Add 4 pin mode for ATTINY85 (much less power and still one pin free)

[ortegafernando]

Add a simple option (CSN = 0) to control RF24 with 4 pins of the ATTINY85.

This way you could use ackPayload functionallity and, more important, less power, as:

1. in 3 pin mode, CE is always HIGH so we will consuming about 12-13 mA
2. in 4 pin mode, CE is control by ATTINY so we will consuming 12-13 mA durings writes and almost nothing between them

NOTE: Sorry if I can't explain well, but I have check y modifications and they are working.

[soligen2010]

I had already made changes for this, but not merged yet. See commit #356

[ortegafernando]

Hi @soligen2010 may be your commit is for keep CE pin HIGH and use CSN pin "properly"

Mine is just other way round. Use CE pin "properly" (so you can sabe power between writtings and use ackPayload feature) and use CSN pin with the 3-pin trick.

Both are compatible, but mine needs the changes that I have submit.

Regards,

[vverdot]

Hi @ortegafernando , sorry to dig up an old thread, I am very interested in your solution (freeing 1 pin on Attiny85 while preserving low power mode of nrf24). I didn't find many other alternatives to achieve the same goal and I don't know why this wasn't merged, but I plan to use it in a personal project. Any advice, caveat, general feedback regarding your approach? Thanks.

[2bndy5]

I'm uneasy about using 0 as a sentinel for this. It might confuse people reading the docs as 0 is also used for the CSN pin on Linux's SPIdev implementation. Is there a problem with using a negative number for this sentinel instead?

[kripton]

typo: funtionallity -> funtionality

[vverdot]

"functionality" even better ;)