5 Logic Levels and Parallel Interfaces Flashcards
1
Q
Voltages that modern devices operate at?
A
- An internal supply that is as low as possible – Power reduced by V2
- An external supply that is compatible with an I/O standard e.g. 3.3 V or 5 V
2
Q
What do you need to consider when connecting two devices?
A
- Will the output pin on one device correctly drive the input of the other device?
- The input device has a VIL and VIH which are the input low and high voltages
- The output has a VOL and VOH which must conform to VOL < VIL and VOH > VIH at the specified current
3
Q
What is Tri-State
A
- A tri -state output is an output that can be driven low or high or not driven at all – a high impedance state.
- This allows more than one output to drive one input (at different times) or a pin to be used as both input and output
4
Q
Why are control signals needed?
A
Control signals are needed to ensure that only one output is ever driving the wire at any particular moment
5
Q
Tri-State diagram?
A
6
Q
How do open drains function?
A
- Open drain outputs only drive a wire low, using a transistor as a switch
- A resistor is used to pull the wire high when no output is driving it low
- There can be any number of outputs driving the wire
- Note that the more connections there are the higher the node capacitance
- CR charging will affect maximum data rates
7
Q
Open Drain circuit diagram?
A
8
Q
Details of using an open drain as a driver?
A
- Open drain outputs only drive a signal low
- If the output transistor is strong enough, this can drive higher voltages than the supply voltage of the logic
- E.g. drive a fuel pump relay from a PIC: Diode prevents transient overvoltages caused by inductive loads
- Effectively this is an inverting output
9
Q
Open drain as a driver diagram?
A
10
Q
What is a parallel interface?
A
A parallel interface provides multi-bit I/O between two devices
11
Q
Explain how a Parallel interface works?
A
- Typically parallel outputs are registered each time the relevant address is written on the system bus
- Parallel inputs are tri-state buffered onto the data bus when there is a relevant read on the system bus
- A Data Direction Register controls whether each I/O is driven as an output or read as an input (TRIS register)
- The system bus has Address lines, Data lines and Control lines
12
Q
What does a system bus contain?
A
The system bus has Address lines, Data lines and Control lines
13
Q
Typical Parallel interface commands?
A
- Read – Indicates that the processor is reading its data lines
- Write – Indicates that the processor is driving its data lines
- Clock – Signals are required to be valid at the positive clock edge
14
Q
Example of parallel interface?
A
LCD display
