NRF Radio issue on shared SPI
I have an arduino pro mini that I have an NRF24 radio hooked up to and six PN532 RFID readers on a shared SPI. I was planning to have each of the readers on a two and a half feet long ribbon cable. The radio only works, if there is a maximum of 2 RFID-s hooked up on short (less then 8 inches) jumper cables. If I hook up a third one, or I just hook up the long ribbon cable w/o the RFID reader, the radio gives me a check wires error. The fact that it does not work when the long cable is attached points me to it being interference issue, but I’d expect that only to affect sending/receiving data between the radio and the gateway, not to make it impossible to even start the radio up and communicate with it. So that points to it being the length of wire so maybe a resistance issue. I am thinking of maybe using bus transcievers between the MISO, SCK and Mosi lines, but that is just a best guess. Please, point me in the right direction with what the issue may be. Thanks very much!
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@Julia-J. the recommendations in https://www.dorkbotpdx.org/blog/paul/better_spi_bus_design_in_3_steps might be useful to make the bus more stable.
You do have a capacitor for close to the nrf right? I know it shouldn't matter but unstable power often causes strange things to happen.
Another alternative could be to use softspi to place the nrf on separate pins. An example is available at https://www.mysensors.org/build/ethernet_gateway
As suggested in http://electronics.stackexchange.com/questions/166832/multiple-spi-busses-in-ribbon-cable?rq=1 connecting every second wire in the ribbon to ground can shield the signals from interference.
Maybe lowering the SPI speed helps. I think that is done by setting
//#define MY_RF24_SPI_MAX_SPEED 4000000
to something lower, maybe try 1000000 instead.