Selection
If a bus line has more than one driver, a driver must be implicitly or explicitly selected.
See Section VI Part V
Explicit: Technique selects a unique device using bus signals
Implicit: Technique resolves conflicts in a predictable manner using wired logic (must have a passive component to driver)
Tri-State Explicit Selection
The enable signals must be mutually exclusive.
A high resistance PU resistor is added to the shared bus line to ensure a stable value when the bus is idle.
Address Decoding
Centralized Decoding
- There could be a centralized address and timing decoder to turn on devices Decentralized Decoding
- Each device may contain the address and timing decode circuitry
Address Aliasing
- The idea that a portion of the address bus signals can be ignored by the system
- Given a circuit providing an address, there will be bits missing which is why aliasing works
- Idea:
- Think “email aliasing here”, basically the idea that we don’t need to check all their bits (so its like a nickname (Dan instead of Daniel))
- We can uniquely identify our devices without checking everything (without checking every bit)
- If you have four addresses at get you to the same address, one of those is the original address (you don’t know which one it is) and three aliases
Diagrams
See LEARN for diagrams