Development Kit Analysis: TI's Piccolo LED Kit Shines

This kitaims to help engineers who plan to use LEDs for architectural illumination, insigns, or for residential or business lighting, and who want control overillumination intensity and power consumption on multiple LED strings. For sometime TexasInstruments has trumpeted the virtues of using its TMS320F2803x/2x Piccolomicrocontrollers (MCUs) in power applications. Now, engineers have a kit thatgives them a 'C28035 MCU, demonstration code and solid hardware so they can digdeeper into the hardware and software used for digital power control. Althoughthis kit controls LEDs, TI has other power-control kits. In spite of a fewglitches, I recommend this kit highly.

A C28035 Piccolo MCU directly controls a single-endedprimary-inductor converter (SEPIC) dc/dc power source that accepts from 12 to48V and "bucks" or "boosts" the input voltage to a desiredlevel. This kit generally produces a 20V output, set via software, as explainedin an app note. The main board routes this digitally controlled power to fourpulse-width modulation (PWM) stages that can each drive two LED strings at 30Weach. (The kit supplies six LED strings on a separate board.) Users have accessto open-source, closed-loop control code for the dc/dc converter and LEDlighting stage. TI also offers developers free schematic diagrams, bills ofmaterials and circuit-board Gerber files.

The kit came with a printed Quick Start Guide (QSG) thatexplained how the power-control hardware and software work, pointed out thesections of the main board, explained how to check switches and jumpers, andnoted how to connect the main board to the LED board. Labels on the main boardmake most components easy to find. Set up required the usual WindowsUSB-device-setup steps and TI provided the drivers on a CD. A USB memory stickcontained a simple graphical test program, and the QSG described how to run itand how to adjust the supply voltage and current through each of the six LEDstrings via slider controls on the PC display. The demo worked and demonstratedmy lab PC (Win XP, SP2, 1 Gbyte of memory) could communicate with and controlthe main board.

The CD includes a version of TI's controlSUITE, a graphicalprogram that simplifies locating documents, schematic diagrams, code, andreference materials for C2000-family MCUs and projects that include the LEDkit. Although controlSUITE comes on the CD, I recommend you install the latestMicrosoft .NET framework and then download the controlSUITE software from TI's website.If you don't receive a printed QSG, use controlSUITE to locate it for the LEDkit.

The demonstration whet my appetite for some hands-on codedevelopment using TI's Code Composer Studio (CCS) tools, also provided on theCD. I ran into problems with the CD and after some troubleshooting with TI'stechnical-support people, I uninstalled CCS, deleted its workspace folders anddownloaded the latest CCS tools from TI's website. I chose the freecode-limited version and performed a "typical" installation.

Before you download theCCS tools, click here reviewsystem requirements.

For the CCS tools, click here andlook under CCS-FREE.

To get to the underlying power-control code and run through threeexercises, you must have the document, "LED Lighting and DC/DC ConversionControl Integrated on One C2000 Microcontroller, " by Brett Larimore.Start the controlSuite program, go to the Kits section and select "LEDLighting DC/DC Kit." In the Documentation window, choose, "SoftwareGuide for DC/DC LED Lighting Kit." Although misnamed, it's the documentyou'll need. You'll find the QSG here, too.

Larimore's helpful 30-page application note explains theobjectives for the document and goes into detail about LED-lighting theory andthe benefits of MCU power control. He also describes the overall operation ofthe main board and the sample code, although most of the app note centers onsoftware and the use of the CCS tools. Right away you'll learn about the 17 keyvariables used in the software and what they control, as well as about the fivemain framework files and the macros used. Every dev kit should provide thistype of information. (I'd still like a list of all files and folders in the LEDproject and what they contain or do.)

To run the app note examples, you must make some changes from thehardware setup used for the basic demonstration. Don't overlook this step. Butthe document misses a few steps you'll need to properly configure the hardwareso the CCS tools and debugger can communicate with the board. I'll describethose later. I also discovered a small bug in the CCS tools that prevented mefrom setting some debug conditions. TI duplicated the problem and should haveit corrected by now.

Before I could use CCS to work on code, I had to configure theproper driver for the emulator connection (via the USB port) on the main board,identify the processor in use and establish a working directory. Theinstructions provide the step-by-step instructions. TI's code example involvesthree separate "builds" of the code and instructions for each explainthe objectives, what the build will do, how to test the code, how to open watchwindows and view and set variables, and how to run the code. A block diagramfor each build shows the relationship between parts of the code and how thehardware and software interact. I liked that information and wish more dev-kitmanufacturers would include something similar in their manuals.

Build 1 and 2 worked well and I hope to go through the Build 3steps at another time. Even if you don't go through the three builds, you canstill use the CCS tools to examine the code, which includes plenty of comments.TI fosters an active community that can helpexplain what the code does and answer questions.

Anyone interested in digital control of LEDs should take aserious look at this kit. If you don't think digital control offers economicand technical benefits, at least look at the bullet points on page 9 of theLarimore app note.

Problems

Every development or evaluation kit has a few glitches orannoyances, but with help from TI and its E2E community,those in this kit generally proved easy to overcome:

1. I had problems with controlSUITE because I had an old versionof Microsoft's .NET framework. The instructions suggest at least .NET 3.0. Iused .NET 3.5.

2. The small characters used to identify jumpers and switches onthe main board are difficult to read. A diagram in the controlSUITE librarywill help. Look in the Documentation section for the LED kit. Choose:"Hardware Guide for DC/DC LED Lighting Kit," and look at the diagramon page 9. The Piccolo board provides tiny switches. You should find themalready in the proper positions, but check their positions against theinformation in the QSG and the Larimore app note. I did not find a diagram forthe Piccolo board.

3. When you connect the main board to your PC - and thus to theCCS tools - through a USB port, ensure you have the USB drivers installed. Ifyou ran the demo program on the USB stick, you shouldn't need another go-aroundwith a Windows USB-device installation.

4. The main board requires different jumper and switch settingswhen you use it with the CCS tools, but the instructions lack some information.The settings and procedures below worked for me. I already had the LED boardconnected to the main board and a working USB connection:

a. Turn off the power (large switch near three uprightcapacitors, switch handle pointing toward you.)
b. Move the small switch into the OFF position - handle pointedaway from you.
c. Remove the connection to the wall-outlet power supply.
d. Confirm you have a jumper on [M1]-J1.
e. REMOVE any jumper on [M2]-J4.
f. Connect a jumper at [Main]-J6. (Below the left end of thePiccolo socket.)
g. Confirm you have the banana-to-banana patch wire connectedbetween the Sepic Out connector (BS2) and the LED BUS connector (BS3).
h. Reconnect the wall-outlet power supply to the board.
i. Switch on power (large switch). Green LEDs in section M1 andLEDs on Piccolo board might flash.
j. Turn the small red switch to ON. The green LEDs in section [M1]go on and remain on.

Now you can connect the CCS tools to the board.

5. The Build 2 and 3 instructions do not remind users to repeatthe "Enable silicon real-time mode" under Realtime Options, and also check thebox that says "Enable polite real-time mode" as explained in the Build 1instructions. If you forget to do these steps, the variables in the Watch-3window will show the message: "Execution State Prohibits Access..."

6. TI has saved schematic diagrams in files with the .schextension used by the free schematic-capture software from ExpressPCB. You can download the software and examine, modifyand print the circuit diagrams. TI starts with a top-level schematic that uses"macros" to identify small circuit blocks that you can find in thesame schematic-diagram folder. If you only want to print the schematics, findthem in a separate PDF document file. Thank you, TI, for supplying readableschematics.