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:


the host will send data to the peripheral. Used for commands.


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


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


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.


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.