New-Tech Europe Magazine | Feb 2017

Using a Complementary Waveform Generator

Mike Gomez, Microchip Technology

How a complementary waveform generator found in 8bit microcontrollers can help in motor driver applications A complementary waveform generator (CWG) can be handy for when half-bridge and full-bridge control is needed, such as in motor driver applications, for example. Even better if the generator also has selectable input sources, polarity control, auto-shutdown and auto- recovery. These functions can be provided as a peripheral in some 8bit microcontrollers with no processor overhead. Fig. 1 shows a block diagram of such a peripheral. Each block in this diagram represents a feature of the CWG, which generates a complementary output from one of several selectable inputs. The output can be modified in different modes of operation, such as push-pull, half bridge, full

logic cell (CLC). First, though, the selected peripheral should be configured as the CWG’s input. For devices that have peripheral pin select (PPS), the CWGxIN input pin can be moved to any other pin with the PPS input selection register (xxxPPS). By changing the “xxx” notation in the register name to CWGxIN, any available IO pin can be selected as CWGxIN. The CWG output can be modified to operate in any of the following modes: half bridge; forward full bridge; reverse full bridge; push- pull; and steering PWM. Mode selection is only available in some devices, where it can be selected by setting the mode selection bits. For example, Fig. 2 shows the output of the CWG in different modes of operation for the Microchip PIC16F161X family. In half-bridge mode, two output signals are generated as true and

bridge and steering PWM. The clock source can be selected and used to insert a dead-band delay between the pair of complementary output waveforms. Each output pin has individual output enable control and the polarity of these pins can be controlled individually. The output can also be terminated immediately during a fault and recovered when the fault is removed. Selections Input sources can be external inputs to the CWGxIN pin or outputs from other internal peripherals. The input source selection bits are used for selecting the input source. The input sources and bit selection settings may vary from device to device. Some of the available peripherals that can be used as input sources are the comparator, capture compare PWM (CCP), numerically controlled oscillator (NCC) and configurable

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