Electronics > Projects, Designs, and Technical Stuff
5V rail noise when updating 2x16 LCD
duak:
Txe, you have a number of digital signals changing state in the same cable at the same time. This charges and discharges the parasitic capacitances of the cable and LCD display and causes short current transients in the +5V and ground signals as well as in the PIC itself. I would increase the rise and fall times of the drive signals from the PIC to reduce the amplitude of the current transients. If this cannot be done by programming, placing resistors of 22R to 100R in series with the signals near the PIC can help. You may find that an RC network of 100R in series with 1n0 from each signal to ground at the display end may help because it damps the reflected signal from the unterminated end.
As already suggested, increase the number of ground signals in the cable to the display. If you can get some copper or aluminium foil, try wrapping the cable with the foil and connect it to circuit ground at both ends. This will form a shield and provide a low impedance return path for current transients.
txescientist:
duak,
It is not the bad probeing. Probed it with soldered coax and 50 ohm termination, still got the noise.
As you said it is the current transients in the cable. Just tested it with 100R, not significant improvement.
With 10k in series it works a lot better.
Many of you know Jim Williams app note on switching regulators:
https://www.analog.com/media/en/technical-documentation/application-notes/an101f.pdf
Think that the best path is to filter out these spikes from the analog part of the circuit. As I said,
these spikes get trough LF50CV. Funny part is that ferrite beads did not helped in this case.
They helped when I used them with the buck switchers and linear post regulators to get nice
clean voltage...
AndersJ:
10K seems a bit high.
Use a smaller R, and add capacitance instead,
as has been previously suggested.
The beads fail because the frequencies are low.
Also,
the source impedance of the drivers is low
which makes it difficult for beads to have an impact.
It is better to not create the transients in the first place,
rather than trying to suppress them afterwards.
MasterT:
I'd suggest to probe a data line across 10k using a scope, see if there is substantial current passing. Could be that even LCD powered correctly by 5V, internal circuits / input buffer of the LCD is running at lower voltage, 4.5V or whatever clever china engineer comes up with. And all transient noise is due to current injection into inputs, so some kind voltage converter may be better remedy than resistors.
mikerj:
--- Quote from: AndersJ on July 31, 2019, 05:23:35 pm ---10K seems a bit high.
--- End quote ---
Agreed, seems too high to me as well. Something in the few hundreds of ohms range should be fine, you only need to slow down the edges a little. Resistors should be close to the micro end of the cable.
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