Function multiplexing minimizes pin count requirements

The Motorola Modular Family (MMF) is currently composed of two major groups of products; the MC68HC16 family, based on the CPU16 core, and the MC68HC300 family, based on the CPU32 core. Each product in the MMF contains a CPU module, a system integration module, which controls internal to external bus cycles, and one or more of the other modules available in this family. A new system integration module has been designed to utilize reduced pin count packages for decreased cost. A multiplexed test mode allows microcontroller's internal signals to be driven or to be tested even when the pin, previously used to carry the data, is not implemented. This test mode allows the minimum pin set to provide the same controllability and observability as was present during the original test pattern development, and thus provides the same fault coverage. The internal bus protocol must be met, and internal timing must be identical to the one used before. The cost to develop the high fault grade test patterns was very high, and the time to write the patterns was very long-up to several years