PICkit 3 Programmer and Breadboard

    Hello world: Arduino was a piece of cake! —That is, when compared to puzzling out how to program a PIC microcontroller using Microchip’s MPLAB X IDE (version 4.05) and the PICkit™ 3 programmer. Looking back on it, the effort should not have been as challenging as it seemed to be at the time, but things that become obvious in retrospect can be difficult to grasp when the language is unfamiliar, or when one lacks confidence. Among the things I did not understand at first were: 1) How to power the microcontroller chip from USB through the Programming interface, 2) How to refer to pins on the chip using names that the compiler would understand, 3) How to select or enable the chip’s internal oscillator, 4) how to make timed delays, without a crystal and finally, 5) how to disable the watchdog timer, or rather that such a thing as a watchdog timer exists, much less how to disable it.

Edit Project Properties - Power check-box

Connections from programmer to microcontroller    Routing USB power to the microcontroller by way of the Programmer involves both wiring and a software setting. The latter is shown above. Google searches did not return much useful information on the chip that I was testing, except of course the datasheet. Later I came to realize that the most relevant information was independent of the specific chip type. Regardless of which microcontroller it is, Vpp goes to Vpp, Vdd to Vdd, ground to Vss, data to data, clock to clock. Well, a point of confusion was that microcontroller pins are multiplexed and serve multiple functions, not just application-defined input or output, but other functions as well.  For example, the COM data input pin on the PIC16LF88 chip is labeled RB7/AN6/PGD/T1OSI and the clock pin says RB6/AN5/PGC/T1OSO/T1CLI. For different chips, the number of pins or the pin numbering scheme may vary, but common functions are similarly named. The answers were there—I just did not realize at first that they were answers.

    With an aim toward clarity, I tried a couple of different ways of illustrating connections from the programmer to the chip. The Visio diagram (left) was about the best I could do. The Vpp connection (orange line) is needed for programming, but not for running (or when disconnected from USB and powered via Vdd). On the microcontroller end, Vpp / master clear is pulled high through a resistor when powered from the target side. The small capacitor from the MCLR pin to ground was recommended in a forum thread. I am not sure that it is needed.

    These are the basic connections and setup required to program or run a program on the PIC16LF88, and probably many other microcontrollers. An external oscillator or crystal would involve a couple of additional pins.  Similarly, for demonstrating anything, such as blinking an LED, at least one additional connection would be needed. I used RA0 for the first blink demo, then added RA1, 2, 3, and 4 just for fun. I have not exercised application input yet.

    My first programming test was intended to make an LED blink once. Instead it blinked on and off continuously, as if the thing had a mind of its own. This was extremely puzzling. There was no way the program could be looping. Even so, I fiddled with the program structure in crazy ways. The answer came from a Google search on the terms pickit3 restarts main() this thread. The following directive fixed the issue once and for all —

Watchdog timer off

    It’s funny how a little thing can gunk up the works so severely. One can
’t be sure that the cause isn’t an opaque programming error, or defective wiring or a bad chip. However, once the watchdog timer was disabled, the test program began to behave in the intended way. The LED only blinked when instructed to. Humans were once again in charge!

Imitate Arduino
 
    For this first PIC exercise I used the free version of the MPLAB X IDE with the XC8 compiler. For no particular reason I decided to make the ‘Hello world’ code imitate an Arduino sketch, initializing IO pins in a function named setup() and running the demo in loop(). Timing was a little funky. (I plan to add a crystal later.) In order to use the XC8 compilers built-in delay functions an oscillator frequency should be specified, according to http://microchipdeveloper.com/faq:26. However, the _delay() function, whose argument is a number of instruction cycles, produces the same duration delay whether or not _XTAL_FREQ has been defined in the way suggested at the ‘Developer Help page. Clearly there is much to learn, and many more experiments to do. The present exercise is no more than a sort of necessary first step.

    PICkit 3 demo: Hello_world.mp4 



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