Building a Digital Thermometer

Now that you understand a little bit about your Stamp and the LCD, we can add another component and create a digital thermometer. To create a thermometer, we will use a chip called the DS1620. This chip contains:

• A temperature-sensing device
• An analog-to-digital (A/D) converter for the temperature-sensing device
• A shift register to read the data out of the A/D converter
• A little EEPROM (electrically erasable programmable read-only memory) to remember settings

The DS1620 has two modes: In one mode, it acts as a stand-alone thermostat chip, and in the other mode you hook it up to a computer and use it as athermometer. The EEPROM remembers the current mode as well as the set temperatures for the thermostat.

Hooking up the DS1620 to the Stamp is very easy. The DS1620 comes in an 8-pin chip. Supply +5 volts from the Stamp to pin 8 of the DS1620. Supply ground to pin 4 of the DS1620. You then use three I/O pins from the Stamp to drive three pins on the DS1620:

• Pin 1 on the DS1620 is the data pin. You read and write data bits on this pin.
• Pin 2 on the DS1620 is the clock pin. You clock data in and out of the shift register with this pin.
• Pin 3 on the DS1620 is the reset/select pin. You set pin 3 high to select the chip and communicate with it.

For this example code, it is assumed that:

• The data pin goes to I/O pin 2 on the Stamp.
• The clock pin goes to I/O pin 1 on the Stamp.
• The reset/select pin goes to I/O pin 0 on the Stamp.

The completed wiring looks like this:

You can get a DS1620 either from Jameco (part number 146456) or Parallax (part number 27917) in an "application kit" that includes the chip, the capacitor, some good documentation and sample code. Or you can buy the chip on its own from Jameco (part number 114382). I would suggest getting the application kit the first time you try using the DS1620 because the documentation is very useful.

You can assemble the DS1620 in the prototype area of the Stamp carrier board or on a separate breadboard. Once you have assembled it, hook your LCD display up to I/O pin 3 of the Stamp, and then load and run the following program:

symbol RST = 0 ' select/reset line on 1620

symbol CLK = 1 ' clock line for shift registers on 1620

symbol DQ = 2  ' data line on 1620

symbol DQ_PIN = pin2 ' pin representation for DQ

symbol LCD = 3 ' data line for LCD

begin:

low RST     ' deselect the 1620 unless talking to it

high CLK    ' clock pin on 1620 should default high

pause 1000  ' wait for the thermometer and LCD to boot

setup:

high RST        ' select the 1620

b0 = $0C        ' $0c is the 1620 command byte

                '   saying "Write Config"

gosub shift_out ' send it to the 1620

b0 = %10        ' %10 is the 1620 command byte

                '   to set thermometer mode

gosub shift_out ' send it to the 1620

low RST         ' deselect the 1620

pause 50        ' delay 50ms for EEPROM

start_convert:

b0 = $EE        ' $EE is the 1620 command byte

                '   to start conversions

high RST        ' select the 1620

gosub shift_out ' send it to the 1620

low RST         ' deselect the 1620

' This is the main loop

' - reads and displays temperature every second

main_loop:

  high RST        ' select the 1620

  b0 = $AA        ' $AA is the 1620 command byte

                  '   for reading temperature

  gosub shift_out ' send it to the 1620

  gosub shift_in  ' read the temperature

                  '   from the 1620

  low RST         ' deselect the DS1620.

  gosub display   ' display the temp in degrees C

  pause 1000      ' wait a second

goto main_loop

' The shift_out subroutine sends whatever is in

' the b0 byte to the 1620

shift_out:

output DQ           ' set the DQ pin to

                    '   output mode

for b2 = 1 to 8

  low CLK           ' prepare to clock the bit

                    '   into 1620

  DQ_PIN = bit0     ' Send the data bit

  high CLK          ' latch data bit into 1620

  b0 = b0/2         ' shift all bits right

                    '   toward bit 0

next

return

' The shift_in subroutine gets a 9-bit

' temperature from the 1620

shift_in:

input DQ            ' set the DQ pin to

                    '   input mode

w0 = 0              ' clear w0

for b5 = 1 to 9

  w0 = w0/2         ' shift input right.

