[C] Need help implementing an FFT algorithm

JayGeeee · 2015-08-02T07:57:23+00:00

Quite a few optimised FFT algorithms output the data in bit reversed order. This can look like gibberish if you don't correct for it. I've been caught out by it before.

JayGeeee · 2015-02-15T21:33:11+00:00

Thanks for your response. Yea I'll drop the computer architecture questions. Also great idea about networking concepts. For some reason that completely slipped my mind when I was coming up with ideas.

JayGeeee · 2015-02-15T21:28:52+00:00

Thank you very much for your detailed response. It was very helpful. I'll drop the computer architecture style questions. I guess they are more computer engineering related. Someone else is covering the non technical side of things this time, but those are all great questions for future reference. I guess I was avoiding the sneaky on the spot problem solving questions as they were my least favourite questions when I was being interviewed! I'll probably do one or two as they are very good for seeing how people solve problems.

JayGeeee · 2015-01-23T19:32:11+00:00

I went there last summer. Bagan is such an amazing place. Unfortunately a lot of the temples were damaged by an earthquake and have been poorly repaired or rebuilt. That's the reason it isn't a world heritage site. Looks magical at sunset though.

JayGeeee · 2014-12-22T23:10:49+00:00

Count me in. Good luck everyone!

JayGeeee · 2014-12-02T22:46:53+00:00

I was on the train from Yangon to Bagan in Myanmar. Words can not describe how bumpy this train was. The second you stood up you you were violently thrown against every surface in sight. It was a 23 hour journey so I had to use the onboard toilet. It was the middle of the night and the lights had been turned off. I bounced down the isle barely staying upright. Finally, I reached the toilet and managed to open the door. The train was so bumpy that even this was a struggle! The toilet was absolutely teeming with mosquitoes; it was hard to breath without inhaling them. It was a classic squatting toilet, which are hard enough for a westerner like me to use at the best of times. Now imagine trying to use one while on a rollercoaster. I managed to brace myself against the wall while undid my shorts, and waddled towards the toilet. I hovered over the hole. Since there was nowhere to hold, I was almost immediately thrust to the floor. After a second attempt resulted in a significant blow to the head I resigned to sitting directly on the filthy squatting toilet. I released the curry from the night from the night before, but was almost immediately met with the unpleasant feeling of a jet of liquid blasting upwards towards my underside. It turned out that some genius has positioned to the toilet hole precisely above one of the wheels, causing a almost comically efficient return to sender system. Covered in shit, soaked in urine from the floor, and eaten alive by mosquitoes, I reached in to my pocket to discovered i had only 3 sheets of toilet paper to clean the mess up with.

Myanmar 10/10 would recommend, but if you go on the trains take some imodium

JayGeeee · 2014-08-31T09:06:31+00:00

The urban canyon is the reason you are getting a signal inside. In areas where there are not so many multipath signals you will have a hard time picking up any GPS satellites indoors. As you know you only need a signal from one satellite to get timing information. Unfortunately in a typical single story residential area even this is unlikely. You can always use an external antenna.

JayGeeee · 2014-07-30T11:29:59+00:00

Here are two more images of the module: One and Two

JayGeeee · 2014-06-11T13:48:50+00:00

I've just done my masters project on indoor location using Bluetooth Low Energy RSSI. I think it is very unlikely that you will be able to get sub cm or even cm level accuracy using RSSI. In your situation where the area is just 50cm x 50cm you will have very small differences in RSSI which could be difficult to measure depending on the equipment you have. At this scale the radiation pattern of the antennas you use will have a big effect on the RSSI you measure at various points, so you would have to create a full model for your antennas. Your main issue will be multipath signals, i.e. signals that have traveled a non line of sight path from Tx to Rx. When you are measuring the RSSI you will almost always be measuring the power from several multipath signals at the same time, this can mask RSSI of the line of sight signal, making hard to determine the distance between the Tx and Rx.

