Internal details
=================


USB transactions are always initiated by the Host. MCP2221 supports Full-Speed USB at 1000Hz polling rate. That is, a transaction every one millisecond.

Each USB transaction can be:

Output: 
    the host will send data to the peripheral. Used for commands.
Input: 
    the host wants the peripheral to send data. Used for replies.

We need 2 USB transactions (2ms) for each command/reply. Thus, the highest DAC update rate for or ADC sampling rate is 500Hz.

The maximum length of a data payload is 64 bytes.


I2C transfers
-------------

Write transfer
~~~~~~~~~~~~~~

.. figure:: img/I2C_write_100bytes_usb3.svg

    Timeline of a 100 bytes I2C **write**. Open the image in a new tab to see it full size.

While the I2C engine is sending ``data1``, subsequent write commands will fail and will be ignored. Only when ``data1`` has already been sent, the device responds with OK and proceed to send the next data chunk ``data2``.


Read transfer
~~~~~~~~~~~~~~

.. figure:: img/I2C_read_75bytes_usb3.svg

	Timeline of a 75 bytes I2C **read**. Open the image in a new tab to see it full size.

While the MCP2221 is reading bytes, subsequent calls to *Read I2C data buffer* (40) will fail. When the buffer is ready (or full), the call succeed, the data is returned and the reading of next chunk begins.


Transfer failure
~~~~~~~~~~~~~~~~

Note that since the USB host only send or requests data at fixed intervals of 1 ms, the state of the last issued command may or may not match the actual state of the I2C transfer.

For example, in a read operation to a nonexistent device. The I2C transfer starts and even ends before the response to the write command ``91`` is read.

.. figure:: img/I2C_read_NAK.svg
   
    I2C read failure. Not acknowledge.

Moreover, like in the above successful reading, the *I2C read command* (91) succeed but the *I2C read data command* (40) fails, exactly as before.

I use internal I2C engine status code to differentiate between both cases. Unfortunately, not all the states are fully documented.