6 Serial Interfaces 1 Flashcards
Where do we need to isolate systems?
- When we are connecting our embedded processor to other systems we sometimes need to consider electrical or galvanic isolation
- We often need to isolate high voltages from low voltages in order to protect parts of a circuit
Describe what is meant by an Opto-isolater?
- Opto-isolators allow a signal to pass across a high voltage differential with total galvanic isolation (i.e no chance for current to flow over the gap!)
- Often available as an LED and photo-transistor in a single package
- Typical drive current is 20mA @2V, and gain < 1. Can be used to pass analogue signals, generally very good frequency response (fast!)
Diagram for an optoisolator?
Details of Transformer isolation?
Typically requires less current than an opto-isolator, no need to drive an LED just a small coil
Cannot pass DC components of signals! What about high frequency?
Requires the use of a DC balanced signal:
Diagram of a Power Supply isolator?
Explain what an power supply isolator does?
- These are called DC-DC converters
- They encapsulate all the functions needed to create an isolated DC supply
- Often feedback is included to reduce power consumption
What are real switching issues?
- When a physical switch opens or closes, there is usually a short period of time during which the state changes rapidly back and forth (contact bounce)
- A typical way to solve this is to feed the smoothed input through a Schmitt trigger
- The Schmitt trigger has hysteresis – a tendency to stay in it’s current state
What is a Schmitt trigger
A typical way to solve this is to feed the smoothed input through a Schmitt trigger
The Schmitt trigger has hysteresis – a tendency to stay in it’s current state
Describe a basic serial interface?
- Serial interfaces connect in parallel to the bus
- The I/O side of the interface feeds from/to a shift register
- Depending on the interface, the unit may perform clock recovery to work out when to read the data bits
- Additional controls and connections can indicate receipt of data and errors
Diagram of a basic serial Interface?
Types of Serial interfaces?
Interfaces may be simplex, half-duplex or duplex
How are devices normally laid out in a serial interface?
- The interface may be between more than one device but generally has one controller (master – slave)
- Additional controls and connections can indicate receipt of data and errors
serial interfaces that are relevent?
- USB
- RS-232
- SPI
- I2C
The types of synchronicity?
synchronous, asynchronous or isochronous
Synchronicity: Synchronous: Explain?
Synchronous – Each bit of data is transferred relative to shared (common) clock reference
Synchronicity: Synchronous: Good/Bad?
• Can be tricky to maintain required data rates, often uses FIFO buffers at either end of the link
Synchronicity: Synchronous: Diagram?
Synchronicity: Asynchronous: Explain?
Asynchronous – Data is transferred based on transitions, associated with start and stop conditions that allow for alignment of bytes
Synchronicity: Asynchronous: Good/Bad?
Allows for variations in timing, quite easy to implement but not ideal for timing critical applications
Synchronicity: Asynchronous: Diagram?
Synchronicity: Isochronous: Explain?
Isochronous – Data is transmitted asynchronously but at regular predefined intervals
Synchronicity: Isochronous: Good/Bad?
Used for high quality audio playback over USB at > 44.1 kHz 32-bit
Synchronicity: Isochronous: Diagram?
Advantages of Parrallel interfaces?
For many years parallel data interfaces were the standard for connecting devices together
Parallel interfaces can transfer all N bits in a single clock cycle, and so in theory should be much faster than serial interfaces that transfer 1 bit per clock cycle