  low CLK           ' ask 1620 for next bit

  bit8 = DQ_PIN     ' read the bit

  high CLK          ' toggle clock pin

next

return

' Displays the temperature in degrees C

display:

if bit8 = 0 then pos      ' if bit8=1

                          '   then temp is negative

  b0 = b0 &/ b0           ' invert b0 by NANDing it

                          '   with itself

  b0 = b0 + 1

pos:

serout LCD, n2400, (254, 1)    ' clear the LCD

serout LCD, n2400, ("Temp = ") ' display "Temp="

                               '   on the display

bit9 = bit0                    ' save the half degree

b0 = b0 / 2                    ' convert to degrees

if bit8 = 1 then neg           ' see if temp is negative

  serout LCD, n2400, (#b0)     ' display positive temp

  goto half

neg:

  serout LCD, n2400, ("-", #b0)' display negative temp

half:

  if bit9 = 0 then even

    serout LCD, n2400, (".5 C") ' display the half degree

    goto done

even:

    serout LCD, n2400, (".0 C") ' display the half degree

done:

return

If you run this program, you will find that it displays the centigrade temperature with an accuracy of one-half degree.

The DS1620 measures temperatures in centigrade half-degrees. It returns the temperature in a 9-bit 2s-complement number with a range of -110 to 250 F (-55 to 125 C). You divide the number you receive by 2 to get the actual temperature. 2s-complement binary numbers are a convenient way to represent negative values. The following list shows the values for a 4-bit 2s-complement number:

0111 : 7

0110 : 6

0101 : 5

0100 : 4

0011 : 3

0010 : 2

0001 : 1

0000 : 0

1111 : -1

1110 : -2

1101 : -3

1100 : -4

1011 : -5

1010 : -6

1001 : -7

1000 : -8

You can see that instead of the 4 bits representing values from 0 to 15, the 4 bits in a 2s-complement number represent the values -8 to 7. You can look at the left-most bit to determine if the number is negative or positive. If the number is negative, you can invert the bits and add 1 to get the positive representation of the number.

Here's what goes on with the digital thermometer program shown here:

1. It uses the symbol keyword to set up several constants that make the program slightly easier to read (and also make it easy for you to move the chip to different I/O pins on the Stamp).
2. It sets the CLK and RST pins on the DS1620 to their expected values.
3. It writes a command byte to the EEPROM on the DS1620 to tell the chip to operate in "thermometer mode." Because the mode is stored in EEPROM, you only have to do it once, so you could technically take this section of the code out of the program after you run the program once (to save program space).
4. The program sends the command $EE ("$" means "hexadecimal number" -- $EE is 238 in decimal) to tell the thermometer to start up its conversion process.

The program then enters a loop. Every second, it sends a command to the DS1620 telling the DS1620 to return the current temperature, and then it reads the 9-bit value that the DS1620 returns into the w0 variable. The Stamp sends and receives data 1 bit at a time by toggling the CLK line on the DS1620. Remember that the w0 (16-bit) variable overlays the b0/b1 (8-bit) variables, which overlay the bit0/bit1/.../bit15 (1-bit) variables, so when you insert a bit from the DS1620 into bit 8 and divide w0 by 2, what you are doing is shifting each bit to the right to store the 9-bit temperature from the DS1620 into w0. Once the temperature has been saved in w0, the display subroutine determines whether the number is positive or negative and displays it appropriately on the LCD as a centigrade temperature. The conversion from degrees C to degrees F is:

dF = dC * 9/5 + 32

At this point, we have succeeded in creating an extremely expensive thermometer. What might you do with it? Here's one idea. Let's say you work for a drug company and you are shipping expensive drugs across the country that MUST remain at a certain temperature the entire way or the drugs will spoil. What you can do with a Stamp is create a data logging thermometer. Both Jameco (part number 143811) and Parallax(part number 27960) sell a device called the "RAM Pack module." It contains a low-power 8-kilobyte (or optionally 32-kilobyte) RAM chip with a serial interface. You could add this component (or something similar) to your Stamp and write code that saves temperature readings to the RAM every minute. You could then slip your Stamp into the drug shipment, and at the other end of the trip retrieve the Stamp. The RAM module would contain the temperature history of the entire trip and you would know whether or not the drugs ever thawed out.

There are all kinds of neat, useful devices like this that you can build with a Stamp now that you know how microcontrollers work!

Reference:

howstuffworks.com