Logic analyser pitfalls for new players - Page 1
Forum section: electronics / beginners

author: spaceface
author_rank: Newbie
timestamp: March 28, 2013, 12:52:33 am
content: So I finally bit the bullet and got myself a cheapo USB logic analyser/scope/signal gen from China :-) (http://cgi.ebay.co.uk/ws/eBayISAPI.dll?ViewItem&item=261158100499&ssPageName=ADME:L:OU:GB:3160) I know it's not the best but I don't have £100's to spend on hobby equipment!I'm probing around the test points of a WiiMote and have found a nice I2C signal coming from the camera however when I connect leads up to other pads at the same time it becomes noisy. I thought a 16ch logic analyser should be able to proble all over a circuit without causing any problems. The other pads will show various things depending on what leads I have connected up else where - sometimes a pad shows what seems like noise but then if I connect another lead up somewhere else it'll turn into a nice ~24kHz clock frequency, all while the wiimote is functioning fine.So, does my logic analyser just suck? Is this normal behavior and I need to pay more attention to the points in the circuit that I'm probing?I've got the wiimote and my laptop running of batteries to avoid any noise from switching power supplies etc.

author: c4757p
author_rank: Super Contributor
timestamp: March 28, 2013, 01:13:42 am
content: This may sound like a stupid question, so forgive me, but have you connected the ground lead? Signals and noise drifting about as you make other connections sounds to me like a floating ground.

author: spaceface
author_rank: Newbie
timestamp: March 28, 2013, 01:20:47 am
content: Yep, connected all four available ground leads to the battery negative terminal!

author: c4757p
author_rank: Super Contributor
timestamp: March 28, 2013, 01:24:22 am
content: Have you measured the voltage? It might be too low for the inputs on the logic analyzer. I don't see any voltage threshold specification for it.

author: c4757p
author_rank: Super Contributor
timestamp: March 28, 2013, 01:27:52 am
content: Of course, for that price it's quite likely that the thing is just a piece of crap.

author: spaceface
author_rank: Newbie
timestamp: March 28, 2013, 01:34:58 am
content: Yep that just occurred to me too, it's a 3.3v circuit so maybe at the threshold of the logic analysers capabilities.

author: c4757p
author_rank: Super Contributor
timestamp: March 28, 2013, 01:41:37 am
content: If a 3.3V circuit is too low then it'sreallya piece of crap. Almost anything will respond to 3.3V levels. Probably something else.

author: updatelee
author_rank: Contributor
timestamp: March 28, 2013, 02:09:39 am
content: did you connect the unused channels to gnd as well? wouldnt hurt. Got your sample rate high enough? 10x the freq is best, 2x at a bare min (poor results) most I2C runs at 100khz or 400khz, but there are other speeds.UDL

author: HackedFridgeMagnet
author_rank: Super Contributor
timestamp: March 28, 2013, 04:41:55 am
content: Take a ground nearer to where you are probing, if you can.

author: marshallh
author_rank: Unknown
timestamp: March 28, 2013, 04:33:50 pm
content: Scope out signals to see what they are (analog, digital, sine clock, square clock) before putting a LA on them. And sorry, the one you bought is a flaming pile of crap.

author: MasterOfNone
author_rank: Regular Contributor
timestamp: March 28, 2013, 07:22:39 pm
content: There is usually an option that allows you to set the input voltage level. Have you got this option on your analyzer?