Nuts and Volts - March 2015

up and perform any SPI master tasks the application would require. The Moray’s strength lies in its ability to communicate via Wi-Fi radio. The Moray’s GPIO capabilities are gravy on the biscuit.

Firmware — From the Beginning

To be successful, we must think out our firmware design in the same way we have to plan and prepare our hardware design. The CCS C compiler is super capable and super easy to use. Thus, our firmware thought process shouldn’t produce a lot of ear smoke.

Let’s take it from scratch. We know our target microcontroller is a PIC18F27J13. According to Schematic 1, there is no external clocking device (such as a crystal) incorporated into the design. So, we can bet on using the PIC’s internal clocking mechanism. Since there are no predetermined crystal frequencies specified in the schematic, we can call the shots ourselves.

The PIC18F27J13 is equipped with an internal oscillator that ranges from 31 kHz to 8 MHz. We can use the PIC18F27J13’s 4x PLL to kick the MCU clock frequency up to a maximum of 32 MHz. Our alternative PLL-boosted frequency is derived from the 4 MHz internal clock setting and is presented to the MCU as 16 MHz. We don’t need lightning in a bottle. So, we’ll drive our PIC18F27J13 at 8 MHz.

We can construct the proper C source by hand to define our PIC selection and the clock speed of 8 MHz. However, that is not necessary. The C compiler comes standard with a Project Wizard that does a lot of this manual coding work for us. Take a look at Screenshot 1. In this screen capture, we enter the PIC type, the desired clock frequency, and clock source. We also specify if we want to debug this project’s code. A watch dog timer (WDT) check box lets us decided if we want to enable the WDT or let is sleep quietly in the dog house.

If we move methodically and logically in our Project Wizard path, the next configuration item will be Communications. Recall that the Moray is perfectly capable of performing any analog functions that we know we may need at this moment. We configure the PIC’s communications resources within Screenshot 2, which is a screen capture of the Project Wizard’s Communications configuration window. All we need is simple RS-232 capability in a three-wire configuration (RX, TX, GND).

As you can see in the screen capture, we have selected the PIC18F27J13’s native RS-232 port at pins C6 and C7. The PIC18F27J13’s serial port configuration is set for eight data bits, no parity, and one stop bit. The Moray defaults to 115200 baud and we have set our PIC serial port up likewise. In the event that we need to use the PIC18F27J13’s other serial port, we can provide another unique Stream name other than MORAY.

Streams allow us to direct serial I/O to a specific serial port by name. In this case, we are only using the default

74 March 2015

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■ Screenshot 1. This is a screen capture of the CCS C Project Wizard. This window is where the clocking, watch dog timer, and debugger information is entered for our selected device. Our entries automatically generate the necessary configuration code, which we will plug into our application's C source code.

■ Screenshot 2. Right now, all we need are the resources provided by the PIC18F27J13's native serial port (Tx1-Rx1) which resides at pins C6 and C7, respectively.