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BUILDING AN EMBEDDED TIMING SYSTEM

by Jamie Pollock

Start ý Problem Solving ý EEPROM Programming ý Some Assemblyý ý Interfacing the LCD ý Sources and PDF

INTERFACING THE LCD

Interfacing the LCD can pose certain challenges. With the Optrex display, the technical manual for the LCD controller is needed. The manual for the Hitachi HD44780 can be found on the Internet. Complete descriptions of all LCD functions are explained in the manual, but a good understanding of the LCD is necessary for reliable operation.

At the beginning of the program, the LCD is initialized. This consists of placing data on the LCD bus and toggling the E pin on the LCD. The code sets up the LCD for an 8-bit data bus, then chooses two lines and a 5 ý 7 font. Next, the display is turned on (in software), then auto-increment and cursor are picked. After these control commands are accepted, the busy flag is in operation.

The busy flag enables the LCD to receive data as quickly as possible. The use of delay loops as opposed to busy-flag checking reduces the efficiency of the code. Busy-flag checking allows the LCD to be updated inside the TOC1 ISR.

Listing 3 is the routine that writes characters to the LCD. This code receives the RAM location for the character in register Y and continues to send characters until the RAM location contains a 0. This arrangement has multiple uses. First, a string of text can be sent, or the result of a Hex to Decimal conversion (derived from the Motorola 6811 manual) can be sent. The routine first sends the data to Port C then toggles the E pin.

Next, Port C is changed to an input port and the busy flag bit is monitored until it clears. Once the busy flag has cleared, the LCD autoincrements its display address. A small delay loop of 120 cycles (by trial and error) allows the LCD to update. This routine can send the seconds, a decimal point, the milliseconds, and a space, five times. The HOME command, which returns the cursor to position 1 without clearing the display, is then sent.

Control commands require more time to complete so the DLY1 delay subroutine is called after a Control command has been sent. This allows the character-write loop to operate at its fastest speed.

Photo 1—Under the LCD is asocketed 68HC11F1, and the other supporting components. The dial is a contrast control which came mounted to the LCD.

Photo 1shows the completed project. Now all that is left is to build a case to protect the components.

With the use of the Internet and freeware, many obstacles for developing microcontroller solutions are eliminated. Wire wrapping prototype boards with few components can allow experimentation on the smallest of budgets. Realistically, $20 and a little time can produce stand-alone single chip products for many different applications.

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