The TPS-1 allows two methods for the clock:
Due to the internal oscillator of the TPS-1 this will probably be the typical way to supply the clock. An example circuit is in the TPS-1 user manual - the 1M resistor is optional. The circuit uses an Epson Toyocom FA-238 Crystal Unit and is actually used on boards from Phoenix Contact for the TPS-1.
2) External oscillator
Such as a 25MHz clock that is already available in the customer's system. In this case, the external clock must be connected to the XCLK1 pin and XCLK2 must be left open.
For the TPS-1 starter kit we have chosen the second method for the clock supply, for the following two reasons:
1) A 25MHz oscillator was already available in the component library that we use for circuit and layout design.
2) Using an external oscillator avoids potential clock start up problems.
In other words, for the TPS-1 starter kit we chose the easier method, we did not choose most economical method. Because of that, the fact that we used an external oscillator on the TPS-1 starter kit should not be considered a recommendation.