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

Alexandros adrcki at gmail.com
Wed Sep 2 22:12:33 CEST 2020

On 2/9/20 7:19 μ.μ., Martin Peach wrote:
> 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

Plugging an Arduino straight into the Pi is surely an easier solution,
but what I'm trying to do is for a more complex setup. There's already a
Teensy connected to the Pi over serial and I want to be able to control
more micro-controllers without definitely knowing the exact number of
them. For such a case I2C is probably the best solution as it uses the
same 2 pins for any number of connected devices on the same bus, whereas
SPI needs at least three pins for a one-direction communication, plus an
additional pin for each additional slave.

In the link you posted, in the answer there's this chunk of code:


|charbuf[size +1];// note: variable length
arrayioctl(file,I2C_SLAVE,address);// real code would need to check for
an errorbuf[0]=subaddress;// need to send everything in one call to
writememcpy(buf +1,data,size);// so copy subaddress and data to a buffer
write(file,buf,size +1);} ``` |and the OP says this worked, where in the OP he mentions he needs to 
send 189 bytes. I don't really understand this bit of code, but I do see
these functions (ioctl() and write()) are used in your code as well. In
the worst case scenario I'll go with 32 bytes... I'll definitely check 
your [pispi] object though, I really like your work for interfacing Pd 
with peripherals!

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