RFM69W voltage dividers?


  • Hardware Contributor

    I ordered a couple of RFM69W radios since I'm having some range issues and like the built in encryption they provide. I just realized that my gateway and most of my pro minis are 5V and the radio can't handle 5V inputs so I need some kind of level shifter. From reading other forums, it sounds like the NSS, MOSI, and SCK lines need to be shifted (but not MISO and IRQ). I've seen resistor networks and several different level shifting chips used for this and they all seem to have various pros and cons listed but it's not obvious which of those reasons apply to the RFM69 radios.

    So before I start ordering parts, I was curious what people are using for 5V->3.3V level conversion on the data lines to the radio? My search turned up the TXB0104 (4 channel) converter as a reasonable option that isn't too expensive and has several different packaging options.


  • Admin


  • Hardware Contributor

    Thanks! Out of curiosity, why is the I2C level shifter better for the radio than non-I2C versions I was looking at?


  • Hero Member

    +1

    BTW, would it maybe be easier to simply power your 5V gateway at 3.3V instead of 5V? I haven't yet tried that experiment, but it seems like it would be the shortest path. Of course, if you have anything 5V that's powered by the gateway, then not, but otherwise, maybe yes?


  • Hardware Contributor

    @NeverDie In general, yes - 3.3V would be easier (this isn't a gateway - it's the sensors but that doesn't matter). However in my case I'm controlling some 5V actuated valves and a 5V sensor so I bought 5V Pro Mini's and I wasn't sure how much CPU horsepower I needed so I thought the 16MHz clock might be also useful.



  • Do we have to use such a divider when using NRF2L01 with pro-mini 5V ?


  • Hardware Contributor

    @fets No - the NRF radio has 5V tolerant inputs (you still need to supply it 3.3V on VCC, but it can handle 5V on the signal lines). The RFM radio does not - it needs 3.3V on the signal lines. So the NRF works fine w/ 3.3V and 5V arduino's. The RFM radio needs this modification when used with 5V arduino's.

    FYI - I found someone who build a small break out board for the radio and had an optional level shifter using a simple resistor network and said it worked great. I liked the idea, but not his board layout so I modified it and ordered some from OSHPark a few days ago. The original board is here. My modified board (no 3.3V regulator or flash support) is here. I wanted minimum width in order to mount the board on the side of a box (thickness dimension). Warning: I haven't tested this yet (order arrives in 2 weeks) so I haven't published schematics or anything else for it yet so I wouldn't order it if anyone is interested. There are tons of RFM69 breakout boards with various features (including one that plugs into an NRF 2x4 socket) if you search for them on OSHPark.


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