Limitations and bugs

Chip or software limitations

USB speed limits

MCP2221’s command rate is limited by USB polling rate. Each command requires two USB slots. One to send the command to the device, and the other to get the response. See Internal details for details.

On xHCI controllers (USB 3.0), polling rate is 1000Hz. So you can issue one command every 2ms. That command could be a DAC update, ADC read, GPIO, I2C operation, etc.

On eHCI (USB 2.0), the maximum update rate I measured is 333 commands per second.

These ratios depend on multiple parameters. Like your USB hardware (including cable and hub), operating system, or the number of devices connected to the same bus.

I2C speed limit

Each I2C interaction requires multiple USB commands. See Internal details for details.

Sending one byte will require: setup, send data, and get the result. Reading one byte will require: setup, finish test, and read data.

Depending on your USB polling rate, each of these commands can take 2ms or more.

I2C speed (100kHz / 400kHz) only matters when you are transmitting a lot of bytes in a row. For a few bytes interaction, speed is limited by the USB polling rate.

Internal reference reset

From MCP2221A’s datasheet (section 1.8):

When the Set SRAM settings command is used for GPIO control, the reference voltage for VRM is always reinitialized to the default value (VDD) if it is not explicitly set.

This is compensated by software. But, due to the calling interval, there will be always a 2ms gap in the DAC output if it is using internal reference (not Vdd) when you change any pin function.

I2C crashes

Eventually, due to a glitch or unexpected timeout, the MCP2221 cancels an I2C transfer. The slave may be in the middle of sending a byte, and expecting some clocks cycles to send the rest of the byte.

MCP2221 is unable to start a new I2C transfer while SDA line is still busy. And the slave won’t release SDA until next clock cycle. So the whole bus hangs.

See LowSDAError.


  • The ADC seems to be always connected. So leakage current for GP1, GP2 and GP3 is greater than for GP0. Think of it as a very weak pull-down resistor on these pins.

  • This library does not work with password protected devices. You cannot use it to set or clear MCP2221’s Flash password.

  • Changing VID/PID not supported.

  • Maximum length for single I2C read or write operations is 65535 bytes.

  • Regardless of the output frequency, MCP2221(A)’s clock output has glitch every 1ms.

  • Cannot clear the Interrupt flag when ADC reference value is 4.096V and GP1 is low.

Software Bugs

Bug tracking system: