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FT260 - Pauses in I2C comunication.

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allenhuffman:
This may not help, but...

Using the FT4222, we noticed timing gaps between groups of data (I seem to recall odd spacing every 3 bytes).

Somewhere (support? random internet searching?) we were told to look at the SetClock speed and set it to SYS_CLK_24.

FT4222_SetClock (ftHandle, clock);

I do not know if this changed anything.

HI_type:
Thank you allenhuffman

I will forward this information to our software engineer and see if he is willing to have a look.

Hopes are still to get a confirmation from somebody that this chip actually can run at 3,4Mbits actual throughput. An alternative could be the FT201X https://www.ftdichip.com/old2020/Support/Documents/DataSheets/ICs/DS_FT201X.pdf but it requires drivers, not a show stopper, but the lesser choice.

Thank you again for the input.

HI_type:
OK, i talked to our software engineer again and he had news: He used a USB package analyzer to see what the LibFT260.dll is sending and proclaims he would not be able to optimize this.

At the same time, the maximal transfer speed for HDI seems indeed to be 64kbit/s. This appears to be a hard limit. Going beyond this limit would again require a custom driver, defeating the point. Unfortunately, the FT260 is deemed unfit for our use case as we need an actual transfer rate of around 2Mbit/s.

Maybe FTDI would like to point this out explicitly in the datasheet? We have also seen the pause when sending a full set 64bits, effectively making a burst meant to be 2Mbit/s to be closer to 1Mbit/s.

Thank you for the support to everybody involved.

FTDI Community:
Hello,

Thanks for your updates and for your inputs too Allen,

Yes unfortunately the USB transfers are limited to 64K on the interrupt endpoints and so although the device can run at 12Mbaud and 3.4M I2C, the actual throughput will not match these.

We specify this on page 12 of the datasheet but it is in amongst the text and is also not directly referring to the baud and I2C rates. We can look at improving the datasheet to make sure that this limitation is clear and stated explicitly as you suggested,

Sorry to hear the FT260 is not suitable. The FT260 is ideal for many uses as it uses HID class but the throughput is limited by this. We have other hi-speed USB devices which use bulk transfers such as the FT4222H which might be an option for you,
Bulk transfers don't offer guaranteed bandwidth at any instant as the bandwidth is shared with other devices and so can depend what other devices you have connected to the PC, but the overall bandwidth is usually much higher than this 64K interrupt transfer on the FT260.

Best Regards, FTDI Community

HI_type:
Thank you FTDI community

I would be looking at the FT201X as we are constraint with space. Do you see any issues with that device regarding speed?

In our setup, this device will be only one in use when it is accessed. Thank you for the heads up.

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