ST AN594 Application note

AN594

Application note

Direct LCD drive with

ST621xx and ST626xx

Introduction

This note describesatechnique for drivingaLiquid Crystal Display (LCD) withastandard ST62
microcontroller (MCU), without any dedicated LCD driver. This technique offersadisplay capa­bility for applications which require a small display atalow cost together with the versatile ca­pabilities of the standard ST62xx MCU. Higher display requirements are easily handled by
dedicated members of the ST62 MCU family, for example the ST6240B.

The first part of this note describes the typical waveforms required to drive an LCD correctly
withamultiplexing rate of1or2(duplex). The following parts present two solutions based on
standard ST62 MCUs driving directly the LCD. The first is based on an ST6215 without using
software interrupts and the second on an ST6265 where the LCD is controlled by timer inter­rupts.

In both examples the program size, the CPUtime occupation due to the LCD drive and the number
of surrounding components are minimized. Consequently many additional tasks can be added to
the MCU program.

LCD requirements

Withazero Root Mean Square (RMS) voltage applied to it, an LCD is practically transparent.
The LCD contrast, which makes the segments turn dark or opaque and thus “on”, is caused by
the difference between the RMS LCD voltage applied and the LCD threshold voltage, specific
to each LCD type.

The applied LCD voltage must alternate to giveazero DC value in order to ensurealong life

the LCD. The higher the multiplexing rate is, the lower the contrast, also the period of

time of
the signal has to be short enough to avoid visible flickering of the LCD display. The LCD volt­age for each segment equals to the difference between theSand COM voltages (see Figure

1).

Figure 1. Equivalent electrical schematic
of an LCD segment

October 2008 Rev 2

DC value should never be more than

-

100mV. Else time life can be shorten.
Frequency range is 30-2000Hz typically.

-

Less, it flickers; more, consumption
grows.

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Direct LCD Drive

Direct LCD drive

Each LCD segment is connected to an I/O “Segment” and to one backplane common to all the
segments. A display using S segments is driven with S+1 MCU output lines. The backplane is
driven with a signal “COM” controlled between 0 and V

with a duty cycle of 50%.

DD

When selecting a segment “ON”, a signal with opposite polarity to “COM” is sent to the corre­sponding “Segment” pin. When the non-inverted signal “COM” is sent to the “Segment” pin, the
segment is “OFF”. Using an MCU the I/O operate in output mode either at the logic levels 0 or

1.

Figure 2. LCD signals for direct drive

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Duplexed LCD drive

For duplexed drive, two backplanes are used
instead of one. Each LCD pin is connected to
two LCD segments, each one connected on
the other side to one of the two backplanes.
Thus, only (S/2)+2 MCU pins are necessary
to drive an LCD with S segments.

Three different voltage levels have to be gen­erated on the backplanes : 0, V

/2 and VDD.

DD

The “Segment” voltage levels are 0 and Vdd
only. The LCD segment is inactive if the RMS
voltage is below the LCD threshold voltage
and is active if the LCD RMS voltage is
above the threshold voltage. Figure 4 shows
typical Backplane, Segment and LCD wave­forms.

The intermediate voltage V

/2 is only re-

DD

quired for the “Backplane” voltages. The
ST62 I/O pins selected as “Backplanes” are
set by software to output mode for 0 or Vdd
levels and to high impedance input mode for
V

/2. This voltage Vdd/2 is defined by two

DD

equal valued resistors externally connected
to the I/O pin.

By using an MCU with flexible I/O pin config­uration such as the ST6215x or ST6265x,
duplexed LCD drive can be made with only 4
additional resistors.

Direct LCD Drive

Figure 3. Basic LCD segment connec­tion in duplexed mode

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Direct LCD Drive

Figure 4. LCD signals for duplexed mode (used in ST6215 example)

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Direct LCD Drive

Example of duplexed LCD drive with ST6215x

The following example describes the drive in duplexed mode of an LCD with an ST6215. The
software is made in such way that no interrupt is generated and an OFF state for the LCD is
possible. The only components necessary are those to drive the ST62 (oscillator, reset,...) and
four resistors to generate the backplane intermediate voltages.

The ST6215 has 20 I/O pins, thus it is able to drive up to 36 LCD segments and 2 backplanes.
One digit is defined with 8 segments connected to 2 backplanes. Each digit can display 11 val­ues, from 0 to 9 and no display.

Each value to be displayed is associated to a certain LCD waveform. One LCD waveform pe­riod is separated in 4 steps corresponding to the 3 I/O configurations (1-0-input). A look-up ta­ble stores the bytes which relate the I/O configuration to the value to display.

