The XL has a Real Time Clock (RTC)function and a Watch Dog Timer function that are all built into the same Chip. During the boot process, the XL must turn off the Watch Dog Timer until the boot progresses far enough that it can be re-enabled and maintained.

If the RTC cannot be accessed, the boot code will stop the boot process and output a bit pattern on a GPIO pin. In some board Revs, the GPIO pin controls a hearbeat LED and the pattern can be read from there.

The test for accessing the RTC had several flaws in it that prevented the test from doing its job. If the RTC was not functioning the boot code tried forever rather than generating the bit pattern. This has now been resolved.

This bit pattern in primarily useful during repair or initial production of the board as a tool to point to the cause of boot issues.

During initial boot there are a certain level of expected failures as the Chip initializes. The number of expected failures was determined empericaly and then doubled to be conservative.