Hello Bernd,
I've been working on the same Arduino/PCA9626 combination and I
experienced the same problem. (Also with the same code as shown in your
first post)
When counting the number of send commands, I found out that a total of
36 send operations are performed to complete all the register settings.
The total buffer of the Arduino I2C library is set to 32.
So there are 4 operations more required than would fit inside the
buffer; 4 operations times 4 leds per group is 16 leds that couln't be
initialised. That means only the 8 present leds are initialised.
Setting the buffer to 31 makes only 4 leds light up; the problem must be
within the buffer of the Arduino library.
I would suggest to set the buffer to 36 or higher, but even with a
buffer of 33 it won't work. It looks like the buffer is limited to 32
somewhere else than in 'Wire.h' and 'twi.h'.
My working solution right now is to split the initialization in 2 steps.
There must be a way to change the buffer length to 36 or more, but I
haven't found it yet.
Kind regards,
Koen M.