I worked with Anju on picking an FPGA and finding a development board to get her started on writing Verilog before the final PCB is designed and assembled. I looked at some microcontrollers as potential options since she could not find an fpga for us to put on a custom board, and worked with her to determine how to test if using an FPGA dev board will be fast enough.
I specced and ordered lasers and photodiodes to evaluate their performance with high frequency modulation to make sure they are fast enough for our needs. We are ordering a green laser and blue photodiode, and an IR laser and photodiode. The photodiodes have poor performance at the wavelengths of the other laser, since green and IR are far apart. This way, we do not need to do any filtering ourselves to pick out each color laser.
I began considering fiber optic laser drivers for use for driving our free space lasers. They do not meet our design requirements, so I began making a schematic to see if one could still be used with modification to manufacturer recommended support circuitry. It ended up having a lot of potential problems, so we probably will not go this route, and will use discrete circuits to drive the laser. Between doing calculations for component values on my own and TI’s responses to my questions about their drivers, it seems that it might not work once we have the final boards in hand, which is too big a risk for us to take.
I began designing a circuit to drive the laser with discrete components. This is probably what we will end up using.
Schedule
My work is on schedule. I wanted to buy the lasers and photodiodes this week to test their performance at high frequency, which I did. I got a bit further into schematic work this week than I expected as well.
The lack of FPGA chips available means that we have not selected a processor yet, which could put us behind schedule soon. Our fallback plan is to use an MSP430, because Anju and I are familiar with the software and I am familiar with hardware and confident I can make a board with one quickly.
Deliverables
In the next week, I hope to have a complete schematic of the transmitter, and an idea of some part selection for the receiver. I also hope to have come to a decision with Anju about what our processor will be, and a plan with Roger about how to verify that the board will work at our speed.
Searched for FPGA/microcontroller options that were available for purchase
Found out that FPGA chip on a custom PCB would most likely be a difficult option. I was restricting the search to Altera chips (and non-BGA if possible), and found that most chips were out-of-stock or too old so that any recent version of Quartus does not support this.
The search was restricted to Altera due to my prior experience in 341/240.
Looked into a microcontroller that would have UART/SPI, that would enable communication from the FTDI (USB-UART chip) to the lasers. Filled out a purchase form for a MSP430 Launchpad. Estimated day of delivery was Monday if it were ordered yesterday.
KJ and I both have experience using MSP430 in the past, which is why we narrowed down the search to this.
We decided to get a dev board to test out its capability so we can make sure that it can support at least the 4Mbps communication to meet requirements.
Work is on schedule, although the unavailability of FPGA chips were unexpected and I would need to iron out the plan of handling the communication.
By next week, I hope to come up with a way to validate whether the microcontroller and FPGA dev board can support the communication requirements, and choose which design to proceed with based on this.
