Programming for an embedded app

I ran into this: http://pieter-jan.com/node/15

Since you already made the investment in the RPi 2, why not leverage the ideas?  See the description and sample code for low-level GPIO and I2C access.

Another one worth reading: https://projects.drogon.net/raspberry-pi/gpio-examples/

What is the modulation index of your input signal?

This sounds like an application for  a small and fast micro-controller or an FPGA. Unless you can define the input and output signals better then any further speculation is unlikely to be helpful.

Can you explain (ie describe in detail or provide links to) the input and output signals required (with timing constraints !).

MK

I have a 10,000 PPR optical encoder (5 v digital square wave output signal) that I have to reduce to 1024 PPR. I have a frequency divider circuit to divide the signal to 5000 PPR (to reduce processing time) and want to use that as the input to generate the 1024 output. Therefore, I have to generate an output pulse every 4.88 input pulses. I could reduce the input to 2500 but I am using the 2nd input pulse after I turn the digital output high as the trigger to turn the output low generating a square wave output.The encoder will rotate at various speeds that will produce a pulse train signal up to 200 kHz - I will need to handle a 100 kHz input.

I do plan on leveraging the ideas. Nothing will be running on the system except my code so, therefore, I believe it should be near real time. Fingers crossed.

How do I get a display to work with gedit? I get an error "could not open X display".  I don't know what the possible values for X in "gedit --display=X" are.

The way I would do it is this:

to save typing: z = 10000/1024

Every input pulse add 1 to an accumulator, when ever the accumulator exceeds z, make an output pulse and subtract z from the accumulator.

The fast way to do this is multiply everything by 1024 so every input pulse add 1024 to the accumulator and when it exceeds 10000 subtract 10000 and make an output pulse.

If you work on both rising and falling input edges the maths will be similar and it'll work better.

I'm not sure if you need to cope with 100k or 200k pulses per second  - let's assume 200k so you have 2.5uS per edge  - you can do this with a fast micro that does nothing else (STM32F429 springs to mind 180MHz clock, 90MHz output toggle rate) - if you don't mind getting into assembler and very close the metal you could use something slower. An FPGA would be best (no bother clocking at 50MHz for only 20nS max additional jitter) - a tiny one would do. If you only need to make one of these then you could buy a little demo board quite cheaply.

I won't say you can't do this with an RPI (I've seen singing dogs on TV)  but the RPI is totally the wrong part (and an Arduino would be no better  - it's little AVR  is a bit too slow so you would get a lot of jitter on the output pulse edges).

Is this a home job or  a work job ?

MK

It is a work job.   Good clean method to the math. Thanks.

What OS did you install?  Most folks use Raspbian (Raspberry Pi flavor of Debian) or a flavor of Ubuntu.  I mentioned that above.

See https://www.raspberrypi.org/downloads/

Raspian