How to guarantee "grid-looping clock" is running when dynamically driving VFD tubes
I see the following options, depending on what level of protection you want to achieve:


Use AVR's internal watchdog. Kick the watchdog in the same place where you send data to the driver chip. If you fail to kick watchdog for some reason, MCU will reset and presumably software will work properly again. Alternatively, you can detect that the reset was due to watchdog and disable the driver and just hang there.
Previous option will not protect you if MCU is kept in reset (e.g. during programming) or if the power of MCU is lost. To detect this situation, you need external watchdog chip. During normal operation you would send pulses to the watchdog chip. If watchdog does not receive those pulses, it will disable the driver chip.
However, previous options will not protect you from the bug in software which would send "bad data" to the driver, potentially stopping the looping. To detect this situation, you can use second MCU to monitor outputs of the driver chip and disable the chip, if outputs are stuck.