Valvano Vision
Last summer, my friend Josh Payne and I decided we wanted to learn how to build wireless capabilities into our circuits. We did a little research and started with a pair of Linx wireless modules on breakout boards. The following semester, for our final project in our Microprocessor Applications class we had to design and build a circuit board. We decided to implement wireless into this board.
Requirements
For this project, we were required to interface a 6811 microcontroller with a ROM. Beyond that, the project was open ended.
Circuit Design / Schematic
We made the schematic with ExpressSCH. The following includes various design aspects we encountered.
- The wireless chips required 3.3 volt LVTTL logic while the 6811 microcontroller used 5 volt TTL logic. Thus we had to implement a level shifter.
- We connected the output from the RX chip pad to a pin on the microcontroller that had input capture interrupts. This way we were able to interrupt when a signal started to come in from the wireless receiver.
- We covered all remaining area on the bottom layer of the board with ground. This was to help shield our signals.
PCB Layout
We used ExpressPCB for the board layout. This was a little tricky since there are quite a few traces between the microcontroller and the EEPROM. However we were still able to implement the ROM, the wireless chips, and a 20 pin dual row header with all unused I/O pins from the microcontroller.
Software
For a wireless protocol we used an implementation of the SCI UART protocol. We modified some code written by our professor which uses interrupts to create a virtual serial port from an input pin that supports input capture and an output pin. Our professor had some starter code for the Freescale HC12 microcontroller that did this. We had to port that code to the HC11, while at the same time learn the GCC compiler with which we would be programming the HC11. It was difficult to figure out how to program for interrupts on both a new compiler and a new platform but after reading a handful of examples, we finally got it to work. We also introduced redundancy into our data transmission since we did not need high bandwidth to send control signals.
Application
We decided to use the webcam turret I developed for RoboAshley since it would be a flashy application of our circuit boards. I resized the web interface to work on my friend Christopher’s iPod Touch. Also for demonstration with the webcam, I used AJAXCam to send the image to the iPod Touch’s browser. Unfortunately, we weren’t able to afford a wireless camera so we were still limited by the webcam cable. When we disconnected the camera however, we were able to demonstrate that the system in fact did work wirelessly.
Later I went down to Wolf Stuntworks and tried out the remote with a propane flame effect created by Bob Trevorrow.
Let it be known that I am a licensed Pyrotechnic Special Effects operator in the great state of Texas.
Conclusions
I learned a whole lot about PCB layout and wireless communication on this project. It was a bit cumbersome to have to make a board with an outdated microprocessor and ROM, but a good learning experience all the same.
I’ve left out some details on this project for the sake of space. If you have any questions feel free to comment, or email me at jhonboiles at gmail dot com (no jhonboiles is not a typo).
Write a comment