2 days and 5 forum posts later and the picture is clearer. Let’s start from the beginning. To simplify I’ll refer to the Raspberry pi rev. 2 board and ignore rev. 1 and the A+/B+ (** check the notes for details).
The Raspberry pi has the i2c-0 and i2c-1 busses. The former** is used by the GPU to talk to the camera while the latter can be used by the user however she/he wants. Each of the 2 busses can be redirected to several physical pins by changing the mode of these pins:
GPIO 0/1 – ALT0 mode
GPIO 28/29 – ALT0 mode
GPIO 44/45 – ALT1 mode
GPIO 2/3 – ALT0 mode
GPIO 44/45 – ALT2 mode
Only one of these pairs can be activated at any moment per bus.
The camera is physically connected to GPIO 0/1 **. These pins are setup as inputs by default and the GPU will change them to ALT0 whenever it needs to talk to the camera but _switches them back_ to INPUT immediately after. If you monitor the mode of GPIO 0/1 you’ll see that most of the time they are INPUT with random switching to ALT0 0-30 times per second. It seems that the more movement the camera sees the more it talks to the GPU. It seems to be related to AWB and shutter speed.
So far – no problem. It’s pretty clear from the design of the Raspberry pi that i2c-0 is off limits and there is no way to synchronize the GPU access with the CPU – so i2c-0 cannot be shared between the camera and any other device. If one attempted to use i2c-0 and started ‘raspivid -t 0’ he would see weird things ranging from i2c errors, the image freezing for seconds, random noise on the screen or even the board completely freezing.
The OdroidW has a nice PMC 5t619 chip that provides 2 free ADC pins that I intended to use to monitor voltage and current of silkopter. The PMC uses i2c-0 to talk to the CPU so care must be taken to synchronize somehow with the GPU. This is what I’ve been trying to achieve the whole weekend…
– 1st try: After every use of mmal I switched GPIO 1/2 mode from IN back to ALT0 so I can talk to the PMC. Didn’t work as it seems the GPU uses the bus even between calls to mmal (in hindsight it makes sense as the camera has to inform the GPU about starting and finishing transfers and the GPU has to set awb, gains and shutter speeds).
– 2nd try: Use a semaphore to trigger the ADC measurement in the mmal callbacks – hoping that after the callback I get a period of silence from the GPU. No such luck
– 3rd try: Give up using the hw i2c-0 bus and try bitbanging on GPIO 0/1. This was such a nice idea – let the GPU use the hardware i2c-0 bus and I’ll use bitbanging… Didn’t work as both the GPU and the PMC are connected to GPIO 0/1 pins – so they share the same physical pins….
So basically it looks like the OdroidW is not capable of using both the camera and the PMC at the same time because they share the i2c-0 bus but _also the pins_!!!. This could have been easily avoided by putting the PMC on some other GPIO and bit banging, or at the very lease putting it on i2c-1.
So now I’m back to the drawing board and started considering an Arduino mini board as ADC + PWM generator. Connected through i2c-1 probably..
** Rev.1 boards have i2c-0 and i2c-1 reversed. So i2c-0 is free while i2c-1 is the camera one. ** A+ and B+ have the camera using GPIO 28/29 pins to access i2c-0.