Sharing the SPI bus for ISP and a nRF24L01

Development Boards MicroMod
1

I’m putting together an all-on-one-board project, and one thing I haven’t been able to figure out is how to isolate the SPI interface from the peripherals (in this case a nRF24L01) when doing ISP.

I know I could jumper the bus, but as that’s rather fiddly, I’d prefer to use series resistors to do the isolation. My question is, how would I know what value resistor to use? I’ve found out that the programmer I’m using can handle pull-up down to 1 kOhm and pull-down down to 10 kOhm. Using the latter would mean I’d be safe, but then what’s to stop that same resistor from interfering with normal SPI operation?

Or do I not need to worry about isolating the SPI bus at all?

2

I think that as long as the nRF24L01’s select pin isn’t asserted, it shouldn’t interfere with other traffic on the SPI bus. :?:

3

Thanks. Took me a while to find, but the SPI timing diagrams do imply that MISO is hi-Z when CSN is high.

4

Just add a pull-up resistor (10K to 100K) to the nRF24L01 chip select pin. So when you program the AVR the CS pin will be high. This ensures it won’t cause troubles.