Spency's digital-diy 16F PIC micro Tutorial

16F LM35DZ Example

The LM35DZ is an accurate temperature sensor that will provide an analogue output of 10mV per degree C within 0.01% Its not hard to interface one with a PIC, and the following example allows for a temperature range of -55 to 150 deg C.

The datasheet shows many ways to use the LM35DZ, but I'm using the example on page 7 with the -55 to +150 degree C application. It requires 3 additional components, 2 * 1N914 (Or 1N4148), and 1 * 18K resistor.

Here's the circuit

Note the PIC's power supply/oscillator are not shown

ISIS doesn't have a model for the LM35DZ, so I put a signal of 0-2.05V on the output of the LM35DZ to simulate the voltage swing. This is not how the device behaves in real life, and rather has two output references that need to be summed to generate the correct temperature value.

Now its a matter of sampling the voltage, and displaying it on the screen, I also have a custom character made so that it displays the degree symbol on the LCD.

Device 16F877A
Declare XTAL 4

DECLARE ADIN_RES 10         ' 10-bit result required 
DECLARE ADIN_TAD 8_FOSC     ' Set the ADC's clock source
DECLARE ADIN_STIME 50       ' Allow 50us sample time

Declare LCD_TYPE 0          ' Type of LCD Used is Alpha
Declare LCD_DTPIN PORTB.4   ' The control bits B4,B5,B6,B7
Declare LCD_RSPIN PORTB.2   ' RS pin on B2
Declare LCD_ENPIN PORTB.3   ' E pin on B3
Declare LCD_INTERFACE 4     ' Interface method is 4 bit

Dim ADC_Result As Float
Dim ADC_Total As Float
Dim Temp_Float as Float
Dim ADC_Channel as Byte
Dim ADC_Loops as Word
Dim Temp as Word


Dim Last_Result1 As Float
Dim Last_Result2 As Float

ADCON1 = %10000000          ' Set all to analogue inputs (PORTA)
TRISA = $FF ' Declare porta as all inputs

Delayms 150
Cls

Print $FE,$40,$07,$05,$07,$00,$00,$00,$00,$00 ' Custom character for Degree

ADC_Loops = 200

Main:

  ADC_Channel = 1             ' ADC on first reference
  Gosub ADC_Average           ' Perform an averaging to enhance accuracy

  Temp_Float = ADC_Result     ' Store the result

  ADC_Channel = 0             ' ADC on second reference
  Gosub ADC_Average           ' Perform an averaging to enhance accuracy

  ' Convert values into Volts (with a scale of 1000)
  ADC_Result = ADC_Result * 5000 / 1023 
  Temp_Float = Temp_Float * 5000 / 1023   ' to reduce decimal error
  ADC_Result = ADC_Result - Temp_Float    ' And calculate difference

  ADC_Result = ADC_Result / 10 ' Scale back down remembering 10mV = 1 Deg C

  If ADC_Result <> Last_Result1 Then      ' Check if the data has changed
                                          ' and only update display if it has
    Print At 1,1, Dec1 ADC_Result, 0, "C " 
    Last_Result1 = ADC_Result             ' Store new data
  Endif 

Goto Main ' Loop for ever


ADC_Average: ' Perform an averaging on ADC conversions to reduce errors

  ADC_Total = 0 ' Clear summing register
  
  ' Loop for a pre-determined number of times
  For Temp = 1 To ADC_Loops
    ADC_Result = ADIN ADC_Channel      ' Grab a new ADC value
    ADC_Total = ADC_Total + ADC_Result ' Sum it to the total register
    Delayus 1                          ' Allow internal capacitors to discharge
  Next Temp

  ' Determin the average of all the equations
  ADC_Result = ADC_Total / ADC_Loops 

  Return

Where you can get the components;