Update: After doing further work on ECU I've discovered that the none of the speculation originally reported in this post is accurate and should be ignored. After locating the code that handles clock setup for the OBD-II communications I've been able to determine the crystal is actually 4.0768Mhz and the system clock runs at 16.3072Mhz.
Old content begins here:
I'd come across a comment on a European forum that the ECU uses a 16.77Mhz crystal, so had been keeping that in the back of my mind while working with the code. I decided to have another look at this as it would be very handy to know for sure.
The crystal on the NNN500030 ECU I've been working with is soldered to the board at both ends, and the only visible marking is 23A18. I'd pondered the possibility that this is a hex representation of the frequency but it would have to a very unusual, although not entirely implausible result.
Checking back on the SIM documentation the CPU will accept a clock up to 20Mhz from an external source, but this requires that the clock signal be supplied on a single pin (EXTAL) and the second pin (XTAL) be left floating. This does not reflect the Td5 CPU layout, which appears to have a crystal attached as a reference for the internal Phase-Lock Loop. The allowable frequency range for the crystal in this case is 4.194 - 5.234Mhz.
Assuming the crystal is the 4.194Mhz used as a default value in the manual, then fsys is configured to 16.776Mhz.
It looks like I'll need to actually measure the crystal to see what frequency is being used, but at least it should be within the range that my multimeter can handle.