Limitations and bugs

Chip or software limitations

USB speed limits

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.

  • MCP2221 status is saved only to SRAM, so it is non persistent. Only USB power options are written to Flash. See save_config().

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

Software Bugs

None reported.

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