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BUILDING AN EMBEDDED TIMING SYSTEM

by Jamie Pollock

Start ı Problem Solving ı EEPROM Programming ı Some Assemblyı ı Interfacing the LCD ı Sources and PDF

Single chip solutions using the MC68HC11 series microcontrollers can be cost effective for many applications. For the beginning designer or hobbyist, most development systems are beyond practical consideration due to their high cost.

Although development systems are robust with a great diversity in hardware and software, a few application notes and some wire wrap can go a long way. The system here can be built for under $20, excluding the LCD.

Besides cost, time-to-market is also important. If you canıt see progress in your efforts, it becomes difficult to maintain good motivation. Projects like LCD-displayed temperature, a lawn-sprinkler controller, or a simple stopwatch are inexpensive projects with great learning potentials.

In the petroleum industry, the viscosity of a product is important. The viscosity baths I use hold six tubes. Two baths are needed, one at 100ıC and another at 40ıC, to characterize the fluids.

If I try to test six products then I need 12 timers. This can become confusing and more than once Iıve had to restart the test because of carelessness.

This project will create five independent stopwatches on a 40-character LCD display. The controls for each timer will be directly below the LCD screen. A start/stop button and a clear button for each timer will be sufficient for measuring viscosity time.

The display will show seconds and milliseconds because those increments are necessary for the calculation of viscosity. Four characters are allotted for the seconds and two for the milliseconds. The timers will rollover at 9999.99 or 2.77 hours. A typical viscosity test takes about 300 s.

The minimum configuration for the MC68HC11 consists of three partsıthe microcontroller, a crystal oscillator and a low voltage reset IC. First, the controller must be chosen. For the simple stopwatch project, I chose the MC68HC11F1CFN3 primarily because it has enough I/O ports to easily get the job done.

Second, a 12-MHz crystal oscillator is needed to give an internal clock of 3 MHz. (The last digit in the controller part number is its maximum speed). The ı6811 divides the external oscillator by four to generate the internal clock.

Third, a MC34064 (low-voltage reset) is needed to properly initialize the controller. This is specific to the ı6811 line, other microcontrollers donıt require this.

Finally, pull-up resistors are needed on the XIRQ, IRQ, and any input ports. The extras include an OPTREX DMC 40218 40 ı 2 character LCD and a homemade keypad. The LCD was taken from a dead typewriter and the buttons for the keypad were taken from an old printer. Recycling old parts is extremely important for hobbyists.

Because this project is a single-chip solution, the controller will be used in a special mode called bootstrap mode. This mode allows the controller to execute a 256-byte program in ROM at startup, which Motorola created.

The Motorola code initializes the serial communications interface allowing an RS-232 transceiver like the MC145407 to connect the controller to a PC port.

Bootstrap mode provides a means to program the RAM. This code receives a byte from the RS-232 port and places it into RAM in sequential order, starting at location $0000. The code uses a timer to exit the loop. Once the timeout is activated, the controller jumps to the beginning of RAM executing the downloaded program. Bootstrap code listings are available on the Internet for most of the ı6811 line. Studying the listing is essential to understanding the bootstrap operation.

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