How Microcontrollers Work

Creating a Really Expensive Digital Clock

Spending $79 to flash an LED may seem extravagant to you. What you would probably like to do is create something useful with your BASIC stamp. By spending about $100 more you can create a really nice digital clock! This may seem extremely extravagant, until you realize that the parts are reusable in a variety of other projects that you may want to build later.

Let's say that we would like to use the I/O pins on the BASIC Stamp to display numeric values. In the digital clock article, we saw how to interface to a 7-segment LED display using a 7447 chip. 7447s would work just as well with the BASIC Stamp. You could wire four of the I/O pins straight into a 7447 and easily display a number between 0 and 9. Since the BS-1 Stamp has eight I/O pins, it is easy to drive two 7447s directly like this.


For a clock, we need a minimum of four digits. To drive four 7447s with eight I/O pins, we have to be slightly more creative. The following diagram shows you one approach:

In this diagram, the eight I/O lines from the Stamp enter from the left. This approach uses four lines that run to all four 7447s. Then the other four lines from the Stamp activate the 7447s in sequence ("E" on the chips means "Enable" -- on a 7447, that would be the blanking input on pin 5). To make this arrangement work, the BASIC program in the Stamp would output the first digit on the four data lines and activate the first 7447 by toggling its E pin with the first control line. Then it would send out the value for the second digit and activate the second 7447, sequencing through all four of the 7447s like this repeatedly. By wiring things slightly differently, you could actually do this with only one 7447. By using a 74154 demultiplexer chip and some drivers, you could drive up to 16 digits using this approach.

This is, in fact, a standard way to control LED displays. For example, if you have an old LED calculator, turn it on and shake it while watching the display. You will actually be able to see that only one digit is ever illuminated at once. The approach is called multiplexing the display.

While this approach works fine for clocks and calculators, it has two important problems:

  • LEDs consume a lot of power.
  • 7-segment LEDs can only display numeric values.

An alternative approach is to use an LCD screen. As it turns out, LCDs are widely available and can be easily hooked to a Stamp. For example, the two-line by 16-character alphanumeric display shown below is available from both Jameco (part number 150990) and Parallax (part number 27910). A typical display is shown here, mounted on a breadboard for easier interfacing:

This sort of LCD has several advantages:

  • The display can be driven by a single I/O pin. The display contains logic that lets a Stamp communicate with it serially, so only one I/O pin is needed. In addition, the SEROUT command in Stamp BASIC handles serial communication easily, so talking to the display is simple.
  • The LCD can display alphanumeric text: letters, numbers and even custom characters.
  • The LCD consumes very little power -- only 3 milliamps.

The only problem is that one of these displays costs $59. Obviously, you would not embed one of these in a toaster oven. If you were designing a toaster oven, however, you would likely prototype with one of these displays and then create custom chips and software to drive much cheaper LCDs in the final product.

To drive a display like this, you simply supply it with +5 volts and ground (the Stamp supplies both from the 9-volt battery) and then hook one of the I/O pins from the Stamp to the display's input line. The easiest way I have found to connect the Stamp's I/O pins to a device like an LCD is to use a wire-wrap tool (Jameco part number 34577) and 30-gauge wire wrap wire (Jameco part number 22541 is typical). That way, no soldering is involved and the connections are compact and reliable.

The following BASIC program will cause a BASIC Stamp to behave like a clock and output the time on the LCD (assuming the LCD is connected to I/O pin 0 on the Stamp):

pause 1000                 'wait for LCD display to boot
serout 0, n2400, (254, 1)  'clear the display
serout 0, n2400, ("time:") 'Paint "time:" on the display
                'preset before loading program
b0 = 0                     'seconds
b1 = 27                    'minutes
b2 = 6                     'hours

b0 = b0 + 1                'increment seconds
if b0 < 60 then minutes
  b0 = 0                   'if seconds=60
  b1 = b1 + 1              ' then increment minutes
if b1 < 60 then hours
  b1 = 0                   'if minutes=60
  b2 = b2 + 1              '  then increment hours
if b2 < 13 then show
  b2 = 1                   'if hours=13 reset to 1

serout 0, n2400, (254, 135) 'position cursor on display,
                            'then display time
serout 0, n2400, (#b2, ":", #b1, ":", #b0, "    ")
pause 950                     'pause 950 milliseconds
goto again                    'repeat

In this program, the SEROUT commands send data to the LCD. The sequence (254, 1) clears the LCD (254 is the escape character and 1 is the command to clear the screen). The sequence (254, 135) positions the cursor. The other two SEROUT commands simply send text strings to the display.

This approach will create a reasonably accurate clock. By tweaking the PAUSE statement you can get the accuracy to within a few seconds a day. Obviously, in a real clock you would like to wire up a push-button or two to make setting it easier -- in this program, you preset the time before you download the program to the Stamp.

Pocket Watch B Module

While this approach is simple and works, it is not incredibly accurate. If you want better accuracy, one good approach would be to wire a real-time clock chip up to your Stamp. Then, every second or so, you can read the time from the chip and display it. A real-time clock chip uses a quartz crystal to give it excellent accuracy. Clock chips also usually contain date information and handle leap year correction automatically.

One easy way to interface a real-time clock to a stamp is to use a component called the Pocket Watch B.

The Pocket Watch B is available from both Jameco (part number 145630) and Parallax (part number 27962). This part is about as big as a quarter and contains the clock chip, crystal and a serial interface so that only one I/O pin is necessary to communicate with it. This component costs about $30 -- again, not something you want to embed in a toaster oven, but easy to play with when constructing prototypes.