A project log for MagiLog: Open Automotive Datalogging

Developing a full featured datalogger on the cheap

nigelNigel 04/21/2018 at 21:460 Comments

I’ve spent the last little while working away on the HDL planning for MagiLog. It’s been a long time since I’ve touched Verilog, so it has been a slow process. Here’s an update on where I’m at in the process.

ADC Module: This will be a fairly straight forward state machine with it cycling through the channel select -> read -> store process with the channel selection incrementing on each round. Since we have so much sampling overhead at 100Hz we’ll oversample our ADC to increase our resolution from 12bits to 14bits. To do this we follow the procedure explained in this TI application report. We take a burst of 16 measurements and sum them up. From here we bitshift the value twice (dividing by 4) to receive our final 14bit number. Once we get the value to save we record it to the correct memory location and then move to the next channel.

Each ADC will have its own module and memory block. This will make scaling up the number of inputs quite easy.

Digital in Module: Since pretty much everything else is using 16bit numbers I figured I would continue the tradition by using them on the digital inputs as well. Instead of encoding multiple lines on a single value what I think I’m going to do is have the MSB be the pin state and then the remaining 15bit will be a counter so that for each pin at any given time I can know the frequency and state without having to rely on reading back older values. Once again, we can encourage modularity by having each pin be on its own module and memory block.

GPS Module: This is going to be the most complex module I think. The data payload is very large as it’s ASCII encoded and there are multiple characters that we want to ignore. I’ll have to create a state machine based on comma and header recognition or something like that to parse that data and put the values in the appropriate memory space. I’ll probably do the conversion into integers on the XMega. I still need to decide if/how I’m going to use the 1PPS output of the GPS module.

IMU Module: This one is going to be like the ADC module. I have to input an address and then read the corresponding byte(s) which then I save to memory. I’ll need to create a table of the addresses I care about and then have the state machine cycle through them.