07-02-2013 04:34 AM
Why must we use FIFO to obtain analog signal from FPGA? I can use a normal method of calling the FPGA using Read/Write Control and the results in a waveform chart is much more smoother.
07-02-2013 05:08 AM
On your FPGA Host, you can read that front panel control once every loop. The fastest you can guarantee that it will run through timed loops is once every 1 ms. That's a sampling rate of 1 kHz.
On your FPGA Target, your acquisition can run as quickly as your sampling rate and the loop in which you're running. My 7842R can acquire analogue inputs at 200 kHz on a 40 MHz clock, so a tick delay of 400 in a single cycle timed loop means I'll take a data point once every 0.01 ms (or 100 kHz)
I'd miss crucial samples if I just read the indicator every 1 ms. So instead, I put the data into a FIFO and read it back in larger chunks - 10 000 datapoints per channel every time, meaning I can run my loop on my host at a 100 ms period and not lose any data.
FIFOs are definitely the way to go for data acquisition. Control/Indicator updates are good for stimuli, such as enabling a digital power line or checking the state of a switch.
07-02-2013 06:13 AM
But when I used FIFO, the waveform chart seems to be going from a value to zero and next to a value. Why is it happening? I connect it with 2 FIFOs and connect 0 to element and timeout. The first FIFO's element remaining connect to the second FIFO's data. I think this method is widely used by when I connected it to a waveform chart vs time, it somehow keeps dropping to zero (within a few miliseconds) before the next value.
07-02-2013 06:22 AM
If you post your VIs, I can try to have a look at it later.
Meanwhile, have a look at this page:
http://www.ni.com/white-paper/4534/en
07-02-2013 08:16 AM
I've uploaded 2 real-time VIs with and without FIFO. MQ convert is the subVI in my computer. Do have a look at it...
My waveform chart is done that way so that I can easily export to excel ( i think) but this is a different problem in a different topic http://forums.ni.com/t5/LabVIEW/3-Waveform-Chart-to-a-Single-Excel-but-in-Different-Spreadsheet/m-p/...