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There are several ways to reduce the pin number:
Put mutually exclusive signals on the same pin, e.g.
Group some control signals into sets and enable one set for each system.
Time multiplexing address and data (or control) information on the same pins.
General purpose I/O ports share pins with other functions. Examples:
- IOC stares Port T pins.
- Ports A and B used for external data and address lines in expanded modes.
One way to reduce the system cost is to reduce the number of pins on the chip. In order to provide more functionality and keep the number of pins low. Most microprocessors have multiplexed signals on pins. Some signals are mutually exclusive, such as read and write. Therefore, it is easy to put these functionalities on the same pin. Some functionalities are not mutually exclusive but user may not need all of them at the same time. Therefore, these functions are multiplexed on the same pins so that only one of the multiplexed functions can be used for a given application. For example, the analog to digital input pin may be multiplexed with general input. This requires the user to configure each pin to its intended usage. Some pins are time multiplexed that means the same pin sends different information at different time periodically. For example, it is common that microprocessors time multiplexes the address information and data information on the same set of pins.