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ST7LITE0xxx MICROCONTROLLED BALLAST

1.2 DIGITAL SECTION

The ST7LITE0xxx microcontroller has been chosen as it is small and easy to use (please refer

flash program memory, 128 bytes of RAM and moreover it has 128 bytes of EEPROM of us-

able memory, for example, for storing lamp parameters. There is no need for any external

clock circuitry, because it has an integrated RC oscillator with an accuracy of 1%. This oscil-

lator provides a clock signal up to 8MHz. What makes this microcontroller a strong tool for the

ballast application are the peripherals: Autoreload Timer, Lite Timer and Analog to Digital

Converter. Foremost, it must be highlighted that the software drivers for all the peripherals are

you do not need to spend time to develop your own.

The Autoreload timer is a peripheral which controls a PWM output from the microcontroller.

The principle of its function is shown in Figure 3. At the heart of the autoreload timer is a free-

running counter, which works absolutely independently from the processor core. For de-

signers, there are only two important values: “Reload Value Register” and “Compare Value

Register”. The counter increments its value to the maximum. When it is reached, it switches

the output pin to the logical 1 (5V) and after that the counter starts incrementing again from the

value stored in the already mentioned “Reload Value Register”. After each increment, the

counter is compared to the “Compare Value Register”. If the match occurs, the output pin is

switched to the logic value 0 (0V).

Figure 3. PWM Function

From Figure 3, it can be seen that the control of the PWM signal is very simple with

ST7LITE0xxx through just the two registers: by writing into the Reload Register, you select the

frequency and by writing into the Compare Register you can select the duty cycle. This way,

the frequency can be selected from 2kHz up to 4MHz. The incremental period change is

125ns with fclock = 8 MHz.

In addition to the autoreload timer, there is also another generic timer available. This timer

(called Lite Timer) is a free running counter generating a software interrupt every 1ms. There

4095

000

Output

pin

Reload Value

Register

Compare Value

Register

t