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I am driving a RGB led strip (ldp6806) with SPI.

Since the led strip does not have a chip select input, I would like to know how I can drive this strip with another SPI item connected to the Pi (like an arduino, another strip, or anything else...), by adding a "chip select"-like circuit between the RPi and the led strip (or any other SPI slave that does not have a chip select line).

Any idea how can I start ? Maybe I could build a simple logic gates circuit to have clock and MOSI flat when chip is not selected?

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Using simple logic gates to kill SCLK for any unselected slave on the bus is exactly the way to go. Turning SCLK flat should suffice, no need to also flatten MOSI.

Since Slave Select (SS) is active low an OR gate, e.g. 74HCT32 (a quad 2-input OR gate) should do the trick to or SS with SCLK. HCT should allow for 3V3 supply voltage and logic levels (please check the AC/ACT, HC/HCT families).

  • I have a bunch of NAND gates, I will make a circuit to disable SCLK. I will check voltages, but as far as I remember, Pi's GPIO are all 3.3V, that should work as input for 5V gates. I will double check. – Dede Dec 31 '15 at 0:59
  • @Dede, thanks for accepting my answer (though I would not insist, an upvote is also nice ;) ). In any case, feel free to report back with your findings and write your own comprehensive answer. – Ghanima Dec 31 '15 at 1:02
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The standard Pi SPI driver has two slave selects (CE0/GPIO8 on pin 25 and CE1/GPIO7 on pin 26).

If you are using SPI then simply connect the other SPI device to the other SPI channel.

  • Yes, but LED strips does not have salve select pins. So if I have two led strips that I want to drive individually, I have to make a circuit connected to the slave select pins (and mosi/clock) for each strip. – Dede Dec 30 '15 at 21:31
  • @Dede You need to add such (important) details to your question. – joan Dec 30 '15 at 21:43
  • I have added that led strip does not have SPI chip select. But I already said "or any other SPI that does not have a chip select line", that means the same thing. I you prefer, I can also add that items are not chained, but are connected directly to master pins. Other slaves could have been a RGB Led strip (no SS pin) with a SPI LCD display... – Dede Dec 30 '15 at 22:03

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