24. July 2009 by pilot.

I know there are gobbles of RAM in a PIC32 (more than the entire FLASH bank in most other PIC16/18… ) but RAM is one of those things in life you can never have enough of!

Plus, I am an 8-bitter by education, as I have already acknowledged before, and having spent most of my professional life working on microcontrollers with very limited resources, I am very sensitive to waste and waste of RAM in particular.

If you check the memory gauges in MPLAB, even an empty PIC32 program with no function calls and practically no code at all in the main(), will give you a minimum memory usage of 1.5K bytes of RAM. To me that’s huge! I need to know where all this RAM is going and how I can control it should I need it all!

The mystery is easily solved, if you look at the .map file, it shows that 1k byte of that RAM is just the default amount “reserved” for the stack.  As we have seen before (the stack so misunderstood…) the way MPLAB manages the stack memory allocation is a bit tricky. By the way, that memory is not literally allocated, nor the SP register is affected directly, but this number is used at compile time, only once, to check if , after allocating all the global variables and the heap (if there is one), there is still room for the requested amount of stack. So it shows in the memory gauges, but it is not representing by any means the “real” amount of RAM that will be used by your application stack, nor the maximum amount available. In fact the stack will take all the memory left and some… if it needs to.

But where is the other 1/2k byte of RAM gone?

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18. March 2009 by pilot.

In the last several months I have seen a couple of postings on the PIC32 forum about users testing the speed of the PIC32 I/O. It is quite normal and healthy for a community of Embedded Control designers to worry about the actual I/O performance of a microcontroller, and even more so when the microcontroller in question claims to be able to operate all its peripherals (I/O pins included) at the same (top) speed of the core: 80MHz!

With such capability and thanks to the special Atomic I/O operations (PORTINV), the theory says that a PIC32 could produce a 40MHz output square wave!

This performance is quite unique among competing microcontrollers (they typically achieve 1/2 the processor top speed), but we have to admit it can be tricky to prove. Here is what I recommend you do to get as close to the theoretical limit as possible… Read the rest of this entry »

Posted in PIC32, Tips and Tricks | 2 Comments »

27. December 2008 by pilot.

I guess we all wait for the Christmas holidays to recover from a long year of hard work, enjoy some more time with family and friends and prepare for the new year at the doors. As I am writing this, it’s real cold and snowy outside, just like in the typical Christmas stories, and I find that this is just the perfect time (after dinner and when the kids are in bed) to … write some fun embedded applications!

I had meant for a while to post some additional projects for those of you who have read the books and tried out (prototyped) the projects using the published schematics or assembling  the AV16/32 kits, but I never got to it.

So, finally ..here they are!  I have just added a new page to the PIC32 explorers’ web site dedicated to the first six new projects.

More will be coming shortly… hope you’ll have fun!

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19. November 2008 by pilot.

As a reader pointed out today, when preparing one of the pictures in Chapter 3,  I did use a little trick, I had almost forgotten…

I did reduce the DELAY constants by a factor of 100 to … speed things up … so to speak. Otherwise the Logic Analyzer window would not be able to contain all the samples required to produce the complete “Hello” message.

I do these things quite a bit when I use the Simulator … Read the rest of this entry »

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15. May 2008 by pilot.

Using the new double buffered graphic library (see previous post) we can now create simple animations. In chapter 12 we have already seen how to visualize a 2D function using a grid and a bit of perspective (isometric). The next obvious step is to look into 3D objects and how to animate them by moving and rotating them in space. Read the rest of this entry »

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10. May 2008 by pilot.

In the good old times, when studying at the University of Trieste, I particularly enjoyed Rational Mechanics, a required class for any EE Master back then, presented by professor Enzo Tonti. One of the things that were always making his lectures a pleasure (and a challenge) was his constant reference to, and practical use of, personal computers to illustrate visually the concept at hand.

Mind those were the early days of the Apple II, the Commodore C64, the Sinclair Spectrum and for the few lucky (and rich) ones among us the first IBM XT personal computers. The graphic capabilities of those personal computers were primitive to say the least. In fact, a single PIC24F128GA010 has today more program memory than any of those personal computers and the resolution of the video output, using the AV16 board we developed in the book (without any hardware assistance), is actually pretty close. Yet prof. Tonti used to teach us how to develop effective demos using little more than a few lines of BASIC or, I should say, of the “soup” of BASIC dialects that were popular at the time… (Actually prof. Tonti developed a brilliant scheme to identify a sort of minimum common denominator among all those disparate systems and Basic dialects that he called Inter-Basic if I recall correctly… there must be a web site somewhere where all this is documented)…

We simulated graphically the motion of planets around a sun, the complex motion of pendulums of various kinds, we animated objects in 2D and 3D! Key to all those graphic exercises was the ability to alternate fast images on the display to obtain the illusion of motion….

