Help with noisy signal in retro-style computer - Page 1
Forum section: electronics / projects

author: rea5245
author_rank: Frequent Contributor
timestamp: March 09, 2025, 02:12:17 pm
content: I'm seeing a noisy signal, as shown in the attached pic. Usually, the signal is a reasonable square wave, but occasionally it has a huge amount of noise - sometimes enough to be interpreted as a transition.The device I'm building is an Altair-style retro computer, i.e. a backplane motherboard with boards plugged in. The blue trace is the memory ~CE signal on the backplane bus. The yellow trace is the same signal on the memory board, at the memory chip. As you can see, the noise gets worse closer to the chip.What is the fix for this? Is there a fix I can easily bodge onto the current board, or am I going to need to make a new PCB?Any thoughts on why the noise would happen on some pulses but not others? It does not appear to be regularly periodic.Thank you.

author: CaptDon
author_rank: Super Contributor
timestamp: March 09, 2025, 02:43:49 pm
content: On memory cards you should have a .1uf bypass capacitor for every I.C. on the board and the cap should be as close to the i.c. as possible. Normally the power rails are parallel and run under the i.c.'s and there is a cap at the end of every I.C. Two things come to mind looking at the scope shot, the first is buss contention where two devices are trying to drive the buss at the same time. The second is a voltage regulator acting up or self oscillating under certain load conditions. Check your bulk capacitors for high ESR. The memory card should have some bulk capacitors also. Is the memory single +5 or are there other voltages involved?

author: rea5245
author_rank: Frequent Contributor
timestamp: March 09, 2025, 03:22:37 pm
content: Single 5V. The power supply is an off-the-shelf ATX PS, so I trust that. There's a 10uF cap on the 5V coming onto the board, and 0.1uF caps at the memory chip's multiple Vcc pins.There's no other circuit that could be trying to drive the bus.

author: tggzzz
author_rank: Super Contributor
timestamp: March 09, 2025, 04:26:14 pm
content: Show:schematic includingallcomponentsconstruction including layout concentrating on power supply tracksphoto of probing techniqueThe overshoots at transitions could be a consequence of probing technique, or construction/layout, or both.The single cycle where there is more noise near 0V is presumably correlated with something happening elsewhere in the circuit - but what?That the "noise" lasts exactly one high period is interesting. It suggests something might be oscillating. Verify that no digital input is left floating, thereby allowing oscillation.Be aware that the highest frequencies in your circuit are completely unrelated to the clock frequency/period: they dependsolelyon the signal rise/fall times. Modern jellybean logic creates significant energy at several GHz. FFIhttps://entertaininghacks.wordpress.com/2018/05/08/digital-signal-integrity-and-bandwidth-signals-risetime-is-important-period-is-irrelevant/

author: rea5245
author_rank: Frequent Contributor
timestamp: March 09, 2025, 04:34:10 pm
content: Here's some new info. Since CaptDon suggested power issues, I put a probe on the +5V right at the memory chip. It's the magenta trace.As you can see, noise in the power corresponds to noise on the ~CE signal. The amplitude of the noise is about 1V!

author: tggzzz
author_rank: Super Contributor
timestamp: March 09, 2025, 04:37:51 pm
content: Quote from: rea5245 on March 09, 2025, 04:34:10 pmWhere did you put the ground, and how long is the lead?For examples of why that is important, seehttps://entertaininghacks.wordpress.com/2015/04/23/scope-probe-accessory-improves-signal-fidelity/

author: rea5245
author_rank: Frequent Contributor
timestamp: March 09, 2025, 04:48:31 pm
content: Quote from: tggzzz on March 09, 2025, 04:37:51 pmThe bus probe is on the bus' ground line and the chip probes are on the chip's ground lines. The probes' ground leads are 6" long.

author: Dhela
author_rank: Newbie
timestamp: March 09, 2025, 05:01:12 pm
content: Ensure that there are adequate decoupling capacitors near the memory chip and other ICs on the memory board. These capacitors help to stabilize the power supply and reduce noise.You can try adding small ceramic capacitors (e.g., 0.1 µF) between the power and ground pins of the memory chip as a quick fix.Adding ferrite beads on the power lines near the memory chip can help filter out high-frequency noise. Adding small series resistors (e.g., 22-100 ohms) on the signal lines can help to dampen reflections and reduce noise.Ensure that the grounding is solid and that there are no ground loops. Poor grounding can often lead to noise issues.Check the continuity of the ground connections between the backplane and the memory board.If the noise is being picked up from external sources, consider shielding the memory board or the entire backplane.Properly terminate the signal lines to prevent reflections. This can be done using series termination resistors or parallel termination resistors, depending on the design.Ensure that the traces on the PCB are designed with proper impedance matching to minimize reflections.Use a solid power plane and ground plane to provide a low-impedance path for the return currents.Route critical signals away from noisy areas and ensure that they have a clear return path.If designing a new PCB, consider using a multi-layer board with dedicated power and ground planes.The noise might be related to the timing of other signals on the bus. Crosstalk from adjacent signals can cause intermittent noise. Variations in the power supply due to other components switching can cause noise on the signal lines. Improperly terminated lines can cause reflections that appear as noise, especially on longer traces.

