mgregg wrote:Turning off i2c_vc=on opens /dev/i2c-1 only.
Correct. i2c_arm=on will enable /dev/i2c-1 on all revisions of the Pi, except on the 256MB rev1 model B from 2012 where it enables i2c-0.
It is there to enable the I2C controller that is available for the ARM to use, hence the name. i2c_vc is there for the I2C bus assigned to the GPU (VideoCore).
mgregg wrote:But that port doesn't find anything on that bus if they are plugged into pins 3&5.
Details of what you've done then. Many many people are using i2c-1 on pins 3&5 without issue.
mgregg wrote:I'll be happy if you can prove me wrong.
You need to provide a load more information then. No one else can see your setup.
- I am assuming this is a stock Raspbian, Raspbian Lite, or NOOBS install. I'm afraid if not then you're on your own/need to go to that distos support forum.
- As Dougie has just asked, what board revision are you dealing with on your old and new boards?
- The pin assignment of pin 13 also changed between rev1 & 2 from GPIO27 to GPIO21. I take it that you aren't using that to gate any of your I2C comms.
- Have you proved this exact board is fully working using a different Pi?
- (suck eggs) Is the board correctly aligned on the header?
- Does i2c-detect -y 1 run at a sensible speed, or is it slow? Slow response would be indicative of a short to ground on SCL enforcing clock stretching.
- Check with "sudo raspi-gpio --get" that GPIOs 2&3 are assigned to ALT0 and therefore SDA1 and SCL1. No other GPIOs should be assigned to the SDA1 or SCL1 function, otherwise you will get issues.
- Stick a scope or Piscope
on pins 3&5/GPIOs 2&3. Are the lines waggling?
mgregg wrote:It looks as though if you have i2c on pins 3&5, without software i2c, you cannot use those devices.
Sorry, that is absolute twaddle.
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