Nuts and Volts - November 2014

capable PIC18F devices. Standard RS-232 serial ports have been eliminated and replaced by USB portals on most — if not all — of the PCs on the market today. In terms of USB portals, microcontrollers large and small are following the PC lead. However, RS-232 capability remains a mainstay in the microcontroller world as many embedded device applications rely on RS-232 connectivity. Microcontrollers like the PIC16F1459 provide the best of both worlds employing on-chip USB functionality coupled with a native UART portal. Don’t let the simplicity of this prototyping stick schematic fool you. The PIC16F1459 is capable of revealing alternate functions on selected I/O pins. For instance, I/O pin C1 can function as a comparator input and an external interrupt pin when it is not busy supporting the ICSP programming function. Depending on the pin, the PIC16F1459 I/O subsystem supports DAC, analog, SPI, I2C, timer input, interrupt on change, UART, and PWM alternate functions.

I’ve popped the covers on our Rapid USB prototyping stick in Photo 3. As you can see, the component count is minimal. The LEDs in Photo 3 are connected in parallel in such a way as to require only a single current-limiting resistor. The logic level applied to the LED pins determines if the red or green LED illuminates. The prototyping stick comes preloaded with a bootloader that is supported by the CCS C compiler IDE. So, don’t be tempted to plug into the Rapid USB stick’s Tag-Connect programming interface, which is located at dead center of its circuit board. The flip side of the prototyping stick is shown in Photo 4. In this view, you can clearly see the Tag-Connect’s ICSP contacts. The six pads directly above the 47K resistor interface with the Tag-Connect cable’s “bed of nails.” The 47K resistor and the trio of capacitors are all that’s needed to support the PIC16F1459.

The shorting blocks jumper-select one of the PIC16F1459 I/O lines on each of the four-position male headers. The pair of selected I/O lines is routed to the prototyping cable connector mounted to the far left of Photo 3. The prototyping cable shown in Photo 2 is loaded with a red, white, and green conductor. The green conductor is grounded. The header pin/conductor logic is laid out in Figure 1. The pushbutton is electrically connected to the PIC16F1459’s C1 pin. The advantage to this connection is that the pushbutton can be used to trigger the external interrupt pin. Naturally, the pushbutton position (open or closed) can also be sensed by reading the logic level presented to pin C1.

Let's Blink an LED

It may seem immaterial, but learning to simply blink an LED provides important information about how to structure our applications. The code captured in Screenshot 1 blinks the prototyping stick’s red LED. If you are familiar with the CCS C compiler, you know that the Project Wizard will build an include file (xxxx.h) that is referenced at the top of your C source code. In the case

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ADVANCED TECHNIQUES FOR DESIGN ENGINEERS

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; Photo 3. This is a top-side aerial view of the raw Rapid USB prototyping stick. The I/O selection headers are top-center.

One I/O line on each four-position header is routed to the prototyping cable connector on the far left.

; Photo 4. The PIC16F1459 is supported by only four external components. All of the USB and CPU clocking is handled internally by the microcontroller.

Header Position White Conductor Red Conductor

1 PIN_CO PIN_C2

2 PIN_C3 PIN_C5

3 PIN_B5 PIN_B7

4 PIN_B4 PIN_B6 ; Figure 1. The header shunts in conjunction with the prototyping cable connector allow two of the PIC16F1459's I/O pins to be routed to an external destination.