Viewing Serial Data on a Scope - Page 1
Forum section: electronics / beginners

author: A.Williamson
author_rank: Contributor
timestamp: March 17, 2023, 09:11:30 pm
content: Hi,I am trying to view serial data on a Siglent SDS 1104X-E scope.I'm probing the data input to an AD8113JSTZ video crosspoint switch.Datasheet:https://www.mouser.com/datasheet/2/609/AD8113-1502114.pdfThe data should come in 80bit serial words. I'm not sure how to set up the scope to decode this data.I have CH1 on the UPDATE pin and CH2 on the DATA IN pin.I'm triggering off of the the update pin and have the decode selected for ch2CE and SER/PAR pins are both low.Thanks for any pointers anybody can offer me.I will try to attach a grab of a word of data.

author: Muessigb
author_rank: Contributor
timestamp: March 17, 2023, 09:15:05 pm
content: You might be able to decode it by using the SPI decoder built into your 1104X-E.Connect the Clock pin to CH1 (instead of update) and Data to Data IN (CH2), then set them up as clock and data in the decoder.Since you have four channels, you can trigger everything from the Update signal.

author: tautech
author_rank: Super Contributor
timestamp: March 19, 2023, 09:33:51 am
content: Quote from: A.Williamson on March 17, 2023, 09:11:30 pmDo a Default and set the scope up again with channels NOT overlapping one another and all other unnecessary stuff OFF.Channels and Trigger on DC coupling. Once the channel to be triggered on is determined press the Trigger Level encoder to set its level to 50%. Note the trigger level indicator is hidden when in AC coupling, you really need to see it so use DC coupling for most usage, channels and trigger.All waveforms should be fully on the display.Use a 3rd channel to find the Clk waveform.Grab screenshots to USB with the blue Print button so to also capture relevant menus so we can better advise with settings. For simple screenshots Print is much better than using the Save/Recall menu.No need to Stop the scope for screenshots unless capturing a particular event.Use a much slower timebase and try to get several packets on the display, we need to see the timing between them for further settings but no harm in attaching a few images by using the More Attachments link in the posting page.First we need you to get scope usage correct then we can guide you through some simple decoding.

author: Gyro
author_rank: Super Contributor
timestamp: March 19, 2023, 10:11:38 am
content: You would probably be better picking up one of those $10 '24MHz' 8 bit USB logic analyser clones from ebay and using it with Sigrok Pulseview, it will offer you more data capture options. Scopes are best for looking at signal integrity and logic analysers for data decoding.

author: tautech
author_rank: Super Contributor
timestamp: March 20, 2023, 07:02:26 am
content: How are you getting on A.Williamson ?Hopefully these examples can head you in the right direction although they are UART and I can't really be sure what you're dealing with.First and foremost we need get rock solid stable triggering and as you can see in the first screenshot we haven't as the horizontal trigger position is halfway through a packet as the trigger has retriggered part way through it.This behaviour we must change by adding Holdoff for the trigger to be restrained until the next byte or packet.After selecting Holdoff you can twiddle the multipurpose encoder to the desired value or press it to pop out the virtual keypad from which to select the appropriate holdoff which in this case I've selected 2ms to prevent retriggering until after the byte which was still not perfect and I should've selected longer than a packet width, some 60ms although I got the screenshots I needed for this example.BTW, see how this protocol is Idle High and the reason to select a falling edge trigger so to trigger on the very first edge.Now we have a stable packet and can engage decoding and set the correct conditions for accurate decoding.First we must determine protocol and channel used and decode thresholds, all pretty straightforward however care must be taken with Decode Config settings to get accurate results.Scope used SDS1104X-E and Siglent's signal board STB-3

author: pdenisowski
author_rank: Super Contributor
timestamp: March 20, 2023, 02:43:29 pm
content: The basic principles for serial decodes with oscilloscopes are basically all the same:1) Connect a probe to each wire you're interested in (for SPI, this would be MISO, MOSI, CS, and CLK)2) Tell the scope which probe is connected to which line3) Set necessary parameters (for SPI, this would be mode, bit order (MSB/LSB), word length, etc.)4) Be sure you set time/div such that you capture enough data - this is the most common issue I see with people using scopes for serial decode5) Be sure you set voltage levels/thresholds correctly (i.e. what is a 1 and what is a 0) - this is the second most common issue6) Set a trigger, either an electrical level (e.g. falling edge on CS in SPI) or a protocol-specific trigger (i.e. trigger on a specific bit pattern)I've used both scopes and standalone logic analyzers (the older boxes and the newer USB ones), and scopes have a lot of benefits over logic analyzers, especially when things go wrong at the physical layer.Hope that helps!P.S. Can't speak to Keysight instruments, but I made a video showing how to do UART on the RTM3000 oscilloscope

author: A.Williamson
author_rank: Contributor
timestamp: March 20, 2023, 05:03:56 pm
content: Quote from: Muessigb on March 17, 2023, 09:15:05 pmThis was the perfect advise, thank you. It worked great. I set my data length to 5 in order to separate the 16 outputs, turned off the MISO and set the MOSI to be CH2.I'm trying to debug a random issue with video glitches. I'm starting to think that the serial data is the issue because I've tried two different chips and the same symptom occurs.Sorry to respond late. I was doing this last thing on Friday and didn't get back to it till this AM.This is a frame grab from a working board. What is interesting is that the purple data trace is going to -5v on this board. Same exact operating conditions otherwise.

author: tautech
author_rank: Super Contributor
timestamp: March 20, 2023, 07:05:14 pm
content: HintUse the blue Print button to save screenshots.Its main advantage is leaving the menu visible, something that really helps with guiding users when we can see the scopes settings.

author: A.Williamson
author_rank: Contributor
timestamp: March 20, 2023, 08:43:01 pm
content: Quote from: tautech on March 20, 2023, 07:05:14 pmAh, Nice.At this point the -5v thing is a curiosity for the uninformed (me). Perhaps it has something to do with the source of the serial data. In this system there is one board that switches audio and video; my board - essentially a sub-matrix - is connected to this board through a RS-232 connection. Perhaps the other board sends the -5v signals which are somehow inverted once they reach my board and eventually the AD8113. Not sure.Here is a grab with default AD8113 values after power up - initialized by the chip on the board presumably and another grab after I use a console connected to the other board to switch the outputs.

author: tautech
author_rank: Super Contributor
timestamp: March 20, 2023, 10:47:07 pm
content: Negative going protocols are more normal and I have no idea why you show both.They are deemed Idle High.All channels should be DC coupled and trigger also, there is no reason on this good earth why you need AC couple any of these.

author: A.Williamson
author_rank: Contributor
timestamp: March 22, 2023, 02:09:40 pm
content: lmao good catch. the dc coupling normalized the signal polarity