author: tggzzz
author_rank: Super Contributor
timestamp: March 09, 2025, 05:43:34 pm
content: Quote from: rea5245 on March 09, 2025, 04:48:31 pmThe reference I gave indicates the problems with that.Schematic? PCB layout?Don't expect people to be able to find solutions until after the causes are understood.

author: rea5245
author_rank: Frequent Contributor
timestamp: March 09, 2025, 06:28:55 pm
content: Quote from: tggzzz on March 09, 2025, 05:43:34 pmIf the ringing is a probe artifact, wouldn't it happen on every cycle of the square ware? Also, although I can't guarantee exact timing, the noise on the scope seems to correlate with crashes of the retro computer.I've attached the schematic and PCB for the board showing the problem. The memory is a 7025 2Kx16 dual-port static RAM. I'm using only 16 locations of it; these are the CPU's 16 bit registers. There are also two ATF1508 CPLDs on the board. These are the ALU. The idea is I read registers on the left and right of the dual-port RAM, perform an operation in the ALU, and latch the result into the 74HCT16373. That result gets written back to a register on the next cycle.(The ATF1508s have bypass caps on them, but you don't see them in the schematic. I created custom footprints that have the cap footprints "built in".)The way the result gets written back is that the result goes out on the backplane, a mux board routes the Result to the Left side of the DPRAM, while microcode signals do a write.The misbehaving ~CE signal is actually a ~ValidMicrocodeSignals line. The memory's R/~W and ~OE also come from the microcode. This whole thing started when writes to the registers would occasionally fail - a garbage value would get written. I'm guessing - but I'm not fully convinced - that a noisy ~ValidMicrocodeSignals line could cause trouble like that.In the PCB layout, notice the 7025 and ATF1508s are socketed PLCCs surrounded by pin headers for debugging (which are also part of the custom footprint).I've also attached the KiCad files, zipped up.Thank you.

author: tggzzz
author_rank: Super Contributor
timestamp: March 09, 2025, 07:21:56 pm
content: Ringingdoesoccur on every cycle; the "noise" does not.The ATF devices are fast, with many outputs switching simultaneously.The ATF devices change internal power depending on whether any inputs are changing.The graph 3-11 inhttps://ww1.microchip.com/downloads/en/DeviceDoc/ATF1508AS(L)-5V-128-Macrocell-CPLD-Data-Sheet-20006682A.pdfis strange. I don't understand it since it appears to indicate negative resistance. Negative resistance can lead to oscillation.I suggest you do a calculation to find the ground bounce.CV=Q=It, where V is the output voltage swing, t is the transition time and C the load capacitance. Multiply by the number of outputs switching simultaneously.Then dV=LI/t, where dV is the voltage induced by the current I through inductor L switching in time t. Divide L for one pin (20nH?) by the number of gnd pins.Compare dV with the input voltage margins.I think you will be unpleasantly surprised.It is difficult to tell from your layout, but it appears to be a 2-layer PCB without solid power planes. That is a warning flag at these speeds.So, does the "noise" occur when many outputs are switching simultaneously?

author: PCB.Wiz
author_rank: Super Contributor
timestamp: March 09, 2025, 07:23:40 pm
content: Quote from: rea5245 on March 09, 2025, 02:12:17 pmThat's nasty. Something must be triggering it.Try to find what activity it coincides with.Quote from: rea5245 on March 09, 2025, 06:28:55 pmIs that 4 layer PCB?ATF1508 are power hungry, so need a low impedance supply.Also check you havenofloating pins.

author: PCB.Wiz
author_rank: Super Contributor
timestamp: March 09, 2025, 07:26:37 pm
content: Quote from: tggzzz on March 09, 2025, 07:21:56 pmThat's the pin-keeper action, and it does not really look like that.That's someone in publishing using a curve fit to data they did not understand.

author: rea5245
author_rank: Frequent Contributor
timestamp: March 09, 2025, 11:23:58 pm
content: Well, making sure the inputs to the ALU were not floating, did not fix it. :-(