thanks

 

[Ian Rogers]

Once you post a schematic, I'll draw it up in ISIS and when we come to an agreement on the actual values, I'll help with the formula..

 

[rems]

View attachment 65040
this is my schematic

 

[rems]

i need to convert the adc value to temperature using lookuptable and sent it to pc using RS232 interface.
need to read the value in this protocol
<STX> <channelno>< temperature>< checksum>< etx>.thanks

 

[Ian Rogers]

Do you want to increment the temp by 5°C at a time or work out the in-between temp's?

 

[rems]

i would like to work out the in between temperatures also.
Thanks

 

[ericgibbs]

hi rems,
Looking at your circuit, it appears you using OPA's between the NTC circuit and the PIC, what type of OPA is it, I cannot make it out on the drawing.

By reconfiguring the OPA's you could improve the resolution of the Therm/Temperature operation.

E.

 

[rems]

the opam i used is OPA350 series

 

[Ian Rogers]

Here's a good start... I've enhanced your code

#include<htc.h>
__CONFIG(0x2F84);
__CONFIG(0x1EFF);
#define _XTAL_FREQ 4000000	// I used 4mHz
#define STX 20				// change these to suit
#define ETX 21


void ADCInit()
	{
	ANSELA = 0xF;
	ANSELB = 0;
	ANSELE = 0;
	ADCON1 = 0xA0;	
	}

unsigned int ADCread(unsigned char ch)
	{
	if (ch>13) 		// i have 4 thermistors voltage connected 
		return 0;	// to input of pic16f1937
	ADCON0 = 0;
	ADCON0 = ch<<3; // select ADCchannel
	ADCON0 = 1;		// swich on adc module
	NOP();
	NOP();
	GO=1;			//START CONVERSION
	while(GO); 		//waiting conversion to finish
	ADCON0=0;
	return (int)ADRESH *256 + ADRESL;
	}

long GetADC(unsigned char ch)	// oversample 24 times
	{
	char loop = 23;
	long result = 0;
	while(loop--)
		result += ADCread(ch);
	return result;
	}

void HSerinit()
	{
	TRISC = 0xC0;					// should really be set
	SPBRG = 25;						// 4mhz xtal 9600 BAUD
	TXSTA = 0x24;					// TXEN and BRGH
	RCSTA = 0x90;					// SPEN and CREN
	}

unsigned int getTemp() 	// This will work out the temp
	{					// Its in fixed point so 663 is 66.3 degrees
	int idx;
	long High,Low, Actual;
	float temp1,temp2;
	int tempArray[] = {18436,17305,16101,14845,13570,12288,11048,9874,
						8774,7754,6826,5984,5235,4579,4005,3500};
			// these are oversampled values
	High = Low = tempArray[0];
	Actual = GetADC(0);
	for (idx=0;idx<16;idx++)
		{
		if( Actual < tempArray[idx])
			{
			Low = tempArray[idx+1];
			High = tempArray[idx];
			}
		else
			{
			if(High == Low) return 0;	// this bit works out a percentage
			temp1 = High - Low;			// and fills in the gaps
			temp2 = High - Actual;
			temp1 = temp2 / temp1;
			temp2 = (50 * temp1) + ((idx-1) * 50);
			return (unsigned int) temp2;	// returns 0.0 to 75.0 (fixed )
			}								// remember the decimal point.
		}
	return 750;		
	}

void SendTemp(unsigned int temp)
	{
	unsigned char CRC=0, idx;
	unsigned char sendArray[6];
		// Change this to suit YOUR application
	sendArray[0] = STX;
	sendArray[1] = 2;
	sendArray[2] = temp / 256;
	sendArray[3] = temp % 256;
	sendArray[5] = ETX;
	for(idx = 1;idx<4;idx++)
		CRC^= sendArray[idx];
	sendArray[4] = CRC;
	for(idx = 0;idx<6;idx++)
		{
		while(!TXIF);
		TXREG = sendArray[idx];	// send to PC
		}
	}
	
void main()
	{
	unsigned int temperature;
	OSCCON = 0x3B; 		// internal 4 meg
	NOP();
	NOP();
	HSerinit();			// set up Tx ports
	ADCInit();			// set up adc
	while(1)
		{
		temperature = getTemp();		// get actual temp
		SendTemp(temperature);			// send
		}
	}

 

[rems]

Thank You very much

 

[rems]

could you please explain me how to get the oversampled values.

 

[Ian Rogers]

Oversampling just means... read many times then divide... You take the mean value ie..

I read 24 times so I get a really stable reading then I use that high value to scale down to a useful reading to display

 

[rems]

could you please tell me what will be the interpolation formulae to read temperature using adc values.