Whats the difference
In parallel interfacing we transfer multiple bits vs 1 bit in serial interfacing
Purposes
- Synchronizes device with system
- Alters signal levels
- Ecodes data
- Buffers data Notes on parallel interfacing
- If you are storing something from the system bus, you need to put it in a register because the bus data is transient Status Register
- Status signals coming from the device
- Some sort of control “event” oriented information that it wants to share with the CPU Control Register
- Controls signals to the device
- Tells the device how to operate
Parallel Port important take aways
- All connected are to the data line
- Anything you can read is connected to the system bus through a tri-state
- Anything you can write is to a register connected to the system bus
System Bus Side
- Bi-directional busses
- MMIO selection
- Synchronized controls
Device Bus Side
- Uni-direction busses
- Or Bi-directional busses using data direction registers
- For the bus driver, you could also use passive pull-ups, tri-states
- Fore the direction control, port direction is usually configured during initialization once
Persistant Data
When data lives on a line and doesn’t go away until it is either told to go away, or it lives for a really long time in comparison to other actions happening to the line
Note Reading from a device in parallel systems
- Note that to read data from a device, the data out register needs to be set to 1’s to be able to read device data because the data out drives 0’s actively but passively drives 1’s. Therefore, if the data line wants to provide data to the system bus, it needs to drive something actively that beats the passive 1’s from the data out register