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 a thermometer. 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 the picture.

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!