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Issue #209 December 2007
SD Card Display Controller
Honorable Mention Luminary Micro DesignStellaris2006 contest
by Sylvain Davaine
Start | System Overview | SD Memory Cards | Hardware | SHT11 Sensor | FAT File System | Graphic Files | Library | Host | Ideas | Sources & PDF
SYSTEM OVERVIEW
Real-time display is not always necessary. You don’t usually need to see a temperature display every 100 ms! Therefore, it didn’t matter whether my display was updated every second or two.
The trick was to find a way to input data or to draw on the screen. The digital frame reads only pictures stored on a Secure Digital memory card (SDMC). I would then have to fake only an SD card and give my data, in the form of pictures, when the host requested them.
Because I wanted to keep things simple and have at least 640 × 480 resolution, I chose to use a real SDMC in the design. This way, I would not have to respond to a host the same way an SDMC controller would and the problem of the SRAM size that’s needed to store the pictures would be solved.
I could have modified a prestored BMP file on a preformatted FAT SDMC with just basic commands to write sectors, but I used a complete FAT file system so it would be easier to handle future enhancements such as BMP-to-JPEG file functions or to change the picture resolution. I used a freely available FAT file system that I had already used with only the needed functions (e.g., there is no read file function).
The system is simple. Its purpose is to replace an SDMC. It has an SDMC pinout replica on its PCB, which plugs itself into a host (see Photo 1). Most of the time, the host is connected to a real SDMC via the pinout replica. But the system regularly takes a temperature and relative humidity measurement, isolates the host from the SDMC, and writes the temperature measurement into the SDMC. The measurements are “drawn” on the SDMC in the form of a BMP file. The circuit then reconnects the host to the SDMC. Finally, the BMP file can be read and displayed by the host as if it is a new file.
a) b) |
| Photo 1a—The Luminary Micro LM3S811 evaluation kit is plugged on top of the project’s PCB. The SD card’s access LED indicator goes through one of the fixing holes of the evaluation kit. b—The fake SD card is to the right of the PCB and the real SD card is on the left. The SHT11 temperature sensor is mounted on a DIL converter and located near the middle of the PCB. |
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