Ambiq Secure Bootloader Does Not Like Serial Monitor

[oclyke]

Ambiq Secure Bootloader seems to have some peculiar interactions with the Arduino Serial Monitor... for example:

1. Code uploaded using the ASB while the board's Serial Monitor window is open will hang up the microcontroller and be recoverable only by bootloading again with the Serial Monitor window closed. (Only repeatable on some platforms)
2. Rate of successful ASB loads is significantly lower when the Serial Monitor window is open. Closing the window makes ASB operation much more reliable.

The ASB was never intended to be used in the Arduino IDE and we don't fully understand the interactions that may be taking place. Thanks to the SVL the ASB is only a fallback/failsafe so we are not terribly concerned with fixing this issue. We are planning to remove the ASB option from the Arduino IDE menu to emphasize the "advanced" nature of using it. The ASB can still be used to burn the SVL bootloader onto the boards.

This GitHub issue serves as a record of the problem for anyone else who might stumble across it and wonder what in the world is happening.