I2C Master with limited transmission sizes

I2C Master with limited transmission sizes

[Peter Hüwe]

Hi,

I'm currently writing a driver for a cheap usb-to-i2c master device, which 
unfortunately can only write 31 bytes (+slave addr) and read 32 bytes.

What is the correct behavior if a user requests larger read/writes?
Simply set i2c_msg.len to the bytes transmitted?
(in this case i2cdump -i doesn't honor this length ?)

Or should the master try to split up the requests?
but if so how, as some devices don't support continuous reads.

Thanks,
Peter

Re: I2C Master with limited transmission sizes

[Wolfram Sang]

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> I'm currently writing a driver for a cheap usb-to-i2c master device, which 
> unfortunately can only write 31 bytes (+slave addr) and read 32 bytes.

Which one?

> What is the correct behavior if a user requests larger read/writes?
> Simply set i2c_msg.len to the bytes transmitted?

No!

> Or should the master try to split up the requests?

If you have control over sending start/stop bits, you could omit the
stop condition unless you transfered all of the 32 byte chunks and issue
the stop at the end.

If you can't, don't advertise I2C_FUNC_I2C, but SMBUS functions (without
the block functions again). That will work for most devices.

Hope that helps,

   Wolfram


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Re: I2C Master with limited transmission sizes

[Peter Hüwe]

Hi Wolfram,

thanks for your answer.
> > I'm currently writing a driver for a cheap usb-to-i2c master device,
> > which unfortunately can only write 31 bytes (+slave addr) and read 32
> > bytes.
> 
> Which one?

CH341 based ones like this one http://wch-ic.com/download/down.asp?id=114
They go for about ~12 USD on ebay so quite affordable.

lsusb shows it as 
ID 1a86:5512 QinHeng Electronics CH341 in EPP/MEM/I2C mode, EPP/I2C adapter


> If you have control over sending start/stop bits, you could omit the
> stop condition unless you transfered all of the 32 byte chunks and issue
> the stop at the end.

This should be possible - I'll give it a try.

Once I've finished my testing I'll post it here for review.


Thanks,
Peter