How do I use an external TTL clock signal to synchronize the BM time tag counter?

Use the function BusTools_TimeTagMode to select the API_TTM_AUTO mode. Provide the period (in microseconds) of your external clock signal in the TTPeriod parameter. The example program EXAMPLE_BM1B.C demonstrates this.

API_TTM_AUTO is preferred over API_TTM_SYNC since it does the update with the microcode on the board while SYNC does the update in software (API) and is much slower. For example the SYNC mode does not work with a 2KHz clock, but the AUTO mode does work.