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4-MULTIPLEXED LCD WITH A STANDARD ST62

2.3 EXAMPLE OF APPLICATION TIMING – A SIMPLE CLOCK

As explained in Section 2.1, the main application is in charge of providing the time base to the

LCD driver. Because this time base must be relatively precise, it is usually generated by a spe-

cific sub-system of the application, either internal to the MCU (timer peripheral) or external

(e.g. external clock source, RC network, etc.). In both cases, MCU resources are dedicated to

it (peripheral if internal, pins if external). This part describes a solution to share a single time

base between the LCD driver and the main application. Sharing is usually necessary if the

ST6 device has few resources.

The main application is a real time 24-hour clock to be displayed on the LCD. In order to use

as few pins as possible, the time base is generated by a standard 8-bit timer clocked by the

MCU internal clock.

The real time clock requires frequencies far lower than the LCD driver. Consequently, the time

base runs at the frequency required by the LCD driver. Then, this frequency is divided by soft-

ware counters to reach a period of 125 ms, then divided again to reach periods of 0.5 second,

1 second, 1 minute and 1 hour. The half-second period is involved in making some LCD seg-

ments flash at 1 Hz. For example, when the clock is running, the colon flashes at that fre-

quency.

The standard timer is configured in output mode with interrupts enabled, so that the timer In-

terrupt Service Routine (ISR) is called every 1.5 ms. This routine calls the LCD_Do routine. As

a result, an LCD cycle (8 phases) lasts 1.5*8 = 12 ms, so LCD voltage frequency is 83 Hz,

which is in the required range.

Reaching a period of 125 ms requires dividing the timer interrupt frequency by 250/3. To do

so, the timer ISR decrements a counter (RAM variable) three times. After each decrement, it

checks if the counter has reached 0 or not. If it has, the ISR calls an RTC subroutine. This sub-

routine reloads the counter with 250.

The RTC subroutine, called once every 1/8 s, performs several frequency divisions to update

the second, minute and hour counters. Also, it calls LCD_Flash once every half second.