AN1353

Main program

Rev 2

3/17

3

Main program

3.1

Mains period measurement

As the board does not embed an oscillator or resonator, the internal resonator of the MCU is

used to achieve the clock. But, in this case, the running frequency is given within a range of

20%. This is not enough to ensure an optimum pulse gate current control with a power

consumption as little as possible.

To increase the timer accuracy, the MCU uses the Zero Voltage Crossing (ZVC) events to

have time infor-mation. The LINE voltage is connected to the NMI pin through a high

impedance resistor. An interruption will then occur at each ZVC event. The MCU has just to

launch the timer decrementation between two NMI interrupts to calculate how much one

must load the TCR register to count down 20 ms.

Of course, in normal operation, the timer can be used for other tasks than counting the

mains period. The period measurement will then be based on the rest of time from the last

timer utilization and the next NMI interrupt. This measured time is saved as DELTAT (see

Figure 2) by the software. The 20 ms will then equal the DELTAT, plus the sum of times T1 to

T3, plus the time lost due to calculator operations between each timer stop and launch (see

Section 4.3).

Note:

Such a method is only valid when the mains frequency is know in advance; i.e. for a board

dedicated to one range of AC mains voltage. In our case, the software and hardware are

dedicated to 220/240 V 50 Hz applications.

Table 2.

Port A configuration registers

Pin Name

USE

DDR

OR

DR

PA1

START ACST6 ON / START ACST6 OFF

1

1

0/1

PA2

RUN ACST6 ON / RUN ACST6 OFF

1

1

0/1

PA3

LIGHT BULB ON / LIGHT BULB OFF

1

1

0/1