Development Kit Analysis: TI's Piccolo LED Kit Shines

This kit aims to help engineers who plan to use LEDs for architectural illumination, in signs, or for residential or business lighting, and who want control over illumination intensity and power consumption on multiple LED strings. For some time Texas Instruments has trumpeted the virtues of using its TMS320F2803x/2x Piccolo microcontrollers (MCUs) in power applications. Now, engineers have a kit that gives them a 'C28035 MCU, demonstration code and solid hardware so they can dig deeper into the hardware and software used for digital power control. Although this kit controls LEDs, TI has other power-control kits. In spite of a few glitches, I recommend this kit highly.

A C28035 Piccolo MCU directly controls a single-ended primary-inductor converter (SEPIC) dc/dc power source that accepts from 12 to 48V and "bucks" or "boosts" the input voltage to a desired level. This kit generally produces a 20V output, set via software, as explained in an app note. The main board routes this digitally controlled power to four pulse-width modulation (PWM) stages that can each drive two LED strings at 30W each. (The kit supplies six LED strings on a separate board.) Users have access to open-source, closed-loop control code for the dc/dc converter and LED lighting stage. TI also offers developers free schematic diagrams, bills of materials and circuit-board Gerber files.

The kit came with a printed Quick Start Guide (QSG) that explained how the power-control hardware and software work, pointed out the sections of the main board, explained how to check switches and jumpers, and noted how to connect the main board to the LED board. Labels on the main board make most components easy to find. Set up required the usual Windows USB-device-setup steps and TI provided the drivers on a CD. A USB memory stick contained a simple graphical test program, and the QSG described how to run it and how to adjust the supply voltage and current through each of the six LED strings via slider controls on the PC display. The demo worked and demonstrated my lab PC (Win XP, SP2, 1 Gbyte of memory) could communicate with and control the main board.

The CD includes a version of TI's controlSUITE, a graphical program that simplifies locating documents, schematic diagrams, code, and reference materials for C2000-family MCUs and projects that include the LED kit. Although controlSUITE comes on the CD, I recommend you install the latest Microsoft .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 LED kit.

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

Before you download the CCS tools, click here review system requirements.

For the CCS tools, click here and look under CCS-FREE.

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

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

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

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

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

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

Problems

Every development or evaluation kit has a few glitches or annoyances, 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 version of Microsoft's .NET framework. The instructions suggest at least .NET 3.0. I used .NET 3.5.

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

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

4. The main board requires different jumper and switch settings when 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 board connected to the main board and a working USB connection:

a. Turn off the power (large switch near three upright capacitors, switch handle pointing toward you.)
b. Move the small switch into the OFF position - handle pointed away 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 the Piccolo socket.)
g. Confirm you have the banana-to-banana patch wire connected between 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 and LEDs 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 repeat the "Enable silicon real-time mode" under Realtime Options, and also check the box that says "Enable polite real-time mode" as explained in the Build 1 instructions. If you forget to do these steps, the variables in the Watch-3 window will show the message: "Execution State Prohibits Access..."

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