UPC EETAC Bachelor's Degree in Telecommunications Systems and in Network Engineering EEL


Lecture 1

L10. 1. FSM adaptation. External interrupts (INT)

[P10] FSM design programming in C. Detecting events (signal edges) using external interrupts.



3.5. FSM implementation in C language

3.5.1. Specifications FSM concept adaptation to µC.

Fig. 1 shows Chapter 2 FSM architecture as it was studied and applied in many applications to solve sequential systems. 

FSM architecture

Fig 1. FSM architecture.

Now, we propose to adapt the same architecture to software environment as shown in Fig. 2.

Adapting the concept of FSM

Fig 1. FSM adaptation rec: interrupts will allow detection of events (signal edges) and will make it possible to run state diagrams systematically programmed in C, as we did in Chapter 2.

Current state as RAM memory variable. CLK interface using interrupts

How to detect signal edges.

3.5.2. Planning Hardware-software diagram Interrupts: interrupt service routine ISR(). External event detection (CLK and push-button interface)

The key concept of external interrupt (INT) to the main program. Pay attention to the idea of external interrupt to detect a signal edge at the input pins and how they are handled by the μC in hardware (INT0IF, INT0IE, GIE) and once acknowledged at the ISR() software (Var_CLK_flag). The concept of the interrupt vector to execute the ISR() special function. The concepts of nested interrupts, stack memory and stack overflow.

Enabling and disabling interrupts in init_system(). output_logic() and write_outputs(). state_logic() and read_imputs()


3.5.3. Development & testing



- Carmely, T., Using finite state machines to design software, EDN, March 30, 2009

- Bergfeld, D., Software design of state machines , Embedded.com, April 16, 2019


Exercise: Detect the duration of an active-low push-buttons PB_L click using two interrupt flags. How to configure int_system(), what is the software flowchart for ISR()? What kind of processing the FSM can do?