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Topic: Led Strip APA102C - 30led/m - White R1/R2/R3/R4??? (Read 135 times) previous topic - next topic

KFMAKR

Hi Everyone,
Is there someone who know the use of R1/R2 & R3/R4???
A 1m strip is split in 2 sections: 50cm for 15xSMD5050
It's an SPI communication.
R1 connected to DI to the first SMD5050
R2 connected to CI to the first SMD5050
R3 connected to DO from the last SMD5050
R4 connected to CO from the last SMD5050

160Ohm for all resistors.

WHY?

each led SMD can be cut individually but what is the need related to these resistors on SDI and CLK??
Resistors are serial .... it's not a pull-up or pull-down connection

Grumpy_Mike

#1
May 18, 2019, 05:54 am Last Edit: May 18, 2019, 05:54 am by Grumpy_Mike
Quote
but what is the need related to these resistors on SDI and CLK
They are to help to prevent reflections from the impedance mismatches, although I would say they might be a bit too high. This is similar to the seriese resistor you use on the single data line WS2812 strips.


PaulRB

they might be a bit too high.
Why's that, Mike?

Grumpy_Mike

Any series resistor on an input runs the risk of preventing the device bring inputted from seeing a sufficiently low voltage for it to be recognised as a logic low. It also increases the noise floor for the device, even if it still works, making it more susceptible to interference.

Of course it depends on the nature of the device you are driving but I would shy away from values over 1K.

PaulRB

Thanks Mike. The OP did say 160R, wondered why you thought that was too high, that's all. Maybe you mis-read it.

Grumpy_Mike

Quote
160Ohm for all resistors.
:smiley-yell:  :smiley-lol:  :smiley-grin:
Yes mistook the O for an 0
Read that as 1600 Ohm.

KFMAKR

Ok Thanks a lot...
I planned to distributed several led on a support, so I've to keep this in mind and maintain these resistors on the way.
Great

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