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28. April 2008 by pilot.

While working on the porting to the PIC32 of the code developed for the original PIC24 book it occurred to me that there were some obvious optimizations I had yet to explore.

For a starter, the graphic.c library was using several resources including Timer3, the Output Compare 3 module, the Output Compare 4 module and the SPI1 port in addition to one general purpose I/O (RG0 in this case). The OC3 module was used to generate the Horizontal Synchronization pulse portion of the composite video signal, but the module output pin RD2 (active as soon as the module is enabled) was not used. Rather RG0 was “manually” set during the OC3 interrupt service routine and reset during the Timer3 interrupt service routine. This added unnecessary overhead to the application as the OC3 module could easily be configured to do it all by itself…

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15. April 2008 by pilot.

Our brain is wired for image processing and all things visual just reach deep in our mind faster! If there is one moment when I just cannot seem to ever get enough of a “view”, that’s when I am debugging code. The Watch windows, the local variables windows, the stack window are not what I am talking about. The Logic Analyzer window (in MPLAB SIM) and the DCMI (with simulator and emulators) are barely starting to scratch the surface, when I need to debug some complex code, I am willing to use literally everything at my disposal, beyond MPLAB, using hardware (scopes and logic analyzers) but also additional software.

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9. April 2008 by pilot.

There are so many new features being added to MPLAB at each major (and minor) new release that I struggle to keep up. Just a couple of days ago for example, I was discussing with a colleague how it would be nice to have the MPLAB New Project Wizard to use a default linker script in every new project … so that in most standard cases the repetitive  task of adding one (navigating through the many subdirectories inside Program Files) can be spared and … I got stopped mid-sentence and promptly reminded that this feature was already there! In fact since last November, with the introduction of version 8.00, MPLAB has been quietly offering a default linker script when the MPLAB C30 toolsuite is selected.

Thanks !

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12. March 2008 by pilot.

I read with interest today an errata for the Explorer16 demonstration board…

I know, it seems strange to think of an errata for a demonstration board, but it can happen and as in this case, there is a good lesson to learn for all of us, living the dangerous lives of the Embedded Control designers on the edge between 5V legacy technology and the new 3V generations of products.

In order to keep their options as open as possible, the Explorer16 designers arranged for a number of different LCD displays to be used alternatively on the board. Three different layouts were pre-wired and, depending on market conditions (price) and availability, one of several different models of LCD displays (from different manufacturers) will be mounted at any given point in time. Since they are all compatible (or I should say there is a common minimum denominator among all of them) there are virtually no changes required to the demos and code offered with the board.
Interestingly some of these LCD displays are 5V and some are 3V devices. Since most of the PIC24 digital I/O pins are 5V tolerant, theoretically the two technology can be alternated without consequences except for one case that was recently uncovered.

When the Lumex™ LCM-S01602DTR/M module was used (the errata document will teach you how to recognize it) things got more complicated. This 5V LCD module has internal pull-up resistors connected to the 5V rail. When the PIC24 I/Os connected to it (for example PMPA0 which is multiplexed with the AN15 analog input function) is left three stated (very high impedence) it will let 5V get inside the PIC24!

Notice that this will not necessarily damage the part (the pull ups are pretty weak after all), but it will produce a more subtle problem, it will skew the internal analog voltage references and produce a large offset on the A/D module readings for example.

Reading the errata carefully, you will realize how an essential part of the problem is the high impedence of the input pins once three stated, which produces, with the LCD internal pull ups, a partition with a very high output voltage (close to 5V).

Decreasing the impedence, by turning pins to outputs (as you do when you enable the PMP), or by adding pull down resistors (even if of relatively high values) solves the problem as the partition is modified to produce a lower voltage …

The lesson learned: Once more, make sure that when you interface to a 5V device you either:

• use exclusively pins that are pure digital inputs (5V tolerant)
• use open drain outputs if you want to swing a 5V signal

Watch out from all pins that are multiplexed with analog functions, even if you have the analog function disabled in your particular configuration.

The pin-out cross reference table published in the previous posting “The Missing Pinout Table” might help you double check…

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