Main program

AN1353

4/17

Rev 2

Figure 2.

Timing definition

3.2

Subroutines execution time checking

In order not to miss the timer interrupt events, the CPU must be completely free and ready

to check the timer interrupt flag. This means that all subroutines must be completed before

the expected end of the timer decrementation.

Figure 2 shows that the subroutines are placed at different moments, depending on their

length. For exam-ple, the longest CPU action is when the MCU calculates the T1 to T5

delays. This action can last up to more than 7.2 ms for a 4 MHz MCU clock frequency. Then,

there is not enough time available between two timer interrupts to calculate these five

values. This is the reason why the delay calculation subroutine has been split into two parts

(Calcul-Delay_1 and Calcul-Delay_2). These two parts are respectively placed during T4

and DELTAT decounts

Table 3 gives the maximum duration of each subroutine (for a 4 MHz clock frequency, and

the for the lon-gest software loops).

Table 4 gives the code execution maximum times for all the instructions written in the

software, before each subroutine (“code execution time” column). Then, according to the

implemented durations, the time, still available for the CPU, is given with a 0.2 ms safety

margin.

Subroutines

PA1 / PA2

PA3

T1 T2

T3

T4

T5

DELTAT

NMI

Calcul

Delay 1

Motor

Status

Current

Measure

Calcul

Delay 2

Loads

Control

Mains Voltage

Table 3.

Subroutines maximum durations

SUBROUTINE NAME

MAXIMUM TIME

Loads_Control

0.93 ms

Motor_Status

0.26 ms

Current_Measure

0.92 ms

Calcul_Delay_1

1.75 ms

Calcul_Delay_2

5.49 ms