ikalogic

So if i want the data before and after trigger, why not store every thing, and analyse it on the PC... sorry i don't see what would be the use of getting the post trigger data.. can you give an example?

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Ibrahim Kamal
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3v0

You do not have unlimited memory on the AVR. You would have to acquire then send each sample to the PC. You could do that if you had a connection to the PC that was fast enough. It is likely that sending data would take longer then storing it and that would slow down the sample rate.

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ikalogic

i know that that's why i store 1000 samples then send them via RS232

now my questions is, according to you, the trigger would be in the middle of the samples (the the 500'th sample) in such a way that half of the samples are before the trigger, and the other half is after the trigger?

but what is the use of that? can you give me an example where post trigger data is useful?

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Ibrahim Kamal
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ikalogic

from wikiperdia "A logic analyzer can trigger on a complicated sequence of digital events, and then capture a large amount of digital data from the system under test (SUT)."

Also from wikipedia, about LAs "After the mode is chosen, a trigger condition must be set. A trigger condition can range from simple (such as triggering on a rising or falling edge of a single signal), to the very complex (such as configuring the analyzer to decode the higher levels of the TCP/IP stack and triggering on a certain HTTP packet).

At this point, the user sets the analyzer to "run" mode, either triggering once, or repeatedly triggering."

However this example (Tektronix Logic Analyzer) shows that the trigger position can be adjusted, which confirms your point of view..

So i'll see if i can do it..., but i feel that a delayed trigger would considerably reduce the sampling rate! bad! bad! however, i think that for a first version, i'll do it as in wikipedia, where triggering starts the capture sequence...

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Ibrahim Kamal
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3v0

I am sure you can make it work. It was not my intention to upset you.

Collecting data prior to a trigger is useful in finding what led to the trigger condition or event. Problems where you need to know what happened prior to some event. For example if you were using a LA to debug a program you could have it trigger when an address was reached and look at the data prior to the trigger to see how you got there.

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ikalogic

You didn't upset me! it's ok!

I know i can do it, and i already have some ideas, but the problem is, what ever i do, it will slow down the sampling, and i'll never reach the 4 MHz sampling rate that i reached uptill now..

I think, that for a first version i can solve this problem by offering 2 additional options:
1- free running, which would run as slow as 100Khz sampling rate, and store data directly on PC for later analysis
2- Add an option to search the sampled data for the trigger condition and highlight it. (the samples data can be sampled normally at 4 Mhz or in the "free running" method above)

Alternatively the user could set the trigger to a periodic condition that happens many times after the critical point that need to be analysed, then, use the "search feature" described above to monitor a critical transition.

Does that seems like a good start ?

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Ibrahim Kamal
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3v0

When you finish this version I would look into a new processor for REV 2. I do not know much about AVRs. But if they follow in line with what PIC is doing there should be a much faster processor with much more ram in the lineup. It will buy you both faster and deeper at very little cost and very little code change.

The PICkit2 Logic Tool manages 1 MHz with a 20MHz 18F2550. The instruction rate is 1/4 that or 5MHz. You should be able to do better since you AVR with a 12MHz crystal should be more then twice as fast. Carefully code the trigger/acquisition loop in assembly. Use as fast a clock as is pratical (some AVRs run as fast as 40MHz?). 40MHz would give you 10 instructions per loop iteration. You may be able to sample store and a simple trigger in 10 instructions.

I do not much care for sending the data to the PC instead of storing it. I think (math may be wrong) that using a RS232 serail link at 115200 baud would limit you to less then 2KHz. You also have to look at the processor overhead for service the UART so it could be worse. You could do better with USB but I know you do not want to go there.

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ikalogic

cool!

Your right about UART, my math was wrong!

Thanks for the support. If you don't mind, i'll send you a beta version of the software with demo samples loaded so that you test the interface ang give me some of your precious suggestions!

have a nice day

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Ibrahim Kamal
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skyrock

For home made L.A., can I use alligator clips as a probe? (Newbie question)

I saw a very simple L.A. circuit together with pc side software which I wanted to try making. But i'm not sure if I am able to find a decent probe for it.

http://www.geocities.com/ted_rossin/...#LogicAnalyzer

3v0

Good find. That is a lot of tool for $10.

An analog person could tell you at what speed you need a better probe. Given the low speeds I would say a wire with a clip on the end of it would work well enough. The signals may be somewhat distorted but since they are all distorted the same way I doubt it makes much difference in what you see on the display.

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skyrock

I just found a problem with the $10 L.A.

The circuit uses a MAX243 chip but I couldn't find it anywhere. There's only MAX232 available. Do you think it can be replace with 232 in this circuit?

http://www.geocities.com/ted_rossin/...#LogicAnalyzer

3v0

Yes any chip that does RS232 level shifting will work. Just be sure to read the datasheet for the RS232 part you use to find out what size caps it uses if any.

Where do you live ? Need to know that to aid in finding parts. Have you checked the Octopart search engine.

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skyrock

I live in a small town in malaysia. Doesn't have a wide range of supplies over here.

3v0

You may want to go to the user CP and fill in the location.

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skyrock

i've build the simple L.A. in just a few days and it is working well. the little disappointment for me is that it doesn't have a software side buffer to save more data.

I figured even if the PIC doesn't have enough resource to store data in it, it could be done at PC side software to store down the data for later analysis. But there's no source code provided for PC side application, so nothing much can be done.

regards,
SKY

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