I was asking myself, what would a dedicated external trigger add to the project and I could not find a positive answer.

I checked a few documents for expensive LA, and it seems the only purpose of an external trigger input is to have a dedicated BNC or similar connector for interfacing the LA to other devices from the same vendor.

In our case one of the input signals could be used instead of a dedicated trigger input. In any case, to properly use an external trigger, some edge detection is necessary. This and more can be done with generic trigger code, which is applied to all IOs.

The other question is regarding external clock. Do tools from any vendor support external clocking for the SPI protocol, where the clock is silent if slave select is not active. The problem is, if there is no clock, the edge on the slave select signal can not be detected, without clock it is also not possible to push data through input pipeline stages (before clock domain crossing). Maybe this is not the bast example, but the question remains, how sampling on external clock works, if this clock is not available while the system is idle.

Iztok Jeras