An invalid log bit is set when the device detects corrupted log in the flash. The log is stored in an area in the flash. It has a checksum to protect its integrity (do not confuse it with data flash checksum, which protects the integrity of configuration data). Every time a new log is recorded, the checksum needs to be updated. In the event power is disconnected, the UCD90xxx device may not be able to finish writing new logs before the 3.3-V supply vanishes; therefore, the next time UCD90xxx powers up, the log content does not match its checksum, and the invalid log flag is set to warn you.
To avoid an invalid log, there are two approaches:
1. Properly sequence down all the rails before disconnecting power.
2. Implement a brownout circuit as described in the device datasheet.
The brownout circuit includes a Scotty diode and several capacitors to sustain a 3.3-V supply voltage for the UCD90xxx to finish fault logging in the event of power disconnection. The UCD90240, UCD90320, and UCD90320U devices have a dedicated checksum for each log entry. As a result, power disconnection does not generate this flag.
For more FAQ please check the following document.
UCD90xxx Family Frequently Asked Questions and Answers
UCD9090 UCD9090A UCD90120 UCD90120A UCD90124 UCD90124A UCD90160 UCD90160A UCD90240 UCD90320 UCD90320U