Block Diagram
Chapter 3
Implementation
3.1 Block Diagram
The block diagram of the DC fan design is illustrated in Figure 3-1. A 12V low cost bi-phase BLDC motor is used in this application. The MCU performs alternate outputs to the two NPN transistors that drive the motor coils. Open drain output Hall sensor is required and positioned close the rotor. The device responds to magnetic field changes during the motor operation, digitizing output feedback of the rotor position to the MCU for close loop motor control and fault detection. Ambient temperature information is measured from an external temperature sensor. In the faulty situation, such as motor jam, the buzzer alarm is driven by the MCU through a pulse width modulated (PWM) output.
Figure 3-1. DC Fan Design Block Diagram
3.2 Hardware Resources
In this application, the low cost MC9RS08KA2 MCU is used. The device has a built-in 8-bit modulo timer which is used to control the timing for the PWM drive. Bus frequency is chosen to be 4MHz. The design target for the maximum motor speed is 4000 rpm, the timer must have enough resolution to measure the shortest PWM period that is less the 3.75ms per commutation step. Timer prescalar is selected as 256 and the timer resolution becomes 64∝s.
Table 3-1. Hardware Configuration
| Bus Frequency | 4MHz | |
| | | |
| Timer Clock for motor speed monitoring | 4MHz/256 = 16kHz | |
| | | |
| Timer Resolution | 64∝s | |
| | | |
| Variable Speed DC Fan Control using the MC9RS08KA2, Rev. 0 |
| | | |
Freescale Semiconductor | 15 |
