Clock stretching

A project log for STEbus 68B09E

68B09E processor board renovation

KeithKeith 05/27/2022 at 20:180 Comments

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:

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.