Nuts and Volts - March 2015

simple as it gets. As you can see in Schematic 1, our mouthpiece hardware design consists of a PIC, two resistors, three capacitors, a couple of connectors, and an LED for fun. To facilitate easy communications to a terminal emulator that will be running on my Lenovo laptop, J2 is wired to directly interface a Digilent PmodUSBUART module. The PmodUSBUART is shown in

Photo 2.

The heart and soul of our mouthpiece design — a 28- pin PIC18F27J13 — is shown with the PmodUSBUART mounted in the J2 screw terminal in Photo 3. Once we’ve verified that our firmware is working as designed, we can replace the PmodUSBUART module with a connection to the Moray’s serial interface.

Note the crossed Rx and Tx signals between the PmodUSBUART module and the PIC’s serial interface. We must also connect to the Moray’s serial interface in this manner. A serial port is always good to have in any case as it can act as a debug device.

In this design, we’ve added what I call an “activity” LED that can be used for general-purpose debugging or as an “I’m alive!” indicator. The PICkit3 has proven to be a reliable and popular programmer/debugger device. J1 is wired to allow a PICkit3 to be directly plugged into the design for programming and debugging.

The PIC18F27J13 is a very capable microcontroller. However, the objective here is to not have to utilize any of the PIC’s native resources. The Moray can sense analog voltages for us. It can also perform general-purpose I/O functions via commands issued by the PIC. The Moray’s PWM engine allows us to utilize it in applications such as light dimming or digital-to-analog voltage generation. In essence, we are holding the PIC’s resources in reserve. Any task that the Moray cannot perform can be performed by compute and I/O resources native to the PIC.

For instance, the Moray does not support SPI master mode. So, we would call upon the PIC18F27J13 to step

ADVANCED TECHNIQUES FOR DESIGN ENGINEERS

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■ Schematic 1. Absolutely no rocket science is applied here as far as we are concerned. All of the technology is packed into the goo that connects to the PIC's 28-pin frame.

■ Photo 2. Those of you that have previously visited the Design Cycle space will recognize this little device as a workhorse I use very often to add instant USB connectivity to embedded projects.