The designer told me that this board was far harder to design than more advanced processors like the 68020. It stretches the clocks for various reasons:
- Memories of different, but fixed, speeds.
- STEbus access of unknown speed.
- I/O chips that require fixed clock rates.
The Motorola 6809 is an NMOS device and a dynamic design, so the clocks cannot be stretched forever. So this board limits STEbus accesses with a timeout of about 16 microseconds.
The Hitachi 63C09 is a CMOS devices and a fully static design, so the clocks can be stretched indefinitely if this CPU chip is fitted.
The I/O chips need a fixed clock rate (E) to generate baud rate clocks and general timing. This board solves this problem by having separate clocks for the CPU and the I/O chips. The board stretches the CPU clock to synchronise with the I/O clock when accessing I/O chips.