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I got a Humble Pi prototyping board and build a 10-channels logic level converter using this schematic: http://i.stack.imgur.com/gS3SS.jpg

The level converter is connected directly to all communication pins: SPI, I2C, UART and a few other GPIOs.

Everything seems to be working well (I tested the channels with a multimeter and it is working, as well as I tested the I2C using a MPC23008 working at 5V) .

However when I try to use my nRF24L01+ connecting it on the 3.3V side (the 5V side I'm going to use to programme some AVRs later on) I can communicate with it, but the data is getting corrupted and the nRF24L01+ receives the wrong settings...

When I remove the Humble Pi with the level converter and connect my nRF24L01+ directly to the GPIOs it works perfectly.

What might be the problem? Is it because the SPI pins now have a 10K pull-up resistor?

What kind of tests can I do to figure out the problem?

I'm using an Raspberry Pi Model A bought from CPC Farnell less than one month ago with Raspbian.

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    Adafruit is using the same setup in their logic level converter. They say the 10K's do make the interface a little more sluggish. So it could be a speed issue. More info on this forum post: forums.adafruit.com/viewtopic.php?f=8&t=38358
    – Gerben
    Nov 18 '13 at 20:05
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As you suspect the 10Ks are probably the issue, SPI connections are not expected to have either pull-ups or pull-downs on them.

You would probably need to use a different method of logic level conversion. Since the SPI has uni-directional lines you can probably get away with the clock and MOSI lines connected directly to the 5v device, and the MISO line connected using a simple voltage divider into the Raspberry Pi.

If you do connect 3.3v outputs directly to a 5v device just ensure the 5v device does not accidentally drive those pins to 5v otherwise it may damage the GPIO.

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  • Would change the resistors help? Maybe remove the pullup on the 3.3V side? I really want to have a level shifter to protect the Pi...
    – Tiago Queiroz
    Nov 18 '13 at 23:55
  • It might, but there is a chance that the transistor will get hot when TX_LV is 'floating' and TX_HV is high, which may lead to smoke / fire (I would need to check the data-sheet on the FET carefully). SPI is more sensitive then the other pins around this kind of problem, I really can only suggest two ideas:
    – PiBorg
    Nov 19 '13 at 10:05
  • 1. As Gerben commented have you tried using slower SPI speeds, the main problem with using pull-ups or pull-downs is they have the effect of slowing the line down
    – PiBorg
    Nov 19 '13 at 10:06
  • 2. A more complex push/pull type converter, you can get these as single chips from various sources. One example I have used before is a TXB0104 (ti.com/product/txb0104) which is capable of 3.3 to 5 translation in both directions, and should work with SPI from experience.
    – PiBorg
    Nov 19 '13 at 10:09
  • thanks a lot! I reduced the speed from 8MHz to 1MHz and now it's working perfectly!! I'll also think about buying one TXB0104.
    – Tiago Queiroz
    Nov 19 '13 at 23:41

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