4 Drive Signal Generator

The sinusoidal data is divided into six arrays for the DMA controller to easily point its address as the source, as shown in Figure 4-1. The three windings require the same signal, phase-shifted by 120°, therefore a single set of sinusoidal data can be used.
Figure 4-1. Sinusoidal Data Look-Up Table
Each array consists of 32 16-bit elements. These arrays are stored in the Program Flash Memory (PFM) of the MCU. Using the DMA controller, the address of the first element is set as the DMA source address. The data is transferred from the PFM to the PWM Duty Cycle register, which is set as the DMA destination address. The DMA controller requires the size of the destination address to be a multiple of the source address size and vice versa. The DMAxDSZ register determines the destination size, while the DMAxSSZ register determines the size of the source. The PWM Duty Cycle register is a 16-bit register, which makes the value of the DMAxDSZ register equal to 2 bytes. The value of DMAxSSZ depends on the array size stored in the PFM. It can be calculated using Equation 4-1.
Equation 4-1. DMAxSSZ Register Value
D M A x S S Z = ⁡ e l e m e n t ⁢     s i z e ⁢   ( b y t e ) ×     # ⁢     o f   e l e m e n t s

There are two events that access or modify the settings of the DMA controller; one is during Hall state transition, and the other is during motor angular position interrupt. When a Hall state change is detected, the DMA controller source address register is loaded by the firmware, depending on both the sector position and direction. Changing the addresses during operation requires enabling and disabling the DMA channel on the run. A lock and unlock sequence is required to do this. On the other hand, the motor angular position interrupt triggers the DMA controller to increment the DMA source address, pointing to the next element on the array until all elements have been loaded to the destination or another Hall state change is detected.

Each DMA channel's destination address is pointed to the PWM slice register where the variable voltage is set. The switching frequency for motor control applications depends on the motor's inductance. If the motors current can change at a fast rate, the switching frequency should be high enough to avoid core saturation. In this application, the switching frequency was set to 40 kHz, catering the need to provide high frequency and high resolution for motor operation. Equation 4-2 shows the calculation of the PWM period, in reference to the frequency selected. The sinusoidal data that is used in this application are duty cycle values, based on the PWM resolution and PWM period, which can be calculated using Equation 4-3. Center-Aligned mode is selected to produce two pulses line-to-line in each period. By doing this, the ripple current is reduced while not increasing the switching loss in power devices. The PWM registers are double-buffered. It requires an external source or LD bit to be triggered. In this application, DMA is used as an external load event. Every time the DMA channel successfully transfers a message, a request to load is made. The buffer updates every other request.
Equation 4-2. PWM Period Register Value in Center-Aligned Mode PWM
P W M x P R =   C L K ⁢     S o u r c e S w i t c h i n g ⁢     F r e q u e n c y ⁢     ⁢ ×       Pr e s c a l e r ⁢     ×       2 = 64 ⁢     M H z 40 ⁢     k H z ⁢     ×   1 ⁢     ×   2     − 1 ⁢   = 799 ⁢ = 0 x 31 F        
Equation 4-3. PWM Duty Cycles Register Value
P W M x S 1 P 1 = ⁡ P W M x P R ×     d e s i r e d ⁢ ⁢     ⁢ d u t y ⁢     c y c l e ⁢   ⁢   ⁢ % 100
To drive the three-phase motor with sinusoidal currents, the CWG peripheral is used. The MOSFET driver of the three-phase BLDC motor consists of three half-bridges, one for each motor terminal. Each half-bridge consists of two switches, which require complementary PWM signals. Because of this, the CWG modules are configured in Half Bridge mode, where each generates two output signals that resemble the true and inverted versions of the input PWM. The CWGxA input is configured for the high-side switches, while the CWGxB input is configured for the low-side switches of the driver. Figure 4-2 shows the CWG output representation for each half bridge.
Figure 4-2. CWG Output

Since power MOSFETs take a finite amount of time to switch on or off, there may be a time where both switches of a half bridge turn on. It creates a low resistance path from the source to the ground that could cause a short circuit. This event is called shoot-through. In CWG, a dead-band delay is inserted to prevent shoot-through current during an output level transition. Dead-band delay is a time delay inserted between the switch transition to prevent conduction of high and low-side switches at the same time. Dead-band delays can be inserted in the rising or falling edge of the input source. Dead-band counters are set on CWGxDBR and CWGxDBF registers.