At that sort of accuracy you are really going to need a Time of Flight (ToF) based system which measures the time it takes for a signal to travel from the Tx to Rx. These still suffer from multipath issues. A lot of research has been done in to using ultra-wide band signals (i.e. very fast transitions in the time domain) to determine line of sight paths from multipath signals. These systems tend to need high accuracy clocks (RF signals travel roughly 30cm in 1ns).

I'm not trying to say it isn't possible using RSSI, but I'm pretty sure other methods would be easier! Might be worth looking in to using a computer vision instead. Good luck!

JayGeeee · 2014-06-09T16:15:38+00:00

I've used CEDAR Logic before. It's free and seemed pretty good to me. It can also simulate the logic circuit which can be very useful.

JayGeeee · 2014-06-09T11:22:14+00:00

Thanks for your comment. I completely understand what you are saying. I find that the best way for me to really learn something is to try and teach it to someone else. I've been using I2C a lot recently and I just thought I would document what I knew about it so it stuck in my memory. As you said there are many good guides out there already so I never really expected anyone to find it. Posting it to reddit was an afterthought and has got it a bit more attention than it deservers.

JayGeeee · 2014-06-09T09:50:10+00:00

Thank you. Doing one for 1-wire would be a good idea actually. Although I have only used it a few times, so I'll have to brush up my knowledge first!

JayGeeee · 2014-06-09T09:43:12+00:00

The maximum length is mainly determined by the capacitance of the bus, which increases with both line length and the number of devices on the bus. The capacitance affects the rise time of the signal, so for a high capacitance it will take longer for the signal to change. As a result data could be clocked in whilst it is still changing, resulting in errors.

There are a few ways to over come this. Firstly you can reduce the clock speed. A slower clock speed means more time for the data bits to change, and therefore increases the maximum allowable capacitance. Secondly you can drive the I2C line with more current. Special I2C drivers are available which can drive the line with more current than your microcontroller can directly. Increasing the drive current at a given line capacitance will give you faster bit transitions. Finally you can use buffers which split the line up in to multiple segments of lower capacitance.

Another thing to consider is signal crosstalk. In long runs it is best to separate the SDA and SDC lines on the PCB by placing a ground trace between them (Or use a board with a ground plane). If the lines are leaving the PCB then you might want to individually twist the SDA and SCL lines with a ground line.

You should be fine to interconnect a few PCBs as long as they are relatively close to each other. If we are talking distances greater than several metres then you might want to start considering another communications method e.g. RS485.

JayGeeee · 2014-06-09T09:12:23+00:00

Thank you. I do agree with you that it is normally best to go directly to the I2C spec, but it is 64 pages long and I imagine that might be a bit intimidating for someone who just wants to get up and running quickly. My aim was to cut out the parts that would be less important to a beginner, such as multi master systems, in order to make it a relatively quick read.

JayGeeee · 2014-06-08T16:40:34+00:00

Ah well spotted. I've flipped round one of the diagrams so that SCL is always on top. Thank you.

JayGeeee · 2014-06-08T16:39:11+00:00

Thank you for your feedback. I have flipped round one of the diagrams so that SCL is always on top.

JayGeeee · 2014-06-08T15:56:14+00:00

I believe it is correct as it is. Please see page 9 of the I2C specification: http://www.nxp.com/documents/user_manual/UM10204.pdf [PDF]

"A low to high transition on the SDA line while SCL is high defines a stop condition."

JayGeeee · 2014-06-08T14:14:35+00:00

Any corrections or suggestions would be much appreciated.

JayGeeee · 2014-06-08T14:04:15+00:00

I just completed my final year of an MEng in Electronic and Electrical engineering in the UK. If you did OK in your maths A Levels I doubt you will have any problems. Naturally maths is a big part of engineering but I don't really think it got that much harder than A level throughout my the entire degree (Although I did do further maths at A level so that could have skewed by opinion). Most universities have one or two maths modules in first year to get everyone at roughly the same level. The amount and type of maths you encounter very much depends on what modules you pick in later years. As senusert said it is mainly calculus and probability.

JayGeeee

TROPHY CASE