Direct LCD Drive

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The software for the LCD display operates in interrupt mode, driving I/O pins by the CPU at in-

stants defined by the Timer1 interrupts. When the LCD drive tasks are finished, the main appli-

cation program can continue.

The major tasks of the LCD display routine are:

The backplane pattern sequences are different between ST6215x and ST6265x examples.

The ST6215x software is based on non-symetrical signals (see Figure 4). The backplane pat-

terns are symetrical in the ST6265x software. Both are equivalent on the LCD viewpoint.

The ROM code size used is 300 bytes for the program and 256 bytes for the tables. With an

CPU frequency of 8MHz, the display task duration is 240

μs and the full task duration including

decimal conversion is 510

μs. With an LCD period signal of 14ms, the CPU duty cycle of occu-

pation is 2.5% for the LCD drive task. The program can be adjusted to other applications by

modifying the timer duration (FASTIM) and the segment drive byte table. The LCD drive phase

may also be synchronized to the mains zero crossing or a software loop duration, saving the

timer for other tasks.

Summary

The examples presented in this note show that a simple LCD can be driven directly by standard

ST621x/2x/6x/9x microcontrollers. The ST62’s flexible I/O configuration and the large voltage

range of operation of the ST62 family MCU allow an LCD driver to be achieved with very few

surrounding components, small CPU time occupation and reduced ROM program size.

Such an approach is a very cost effective solution for simple LCD displays operating with a

multiplexing rate of 1 or 2 and up to 36 segments. For LCDs requiring more segment drive ca-

pability and/or higher multiplexing rates ST624x and ST628x provide highly integrated and

easy to implement solutions.

Such LCD drive features can easily be included in a larger application including keyboard inter-

face, sensor display or motor control.

We thank the company Akotronic for the ST6215x application example they have developed

for this note.

Annex 1: Software of the ST6215x based application

Annex 2: Flowchart and software of the ST6265x based application