[PD] [pii2c] won't send more than 32 bytes

[Martin Peach]

On Tue, Sep 1, 2020 at 1:19 PM Alexandros <adrcki at gmail.com> wrote:
>
> The MAX_I2C_BUF_SIZE macro in pii2c.c is set to 64, but still when I try
> to send 64 bytes to a slave Teensy, only 32 go through, the rest 32 are
> all 0s. Raising this value to an even greater number, like 128 (which is
> something I would like to implement), gives similar results. The same
> behavior occurs whether I set the I2C address as an argument to the
> object, or if I pass it through the "write" message.
>
> I checked the code and couldn't find where this occurs. I added this:
>
> `post("%d", x->x_xferbuf[i-1]);`
>
> to line 348 to make sure the bytes are stored in the object's buffer,
> and they do get stored properly. The only place I found they end up in
> is a "write()" function, called in line 293, inside the pii2c_writer()
> function. I guess this function is from one of the header files included
> in the object, as I couldn't find it in the code.
>
> Long story short, is it possible to send more than 32 bytes over I2C in
> the Pi? I really like [pii2c], and since wiringPi is now deprecated, I'd
> like to stick to this object.
>

It seems to be an issue with the kernel driver. Whoever wrote it got
it into thieir head that there was a 32-byte limit. I found this
discussion:
https://stackoverflow.com/questions/25982525/why-i2c-smbus-block-max-is-limited-to-32-bytes
The same thing happens in Arduino, but there you can edit Wire.h to
get bigger messages.
I guess it will be fixed eventually.
Meanwhile, do you really need to use I2C? SPI is much faster and has
no limit to the message size. I also made a [pispi] external...
I find in general it's easier to plug an arduino into the pi to do
realtime stuff.

Martin



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