Figure 5. ST6215 based example

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Direct LCD Drive

The following tables show an example of linking one LCD digit of the display with the relevant MCU port se
quences. The second digit follows the same scheme. These examples are for the sample LCD display,
please adapt these tables to your particular LCD.

The digit segments are connected to PB0-PB3 lines in this example. Schematic is shown in Figure 5.
Each digit configuration definesasegment status. Each couple of segments defines a timing sequence to
be output by MCU “Segment” lines (see Figure 4). These sequences are coded inside the ST6215.

Table 1: Example of drive ofadigit with ST6215

Digit Segments to Display Segment Connections

Digit

Required

Digit

Required

a

f b

g

e

c

d

Segments

COM1 ON ON ON ON

COM2 OFF ON OFF ON

T1 0000 0h

T2 0101 5h

T3 1010 Ah

T4 1111 Fh

COM1 OFF OFF OFF OFF

COM2 OFF ON OFF ON

T1 1111 Fh

T2 1010 5h

T3 0101 Ah

T4 0000 0h

Status

Timing Sequences

Timing Sequences

Segment Line PB3 PB2 PB1 PB0

COM1 (PA6) d e f a

COM2 (PA7) OFF c g b

Segments

Status

MCU

Output

Sequences

MCU

Output

Sequences

Digit

Required

Blank

Digit

Required

Segments

COM1 OFF OFF OFF OFF

COM2 OFF OFF OFF OFF

Timing Sequences

T1 1111 Fh

T2 0000 0h

T3 1111 Fh

T4 0000 0h

COM1 OFF OFF ON OFF

COM2 OFF ON ON ON

Timing Sequences

T1 1101 Dh

T2 0111 7h

T3 1000 8h

T4 0010 2h

Status

Segments

Status

MCU

Output

Sequences

MCU

Output

Sequences

-

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Digit

Required

Digit

Required

Segments

COM1 ON ON OFF ON

COM2 OFF OFF ON ON

Timing Sequences

T1 0010 2h

T2 0011 3h

T3 1100 Ch

T4 1101 Dh

COM1 ON OFF OFF ON

COM2 OFF ON ON ON

Timing Sequences

T1 0110 6h

T2 0111 7h

T3 1000 8h

T4 1001 9h

Status

Segments

Status

MCU

Output

Sequences

MCU

Output

Sequences

Digit

Required

Digit

Required

Segments

COM1 ON OFF ON ON

COM2 OFF ON ON OFF

Timing Sequences

T1 0100 4h

T2 0110 6h

T3 1001 9h

T4 1011 Bh

COM1 ON ON ON ON

COM2 OFF ON ON OFF

Timing Sequences

T1 0000 0h

T2 0110 6h

T3 1001 9h

T4 1111 Fh

Status

Segments

Status

MCU

Output

Sequences

MCU

Output

Sequences

Direct LCD Drive

Digit

Required

Digit

Required

Segments

COM1 OFF OFF OFF ON

COM2 OFF ON OFF ON

T1 1110 Eh

T2 0101 5h

T3 1010 Ah

T4 0001 1h

COM1 ON ON ON ON

COM2 OFF ON ON ON

T1 0000 0h

T2 0111 7h

T3 1000 8h

T4 1111 Fh

Status

Timing Sequences

Segments

Status

Timing Sequences

MCU

Output

Sequences

MCU

Output

Sequences

Digit

Required

Segments

COM1 ON OFF ON ON

COM2 OFF ON ON ON

Timing Sequences

T1 0100 4h

T2 0111 7h

T3 1000 8h

T4 1011 Bh

Status

MCU

Output

Sequences

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Direct LCD Drive

Example of duplexed LCD drive with ST6265x

In this example the LCD drive tasks are controlled by interrupts from an on-chip timer. This soft­ware block can be easily included in a central task such as motor control, temperature measure­ment or heating. Figure 6 shows the circuit of the application. Only 4 resistors are added to drive
the LCD in the duplexed mode.

Figure 6. ST6265x based example

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Direct LCD Drive

The software for the LCD display operates in interrupt mode, driving I/O pins by the CPU at in­stants defined by the Timer1 interrupts. When the LCD drive tasks are finished, the main appli­cation program can continue.

The major tasks of the LCD display routine are:

generation of the alternate signals which control the backplanes (PB0, PB1)

-

drive of the LCD segments through PortA

-

conversion of the hexadecimal data to decimal data

-

program Timer1 for the next drive sequence

-

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 pat­terns 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 240μs and the full task duration including
decimal conversion is 510μs. With an LCD period signal of 14ms, the CPU duty cycle of occu­pation 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 ca­pability 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 inter­face, 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

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