AN3288

System architecture

Doc ID 18011 Rev 2

5/27

2

System architecture

The main block of the g-Brake Lights platform is made up of the ST MCU STM8AF5168, the

ST MEMS-based inertial sensor AIS326DQ, and step-down switching regulator A5970AD;

its task is to continuously acquire data, elaborate them distinguishing the several phases of

motion, and actuate a proper light response to warn of the braking phase.

The basic functioning of the system is presented in the following paragraphs.

2.1

System description

The g-Brake Lights platform provides an optical representation of the deceleration detected

by the accelerometer AIS326DQ during the braking phase of a vehicle; the light intensity of

the brake lamp, implemented by the two high brightness LED arrays (each of them is made

up of three high brightness LEDs connected in series), varies according to the braking

intensity of the vehicle. The aim is to make the driver's intentions more visible, in order to

provide the following drivers with faster reaction times.

The board has been designed to acquire the acceleration/deceleration data continuously

and independently from the vehicle's braking circuit. This allows an autonomous system

with a fast real-time response to be had. The response time mainly depends on the data

elaboration made by the STM8A microcontroller after the acquisition.

A digital low-pass filter has been implemented to reduce noise corruption during the data

collecting; this noise is the result of a vehicle's high frequency vibrations due to the engine

and the road. The filtered signal is more intelligible and its representation fits with the real

behavior of the vehicle. A mechanical isolation could also be applied, even though it is not

strictly necessary, to further filter the noise.

Finally, a combination of dynamic thresholds enable the variation of light intensity. The

deceleration data, after filtering, are continuously compared with specific ranges of values in

order to discriminate, case by case, the braking forces of the vehicle. The two arrays of HB-

LEDs are used as actuators in order to maximize the visibility of the warning and reduce the

turn-on and turn-off time to a minimum.

2.2

Block diagram

The core of the g-Brake Lights solution is made up of the ST MCU STM8AF5168, the ST

MEMS-based inertial sensor AIS326DQ, and two buck converters using the ST step-down

switching regulators A5970AD for LED driving.

Basically, the MCU continuously elaborates the information provided by the accelerometer

and generates a PWM signal for driving the two buck converters connected to the two HB-

LED arrays. The MCU varies the PWM signal duty cycle, depending on the braking effects,

on the basis of a set of thresholds. So, the brightness of the two HB-LED arrays is directly

proportional to the detected deceleration on the basis of the variation of the PWM signal

duty cycle.

The HB-LED arrays have been mounted on the secondary board in order to have a

separation between the sensing block and the light actuator, and, at the same time, a more

flexible mechanical system for assembling on the test vehicle.