Nuts and Volts - March 2015

serial port. So, we can opt to use MORAY in a stream statement if we wish. Otherwise, we can successfully execute serial operations using the PIC18F27J13’s EUSART without specifying a Stream name.

The selections you see in Screenshot 2 will generate the necessary code to initialize the PIC18F27J13’s EUSART as we have specified. The baud rate calculation will use the 8 MHz clock speed we defined in Screenshot 1. One of the biggest strengths of the CCS compiler is its built-in delay functions. By simply declaring the delay time, the compiler can produce the code necessary to delay in CPU cycles, milliseconds, or microseconds. The delay functions are also based on the CPU clock speed.

Now that we have laid the groundwork for our RS-232 communications portal, we’ll need to build a framework to deal with the data it will handle. The best way to corral incoming RS-232 data is to use an interrupt at the corral gate. The CCS compiler’s Project Wizard will generate the interrupt handler skeleton, but it will not generate our interrupt handler code. We’ll worry about the interrupt handler code later. Right now, let’s just get the posts in the ground.

Our corral post hole digger can be found in the Interrupts shed of the Project Wizard. All we have to do is check the correct box in Screenshot 3. How do we know which check box to select? We are calling the shots, remember? Let’s choose to only interrupt on received characters. There is no real need to interrupt on transmitted characters. We know what we’re sending. It’s the responses from the Moray that are important to us. So, we must build a firmware mechanism to capture and store the Moray’s responses until we’re ready to process them. That’s a to-do we’ll take care of a bit later.

If you add hardware, odds are you’ll also be adding code. If we’re going to use that “activity” LED, we have to announce its presence to the firmware. Normally, we would simply code an alias for the LED and set up the I/O pin that services the LED as an output. Nothing changes with our C compiler except we instruct the Project Wizard

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■ Screenshot 3.

This may seem trivial, but it sure beats looking up the interrupt handler syntax in the CCS C compiler user manual.

■ Screenshot 4. According to Schematic 1, we have an LED hanging on I/O pin A2. So, here we name the LED and specify the I/O pin it is attached to as an output pin.

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