Need help understanding oscilloscope outputs  - Page 1
Forum section: electronics / beginners

author: UnkaHeemee
author_rank: Newbie
timestamp: May 12, 2023, 04:52:12 pm
content: I want to know what to make of the waveforms I am seeing on my scope output.I am working on a Commodore 64 and I want to start with the 6510 CPU. The clock signals are easy for me to interpret but I don't know what to expect for the address/data lines and other pins.I think I am basically looking for a reference I can compare my outputs to. I want to know what it should look like and what it looks like where it has known issues.Any help will be much appreciated.Edit: Typo. Meant to say 6510 but wrote 6502 mistakenly

author: wasedadoc
author_rank: Super Contributor
timestamp: May 12, 2023, 05:11:43 pm
content: The main (and maybe only) thing a scope will reveal about the address and data lines is whether they are jumping up and down and between the correct voltages.

author: tggzzz
author_rank: Super Contributor
timestamp: May 12, 2023, 05:18:33 pm
content: Start with the 6502 (and other) datasheets to understand what you ought to be seeing. They will define the voltages and relative timings required to assure proper operation.Use a scope to see the voltages vs time, especially for signal integrity problems that might violate those stated requirements, leading how inputs not being able to correctly interpret the voltages as digital signals. Understand how poor probing techniques can lead to misleading waveforms being displayed by the scope.Once signal integrity is assured, use a logic analyser to capture the digital signals.

author: RoGeorge
author_rank: Super Contributor
timestamp: May 12, 2023, 05:22:31 pm
content: An oscilloscope can show you if a data or address bus line is in short, or doesn't move. The waveforms will look rather dirty, particularly on a 3-state bus. Apart from telling if a line is stuck there is not much you can interpret by looking at a data line.What do you intend/expect to see?

author: srb1954
author_rank: Super Contributor
timestamp: May 12, 2023, 09:48:40 pm
content: Quote from: UnkaHeemee on May 12, 2023, 04:52:12 pmThere aren't really any reference waveforms for a microprocessor running in a normally operating system. Unless the microprocessor is locked up in a very tight, repetitive loop the data and address lines will just look a complete jumbled mess on the scope. About the only thing you can tell in that case is whether there is activity on the data bus or address bus and that the logic levels are correct. Even that can be confusing because some of the address lines may appear to be be inactive depending on what portion of the S/W is running at the time. The data bus can also appear to have invalid logic levels if no device is currently driving out on the bus.However, you can get some useful information by looking at the timing of the various signals. I recommend that you closely study the timing diagrams in the datasheet before you even hook your scope up to the circuit so that you can interpret what you are seeing on the scope.The 6502 has a relatively simple bus protocol where all the bus activity is timed in relation to the Phi2 clock output. By triggering your scope off the Phi2 signal you can see the timing relation of the data and address bus signals and when peripheral devices respond on the bus. You can get a more detailed look at how any particular device responds by triggering the scope off the CS signal to that device.

author: DavidAlfa
author_rank: Super Contributor
timestamp: May 12, 2023, 09:49:14 pm
content: Normally what you want to see is all the address/data lines toggling. If any is permanently high/low, might indicate a problem.Start by following the schematics, verifying pin-to-pin connection, and searching for shorts between lines.I've repaired some old boards where some vias got open, thus breaking some data/address line!

author: tggzzz
author_rank: Super Contributor
timestamp: May 13, 2023, 12:55:43 am
content: Quote from: srb1954 on May 12, 2023, 09:48:40 pmThere are very few referencewaveformsin a digital circuit. There are many referencedigital signals, as shown in the data sheet.The waveforms definitely shouldnotlook like a jumbled mess; the waveform eye diagrams should indicate whether there is sufficient signal integrity.QuoteWaveforms often have to be ignored, since they are "undefined" or "don't care". Oscilloscopes are very poor at that. Logic analysers are very good at ignoring irrelevant waveform artefacts: they can be set to only record inputs when clocks occur and chip-select and enable and r/w signals are active.So, use the right tool for the job: scopes for signal integrity, logic analysers for digital signals.QuoteUse an LA to capture the bus state on the phi2 clock and when various signals active. A simple use case is only capturingdataat the instant (i.e. clock edge) it is written (i.e. r/w=e) to a particular i/o chip (i.e. that chip's CS or enable).That's very difficult with a scope.

author: rstofer
author_rank: Super Contributor
timestamp: May 13, 2023, 01:08:51 am
content: If the board has a replaceable EPROM or EEPROM then you can write short snippets of code where you know what the waveform should look like. For example, just increment the address register (you'll see the Address bus increment) and write data equivalent to the low 8 bits of the address so that the data bus looks like a short version of the address bus (during the write cycle). Obviously, the address and data bus will have real values when they are fetching instructions and branching back to increment the address.When you work on uCs, you need to be creative in debugging.A logic analyzer is a better tool. A Digilent Analog discovery 2 (or Digital Discovery) is an excellent, albeit expensive) tool for uC work..

author: AndyBeez
author_rank: Frequent Contributor
timestamp: May 13, 2023, 09:00:40 am
content: Use Google?Scope outputs:http://tech.guitarsite.de/c64_scope.htmlAnd TV diagnostics :https://www.pictorial64.com

author: Aldo22
author_rank: Super Contributor
timestamp: May 13, 2023, 09:35:37 am
content: This guy is fixing a C64 with a cheap DSO1511G.Jump to 17:30 if you don't want to see the intro.I don't know if it helps.

author: Vovk_Z
author_rank: Super Contributor
timestamp: May 14, 2023, 12:49:21 pm
content: Quote from: Aldo22 on May 13, 2023, 09:35:37 amI've seen at least several his videos, including this one. This guy seem to be very helpful. I learned a lot from him.