Firmware design

AN3288

10/27

Doc ID 18011 Rev 2

3

Firmware design

Once the system architecture is defined, firmware implementation is fundamental because

the MCU controls the several devices and manages the resource optimization; the MCU

sets the system clock and enables the SPI peripheral for the data transferring with the

accelerometer. The choice of a state machine has allowed the three main process steps

(acquisition, elaboration, and actuation) to be put into three states, making the algorithm

more flexible and easy to manage.

The algorithm designed for controlling the system and the data elaboration and light

actuation during braking is described in the following paragraphs.

3.1

State machine

The state machine organizes the operations which take place in this application. It has been

designed as event-driven systems based on three main states called ACQUIRE,

PROCESS, and ACTUATE. At any given time, the application exists in one of these three

states.

The block diagram of the designed state machine is shown in Figure 8.

Figure 8.

State machine block diagram

After the system initialization, the algorithm uses polling within a loop to determine when to

read data. In the first state (ACQUIRE), the data acquisition is performed by the MCU which

continuously receives data from the accelerometer and puts them in a buffer. In the

PROCESS state, first the MCU elaborates data, filtering them from high frequency noise

caused by mechanical vibrations (see Section 3.2), and then compares the filtered values

with a thresholds equation system to determinate the effort of the braking.

Finally, in the ACTUATE state, the MCU performs the light actuation. As anticipated, the

system is event-driven but the MCU executes the third state only if the deceleration value is

higher than the activation level; this is a useful solution in order to avoid accidental actuation

due to harsh road conditions.